text:
Increased induction in FeCo-based nanocomposite materials with reduced early transition metal growth inhibitors
McHenry, Michael E.
Increased induction in FeCo-based nanocomposite materials with reduced early transition metal metal growth inhibitors.2 Nano- crystalline soft magnetic alloys based on the FeCo system called HITPERM- terials with Fe:Co ratios of 50:50 Ref. 3 where low mag- netocrystalline anisotropy is observed in bulk
Density functional plus dynamical mean-field theory of the metal-insulator transition in early transition-metal oxides
NASA Astrophysics Data System (ADS)
Dang, Hung T.; Ai, Xinyuan; Millis, Andrew J.; Marianetti, Chris A.
The combination of density functional theory and single-site dynamical mean-field theory, using both Hartree and full continuous-time quantum Monte Carlo impurity solvers, is used to study the metal-insulator phase diagram of perovskite transition-metal oxides of the form ABO3 with a rare-earth ion A =Sr, La, Y and transition metal B =Ti, V, Cr. The correlated subspace is constructed from atomiclike d orbitals defined using maximally localized Wannier functions derived from the full p-d manifold; for comparison, results obtained using a projector method are also given. Paramagnetic DFT + DMFT computations using full charge self-consistency along with the standard "fully localized limit" (FLL) double counting are shown to incorrectly predict that LaTiO3, YTiO3, LaVO3, and SrMnO3 are metals. A more general examination of the dependence of physical properties on the mean p-d energy splitting, the occupancy of the correlated d states, the double-counting correction, and the lattice structure demonstrates the importance of charge-transfer physics even in the early transition-metal oxides and elucidates the factors underlying the failure of the standard approximations. If the double counting is chosen to produce a p-d splitting consistent with experimental spectra, single-site dynamical mean-field theory provides a reasonable account of the materials properties. The relation of the results to those obtained from "d-only" models in which the correlation problem is based on the frontier orbital p-d antibonding bands is determined. It is found that if an effective interaction U is properly chosen the d-only model provides a good account of the physics of the d1 and d2 materials.
Catalytic properties of early transition metal nitrides and carbides: n-butane hydrogenolysis, dehydrogenation and isomerization
M. K. Neylon; S. Choi; H. Kwon; K. E. Curry; L. T. Thompson
Phase-pure early transition metal nitrides and carbides were prepared via the temperature programmed reaction of metal oxides with NH3 or a CH4\\/H2 mixture. The nitrides and carbides were mostly mesoporous with surface areas up to 81m2\\/g. Their gravimetric butane conversion rates were generally higher than those for a Pt¢Sn\\/Al2O3 catalyst. Activities for the nitrides and carbides ranged from 0.4ū1012 to
Site preference of early transition metal elements in C15 NbCr 2
Chao Jiang
The site preference of early 3d (Ti, V), 4d (Zr, Mo) and 5d (Hf, Ta, W) transition metal elements in C15 NbCr2 Laves phase was studied using first-principles calculations. According to the present calculations, at T=0K, Zr, Hf and Ta consistently have a preference for the Nb sites in Nb-rich, Cr-rich and stoichiometric NbCr2, while the site preference of Ti,
J. Phys. Chem. 1988, 92, 2109-2115 2109 Early-versus Late-Transition-Metal-Oxo Bonds: The Electronlc Structure of VO' and
Goddard III, William A.
oxides. I. Introduction While the electronic structureof neutral transition-metal oxides has been of empirical properties of transition-metal oxides reveals that early metal-oxo compounds exhibit highJ. Phys. Chem. 1988, 92, 2109-2115 2109 Early- versus Late-Transition-Metal-Oxo Bonds
Studies of high temperature ternary phases in mixed-metal-rich early transition metal sulfide and phosphide systems
Marking, G.A.
Investigations of ternary mixed early transition metal-rich sulfide and phosphide systems resulted in the discovery of new structures and new phases. A new series of Zr and Hf - group V transition metal - sulfur K-phases was synthesized and crystallographically characterized. When the group V transition metal was Nb or Ta, the unit cell volume was larger than any previously reported K-phase. The presence of adventitious oxygen was determined in two K-phases through a combination of neutron scattering and X-ray diffraction experiments. A compound Hf{sub 10}Ta{sub 3}S{sub 3} was found to crystallize in a new-structure type similar to the known gamma brasses. This structure is unique in that it is the only reported {open_quotes}stuffed{close_quotes} gamma-brass type structure. The metal components, Hf and Ta, are larger in size and more electropositive than the metals found in normal gamma brasses (e.g. Cu and Zn) and because of the larger metallic radii, sulfur can be incorporated into the structure where it plays an integral role in stabilizing this phase relative to others. X-ray single-crystal, X-ray powder and neutron powder refinements were performed on this structure. A new structure was found in the ternary Nb-Zr-P system which has characteristics in common with many known early transition metal-rich sulfides, selenides, and phosphides. This structure has the simplest known interconnection of the basic building blocks known for this structural class. Anomalous scattering was a powerful tool for differentiating between Zr and Nb when using Mo K{alpha} X-radiation. The compounds ZrNbP and HfNbP formed in the space group Prima with the simple Co{sub 2}Si structure which is among the most common structures found for crystalline solid materials. Solid solution compounds in the Ta-Nb-P, Ta-Zr-P, Nb-Zr-P, Hf-Nb-P, and Hf-Zr-S systems were crystallographically characterized. The structural information corroborated ideas about bonding in metal-rich compounds.
Cooperative reactivity of early-late heterodinuclear transition metal complexes with polar organic substrates
Gade; Memmler; Kauper; Schneider; Fabre; Bezougli; Lutz; Galka; Scowen; McPartlin
A comprehensive investigation into the cooperative reactivity of two chemically complementary metal-complex fragments in early-late heterodinuclear complexes has been carried out. Reaction of the partially fluorinated tripodal amidozirconium complexes [HC-(SiMe2NR)3Zr(mu-Cl)2Li(OEt2)2] (R = 2-FC6H4: 2a, 2,3,4-F3C6H4: 2b) with K[CpM(CO)2] (M=Fe, Ru) afforded the stable metal-metal bonded heterodinuclear complexes [HC[SiMe2NR]3-Zr-MCp(CO)2] (3-6). Reaction of the dinuclear complexes with methyl isonitrile as well as the heteroallenes CO2, CS2, RNCO and RNCS led to insertion into the polar metal-metal bond. Two of these complexes, [HC[SiMe2N(2-FC6-H4)]3Zr(S2C)Fe(CO)2Cp] (9a) and [HC-[SiMe2N(2-FC2H4)]3Zr-(SCNPh)Fe(CO)2-Cp] (12), have been structurally characterized by a single crystal X-ray structure analysis, proving the structural situation of the inserted substrate as a bridging ligand between the early and late transition metal centre. The reactivity towards organic carbonyl derivatives proved to be varied. Reaction of the heterobimetallic complexes with benzyl and ethylbenzoate led to the cleavage of the ester generating the respective alkoxozirconium complexes [HC[SiMe2N(2-FC6H4)]3ZrOR] (R = Ph-CH2: 13a, Et: 13b) along with [CpFe-[C(O)Ph](CO)2], whereas the analogous reaction with ethyl formate gave 13b along with [CpFeH(CO)2]; this latter complex results from the instability of the formyliron species initially formed. Aryl aldehydes were found to react with the Zr-M complexes according to a Cannizzaro disproportionation pattern yielding the aroyliron and ruthenium complexes along with the respective benzoxyzirconium species. The transfer of the aldehyde hydrogen atom in the course of the reaction was established in a deuteriation experiment. [HC[SiMe2-N(2-FC6H4)]3Zr-M(CO)2Cp] reacted with lactones to give the ring-opened species containing an alkoxozirconium and an acyliron or acylruthenium fragment; the latter binds to the early transition metal centre through the acyl oxygen atom, as evidenced from the unusuallly low-field shifted 13C NMR resonances of the RC(O)M units. Ketones containing a-CH units react with the Zr-Fe complexes cooperatively to yield the aldol coupling products coordinated to the zirconium complex fragment along with the hydridoiron compound [CpFeH(CO)2], whereas 1,2-diphenylcyclopropenone underwent an oxygen transfer from the keto group to a CO ligand to give a linking CO2 unit and a cyclopropenylidene ligand coordinated to the iron fragment in [HC-[Si(CH3)2N(2,3,4-F3C6H2)]3Zr(mu-O2C)-Fe(CO)[C3Ph2)Cp] (19). The atom transfer was established by 17O and 13C labelling studies. Similar oxygen-transfer processes were observed in the reactions with pyridine N-oxide, dimethylsulfoxide and methylphenylsulfoxide. PMID:10807180
Cyclopentadienyl-Containing Low-Valent Early Transition Metal Olefin Polymerization Catalysts
Marks, Tobin J. (Evanston, IL); Luo, Lubin (Baton Rouge, LA); Yoon, Sung Cheol (Evanston, IL)
A catalyst system useful to polymerize and co-polymerize polar and non-polar olefin monomers is formed by in situ reduction with a reducing agent of a catalyst precursor comprising {Cp*MRR'.sub.n }.sup.+ {A}.sup.- wherein Cp* is a cyclopentadienyl or substituted cyclopentadienyl moiety; M is an early transition metal; R is a C.sub.1 -C.sub.20 hydrocarbyl; R' are independently selected from hydride, C.sub.1 -C.sub.20 hydrocarbyl, SiR".sub.3, NR".sub.2, OR", SR", GeR".sub.3, SnR".sub.3, and C.dbd.C-containing groups (R"=C.sub.1 -C.sub.10 hydrocarbyl); n is an integer selected to balance the oxidation state of M; and A is a suitable non-coordinating anionic cocatalyst or precursor. This catalyst system may form stereoregular olefin polymers including syndiotactic polymers of styrene and methylmethacrylate and isotactic copolymers of polar and nonpolar olefin monomers such as methylmethacrylate and styrene.
Cyclopentadienyl-containing low-valent early transition metal olefin polymerization catalysts
Marks, Tobin J.; Luo, Lubin; Yoon, Sung Cheol
A catalyst system useful to polymerize and co-polymerize polar and non-polar olefin monomers is formed by in situ reduction with a reducing agent of a catalyst precursor comprising {Cp*MRR'.sub.n}.sup.+{A}.sup.- wherein Cp* is a cyclopentadienyl or substituted cyclopentadienyl moiety; M is an early transition metal; R is a C.sub.1 C.sub.20 hydrocarbyl; R' are independently selected from hydride, C.sub.1 C.sub.20 hydrocarbyl, SiR''.sub.3, NR''.sub.2, OR'', SR'', GeR''.sub.3, SnR''.sub.3, and C.dbd.C-containing groups (R''=C.sub.1 C.sub.10 hydrocarbyl); n is an integer selected to balance the oxidation state of M; and A is a suitable non-coordinating anionic cocatalyst or precursor. This catalyst system may form stereoregular olefin polymers including syndiotactic polymers of styrene and methylmethacrylate and isotactic copolymers of polar and nonpolar olefin monomers such as methylmethacrylate and styrene.
Cyclopentadienyl-containing low-valent early transition metal olefin polymerization catalysts
Marks, Tobin J. (Evanston, IL); Luo, Lubin (Baton Rouge, LA); Yoon, Sung Cheol (Evanston, IL)
A catalyst system useful to polymerize and co-polymerize polar and non-polar olefin monomers is formed by in situ reduction with a reducing agent of a catalyst precursor comprising {Cp*MRR'.sub.n }.sup.+ {A}.sup.- wherein Cp* is a cyclopentadienyl or substituted cyclopentadienyl moiety; M is an early transition metal; R is a C.sub.1 -C.sub.20 hydrocarbyl; R' are independently selected from hydride, C.sub.1 -C.sub.20 hydrocarbyl, SiR".sub.3, NR".sub.2, OR", SR", GeR".sub.3, and SnR".sub.3 containing groups (R"=C.sub.1 -C.sub.10 hydrocarbyl); n is an integer selected to balance the oxidation state of M; and A is a suitable non-coordinating anionic cocatalyst or precursor. This catalyst system may form stereoregular olefin polymers including syndiotactic polymers of styrene and methylmethacrylate and isotactic copolymers of polar and nonpolar olefin monomers such as methylmethacrylate and styrene.
Cyclopentadienyl-containing low-valent early transition metal olefin polymerization catalysts
Marks, Tobin J.; Luo, Lubin; Yoon, Sung Cheol
A catalyst system useful to polymerize and co-polymerize polar and non-polar olefin monomers is formed by in situ reduction with a reducing agent of a catalyst precursor comprising {Cp*MRR'.sub.n}.sup.+{A}.sup.- wherein Cp* is a cyclopentadienyl or substituted cyclopentadienyl moiety; M is an early transition metal; R is a C.sub.1 C.sub.20 hydrocarbyl; R' are independently selected from hydride, C.sub.1 C.sub.20 hydrocarbyl, SiR''.sub.3, NR''.sub.2, OR'', SR'', GeR''.sub.3, SnR''.sub.3, and C.dbd.C-containing groups (R''=C.sub.1 C.sub.10 hydrocarbyl); n is an integer selected to balance the oxidation state of M; and A is a suitable non-coordinating anionic cocatalyst or precursor. This catalyst system may form stereoregular olefin polymers including syndiotactic polymers of styrene and methylmethacrylate and isotactic copolymers of polar and nonpolar olefin monomers such as methylmethacrylate and styrene.
Cyclopentadienyl-containing low-valent early transition metal olefin polymerization catalysts
Marks, Tobin J.; Luo, Lubin; Yoon, Sung Cheol
A catalyst system useful to polymerize and co-polymerize polar and non-polar olefin monomers is formed by in situ reduction with a reducing agent of a catalyst precursor comprising wherein Cp* is a cyclopentadienyl or substituted cyclopentadienyl moiety; M is an early transition metal; R is a C.sub.1 -C.sub.20 hydrocarbyl; R' are independently selected from hydride, C.sub.1 -C.sub.20 hydrocarbyl, SiR".sub.3, NR".sub.2, OR", SR", GeR".sub.3, SnR".sub.3, and C.dbd.C containing groups (R".dbd.C.sub.1 -C.sub.10 hydrocarbyl); n is an integer selected to balance the oxidation state of M; and A is a suitable non-coordinating anionic cocatalyst or precursor. This catalyst system may form stereoregular olefin polymers including syndiotactic polymers of styrene and methylmethacrylate and isotactic copolymers of polar and nonpolar olefin monomers such as methylmethacrylate and styrene.
NASA Astrophysics Data System (ADS)
Khomskii, Daniel I.
1. Localised and itinerant electrons in solids; 2. Isolated transition metal ions; 3. Transition metal ions in crystals; 4. Mott¢Hubbard vc charge-transfer insulators; 5. Exchange interaction and magnetic structures; 6. Cooperative Jahn¢Teller effect and orbital ordering; 7. Charge ordering in transition metal compounds; 8. Ferroelectrics, magnetoelectrics and multiferroics; 9. Doping of correlated systems and correlated metals; 10. Metal-insulator transitions; 11. Kondoeffect, mixed valence and heavy fermions; Appendix A. Some historical notes; Appendix B. A layman's guide to second quantization; Appendix C. Phase transitions and free energy expansion: Landau theory in a nutshell.
Electronic and magnetic properties of early transition-metal substituted iron-cyclopentadienyl sandwich molecular wires: Parity-dependent half-metallicity
NASA Astrophysics Data System (ADS)
Li, Yuanchang; Zhou, Gang; Wu, Jian; Duan, Wenhui
Electronic and magnetic properties of early transition metals (V, Ti, Sc)-FekCpk + 1 sandwich molecular wires (SMWs) are investigated by means of ab initio calculations. It is found that all SMWs favor a ferromagnetic ground state. Significantly, V-FekCpk + 1 SMWs are either half-metallic or semiconducting, dependent upon the parity (even or odd) of the number (k) of Fe atoms in the unit cell of SMWs. This parity oscillation of conductive properties results from the combined effects of the band-folding and gap-opening at the Brillouin-zone boundary of one-dimensional materials. In contrast, Sc-FekCpk + 1 and Ti-FekCpk + 1 SMWs are always semiconducting. Our work may open up the way toward half metal/semiconductor heterostructures with perfect atomic interface.
Paris-Sud XI, Universitķ de
TRANSITION DE MOTT METAL-INSULATOR TRANSITIONS IN TRANSITION METAL OXIDES by D. B. McWHAN, A. MENTH with increasing number of d elec- trons in the transition metal oxides. In the (V1-~Cr~)203system a Mott predominantly non-trivalent ions favor the metallicstate. I. Introduction. -Transition metal oxides form a class
Masatoshi Imada; Atsushi Fujimori; Yoshinori Tokura
Metal-insulator transitions are accompanied by huge resistivity changes, even over tens of orders of magnitude, and are widely observed in condensed-matter systems. This article presents the observations and current understanding of the metal-insulator transition with a pedagogical introduction to the subject. Especially important are the transitions driven by correlation effects associated with the electron-electron interaction. The insulating phase caused by
Oligocyclopentadienyl transition metal complexes
de Azevedo, Cristina G.; Vollhardt, K. Peter C.
Synthesis, characterization, and reactivity studies of oligocyclopentadienyl transition metal complexes, namely those of fulvalene, tercyclopentadienyl, quatercyclopentadienyl, and pentacyclopentadienyl(cyclopentadienyl) are the subject of this account. Thermal-, photo-, and redox chemistries of homo- and heteropolynuclear complexes are described.
Electronic structure and physical properties of early transition metal mononitrides: Density-functional theory LDA, GGA, and screened-exchange LDA FLAPW calculations
C. Stampfl; W. Mannstadt; R. Asahi; A. J. Freeman
The desirable physical properties of hardness, high temperature stability, and conductivity make the early transition metal nitrides important materials for various technological applications. To learn more about the nature of these materials, first-principles density-functional theory calculations using the full-potential linearized augmented plane wave (FLAPW) method within the local-density approximation (LDA) and with the generalized gradient approximation (GGA) have been performed.
Transition Metal Switchable Mirror
The switchable-mirrors technology was developed by Tom Richardson and Jonathan Slack of Berkeley Lab's Environmental Energy Technologies Division. By using transition metals rather than the rare earth metals used in the first metal-hydride switchable mirrors, Richardson and Slack were able to lower the cost and simplify the manufacturing process. Energy performance is improved as well, because the new windows can reflect or transmit both visible and infrared light. Besides windows for offices and homes, possible applications include automobile sunroofs, signs and displays, aircraft windows, and spacecraft.
Transition Metal Switchable Mirror
The switchable-mirrors technology was developed by Tom Richardson and Jonathan Slack of Berkeley Lab's Environmental Energy Technologies Division. By using transition metals rather than the rare earth metals used in the first metal-hydride switchable mirrors, Richardson and Slack were able to lower the cost and simplify the manufacturing process. Energy performance is improved as well, because the new windows can reflect or transmit both visible and infrared light. Besides windows for offices and homes, possible applications include automobile sunroofs, signs and displays, aircraft windows, and spacecraft. More information at: http://windows.lbl.gov/materials/chromogenics/default.htm
Mott Metal Insulator Transitions
Budker, Dmitry
Physics Letters 95, no. 4 (2009) 14 #12;Mott Insulators: Actuators Bimorph Actuators Thermal Sensing Liu, despite being a "toy" model, is used in active research Crystal structures matter! Mott insulators haveMott Metal Insulator Transitions Satej Soman, Robert Tang-Kong March 21, Physics 141A 1 #12
Calixarene supported transition metal clustersĀ
Taylor, Stephanie Merac
This thesis describes a series of calix[n]arene polynuclear transition metal and lanthanide complexes. Calix[4]arenes possess lower-rim polyphenolic pockets that are ideal for the complexation of various transition metal ...
Metal to semiconductor transition in metallic transition metal dichalcogenides
Li, Yan; Kang, Jun; Li, Jingbo, E-mail: email@example.com [State Key Laboratory of Superlattice and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083 (China); Tongay, Sefaattin; Wu, Junqiao [Department of Materials Science and Engineering, University of California, Berkeley, California 94720 (United States); Yue, Qu [College of Science, National University of Defense Technology, Changsha 410073 (China)
We report on tuning the electronic and magnetic properties of metallic transition metal dichalcogenides (mTMDCs) by 2D to 1D size confinement. The stability of the mTMDC monolayers and nanoribbons is demonstrated by the larger binding energy compared to the experimentally available semiconducting TMDCs. The 2D MX{sub 2} (M?=?Nb, Ta; X?=?S, Se) monolayers are non-ferromagnetic metals and mechanically softer compared to their semiconducting TMDCs counterparts. Interestingly, mTMDCs undergo metal-to-semiconductor transition when the ribbon width approaches to ?13?┼ and ?7?┼ for zigzag and armchair edge terminations, respectively; then these ribbons convert back to metal when the ribbon widths further decrease. Zigzag terminated nanoribbons are ferromagnetic semiconductors, and their magnetic properties can also be tuned by hydrogen edge passivation, whereas the armchair nanoribbons are non-ferromagnetic semiconductors. Our results display that the mTMDCs offer a broad range of physical properties spanning from metallic to semiconducting and non-ferromagnetic to ferromagnetic that is ideal for applications where stable narrow bandgap semiconductors with different magnetic properties are desired.
Semiconducting transition metal oxides
NASA Astrophysics Data System (ADS)
Lany, Stephan
Open shell transition metal oxides are usually described as Mott or charge transfer insulators, which are often viewed as being disparate from semiconductors. Based on the premise that the presence of a correlated gap and semiconductivity are not mutually exclusive, this work reviews electronic structure calculations on the binary 3d oxides, so to distill trends and design principles for semiconducting transition metal oxides. This class of materials possesses the potential for discovery, design, and development of novel functional semiconducting compounds, e.g. for energy applications. In order to place the 3d orbitals and the sp bands into an integrated picture, band structure calculations should treat both contributions on the same footing and, at the same time, account fully for electron correlation in the 3d shell. Fundamentally, this is a rather daunting task for electronic structure calculations, but quasi-particle energy calculations in GW approximation offer a viable approach for band structure predictions in these materials. Compared to conventional semiconductors, the inherent multivalent nature of transition metal cations is more likely to cause undesirable localization of electron or hole carriers. Therefore, a quantitative prediction of the carrier self-trapping energy is essential for the assessing the semiconducting properties and to determine whether the transport mechanism is a band-like large-polaron conduction or a small-polaron hopping conduction. An overview is given for the binary 3d oxides on how the hybridization between the 3d crystal field symmetries with the O-p orbitals of the ligands affects the effective masses and the likelihood of electron and hole self-trapping, identifying those situations where small masses and band-like conduction are more likely to be expected. The review concludes with an illustration of the implications of the increased electronic complexity of transition metal cations on the defect physics and doping, using as an example the diversity of possible atomic and magnetic configurations of the O vacancy in TiO2, and the high levels of hole doping in Co2ZnO4 due to a self-doping mechanism that originates from the multivalence of Co.
Semiconducting transition metal oxides.
Lany, Stephan
Open shell transition metal oxides are usually described as Mott or charge transfer insulators, which are often viewed as being disparate from semiconductors. Based on the premise that the presence of a correlated gap and semiconductivity are not mutually exclusive, this work reviews electronic structure calculations on the binary 3d oxides, so to distill trends and design principles for semiconducting transition metal oxides. This class of materials possesses the potential for discovery, design, and development of novel functional semiconducting compounds, e.g. for energy applications. In order to place the 3d orbitals and the sp bands into an integrated picture, band structure calculations should treat both contributions on the same footing and, at the same time, account fully for electron correlation in the 3d shell. Fundamentally, this is a rather daunting task for electronic structure calculations, but quasi-particle energy calculations in GW approximation offer a viable approach for band structure predictions in these materials. Compared to conventional semiconductors, the inherent multivalent nature of transition metal cations is more likely to cause undesirable localization of electron or hole carriers. Therefore, a quantitative prediction of the carrier self-trapping energy is essential for the assessing the semiconducting properties and to determine whether the transport mechanism is a band-like large-polaron conduction or a small-polaron hopping conduction. An overview is given for the binary 3d oxides on how the hybridization between the 3d crystal field symmetries with the O-p orbitals of the ligands affects the effective masses and the likelihood of electron and hole self-trapping, identifying those situations where small masses and band-like conduction are more likely to be expected. The review concludes with an illustration of the implications of the increased electronic complexity of transition metal cations on the defect physics and doping, using as an example the diversity of possible atomic and magnetic configurations of the O vacancy in TiO2, and the high levels of hole doping in Co2ZnO4 due to a self-doping mechanism that originates from the multivalence of Co. PMID:26126022
Transition metal transporters in plants
J. L. Hall; Lorraine E. Williams
Transition metals such as Fe, Cu, Mn, and Zn are essential minerals for normal plant growth and devel- opment, although they can be toxic when present in excess. Thus, for healthy plant growth, a range of transition metals must be acquired from the soil, dis- tributed around the plant, and their concentrations carefully regulated within different cells and orga- nelles.
Living Ziegler-Natta polymerization by early transition metals: synthesis and evaluation of cationic zirconium alkyl complexes bearing beta-hydrogens as models for propagating centers.
Harney, Matthew B; Keaton, Richard J; Fettinger, James C; Sita, Lawrence R
The synthesis and characterization of a series of cationic zirconium and hafnium complexes with alkyl substituents bearing beta-hydrogens of general formula {(eta(5)-C5Me5)MR[N(Et)C(Me)N(t-Bu)]}[B(C6F5)4] [M = Zr; R = Et, n-Pr, i-Pr, n-Bu, i-Bu, and 2-ethylbutyl (5a-f) and M = Hf; R = i-Bu and t-Bu (6 and 7, respectively)] is described, including several isotopically labeled derivatives. The ability of these complexes to serve as model complexes for the living Ziegler-Natta polymerization of olefins that can be effected using the initiator 2a (R = Me in 5) has been addressed. The results obtained shed additional light on the steric and electronic factors that can contribute to the living character of a Ziegler-Natta polymerization based on an early transition metal initiator. PMID:16522123
Selenophene transition metal complexes
White, C.J.
This research shows that selenophene transition metal complexes have a chemistry that is similar to their thiophene analogs. Selenophene coordination has been demonstrated and confirmed by molecular structure in both the {eta}{sup 5}- and the {eta}{sup 1}(Se)-coordination modes. The reaction chemistry of selenophene complexes closely resembles that of the analogous thiophene complexes. One major difference, however, is that selenophene is a better donor ligand than thiophene making the selenophene complexes more stable than the corresponding thiophene complexes. The {sup 77}Se NMR chemical shift values for selenophene complexes fall within distinct regions primarily depending on the coordination mode of the selenophene ligand. In the final paper, the C-H bond activation of {eta}{sup 1}(S)-bound thiophenes, {eta}{sup 1}(S)-benzothiophene and {eta}{sup 1}(Se)-bound selenophenes has been demonstrated. The deprotonation and rearrangement of the {eta}{sup 1}(E)-bound ligand to the carbon bound L-yl complex readily occurs in the presence of base. Reprotonation with a strong acid gives a carbene complex that is unreactive towards nucleophilic attack at the carbene carbon and is stable towards exposure to air. The molecular structure of [Cp(NO)(PPh{sub 3})Re(2-benzothioenylcarbene)]O{sub 3}SCF{sub 3} was determined and contains a Re-C bond with substantial double bond character. Methyl substitution for the thienylcarbene or selenylcarbene gives a carbene that rearranges thermally to give back the {eta}{sup 1}(E)-bound complex. Based on these model reactions, a new mechanism for the H/D exchange of thiophene over the hydrodesulfurization catalyst has been proposed.
Quantum Transport in Transition Metal Oxides
Takada, Yasutami
Quantum Transport in Transition Metal Oxides Anomalous transport properties in transition metal to strong correlation. Another aspect of the transition-metal oxides, in particular in manganese the anomalous current in transition-metal compounds from a viewpoint of the band theory, we have recently
Pressure-Induced Electronic Phase Transitions Transition Metal Oxides and Rare Earth Metals
Islam, M. Saif
Pressure-Induced Electronic Phase Transitions in Transition Metal Oxides and Rare Earth Metals Metal Oxides and Rare Earth Metals by Brian Ross Maddox Electron correlation can affect profound changes transition in a transition metal monoxide. iv #12;The lanthanides (the 4f metals also known as rare-earths
Transition metal sulfide loaded catalyst
Maroni, Victor A. (Naperville, IL); Iton, Lennox E. (Downers Grove, IL); Pasterczyk, James W. (Westmont, IL); Winterer, Markus (Westmont, IL); Krause, Theodore R. (Lisle, IL)
A zeolite based catalyst for activation and conversion of methane. A zeolite support includes a transition metal (Mo, Cr or W) sulfide disposed within the micropores of the zeolite. The catalyst allows activation and conversion of methane to C.sub.2 + hydrocarbons in a reducing atmosphere, thereby avoiding formation of oxides of carbon.
Transition metal sulfide loaded catalyst
Maroni, V.A.; Iton, L.E.; Pasterczyk, J.W.; Winterer, M.; Krause, T.R.
A zeolite-based catalyst is described for activation and conversion of methane. A zeolite support includes a transition metal (Mo, Cr or W) sulfide disposed within the micropores of the zeolite. The catalyst allows activation and conversion of methane to C[sub 2]+ hydrocarbons in a reducing atmosphere, thereby avoiding formation of oxides of carbon.
Transition metals in superheat melts
NASA Technical Reports Server (NTRS)
Jakes, Petr; Wolfbauer, Michael-Patrick
A series of experiments with silicate melts doped with transition element oxides was carried out at atmospheric pressures of inert gas at temperatures exceeding liquidus. As predicted from the shape of fO2 buffer curves in T-fO2 diagrams the reducing conditions for a particular oxide-metal pair can be achieved through the T increase if the released oxygen is continuously removed. Experimental studies suggest that transition metals such as Cr or V behave as siderophile elements at temperatures exceeding liquidus temperatures if the system is not buffered by the presence of other oxide of more siderophile element. For example the presence of FeO prevents the reduction of Cr2O3. The sequence of decreasing siderophility of transition elements at superheat conditions (Mo, Ni, Fe, Cr) matches the decreasing degree of depletion of siderophile elements in mantle rocks as compared to chondrites.
Basic nanosystems of early 4d and 5d transition metals: Electronic properties and the effect of spin-orbit interaction
NASA Astrophysics Data System (ADS)
Bala, Anu; Nautiyal, Tashi; Auluck, Sushil
There are various possibilities for the structure as well as for the growth of nanosystems, particularly of nanowires. The ultimate one-dimensional materialŚlinear chainsŚare difficult to exploit for applications due to their transient nature. Nonetheless these are a good prototype for studying one-dimensional materials and project the kind of behavior one may expect from ultrathin nanowires. Likewise monolayers are the ultimate two-dimensional materials and their study is helpful in understanding the behavior of two-dimensional materials. We present a theoretical study on basic nanosystemsŚlinear chains and monolayersŚof the 4d (Y, Zr, Nb, Mo, and Tc) and 5d (Hf, Ta, W, and Re) transition metals of groups 3-7 by means of an all-electron density functional approach. We have explored all kinds of magnetic configurations: nonmagnetic, ferromagnetic, and antiferromagnetic, by (i) inclusion and (ii) omission of spin-orbit interaction. We find that though this interaction has a marginal effect on nanosystems of 4d transition metals, its impact becomes stronger with lowering of dimensionality. Further it has a significant effect on properties of nanosystems of 5d transition metals as well as those of bulk. It is interesting to note that the monolayers of 5d transition metals seem reluctant to order magnetically despite the general tendency of nanosystems of 4d transition metals and linear chains of 5d transition metals to woo magnetic ordering. The nanosystems with preference for antiferromagnetic ordering are found to be stable at larger nearest-neighbor distances compared to the ferromagnetic and nonmagnetic phases. Specially, antiferromagnetic monolayers of Nb and Mo are predicted to exhibit larger separations with respect to bulk, a feature observed only for some low-dimensional systems. All the monolayers, except Y, are predicted to have a nonmagnetic state almost degenerate with ferromagnetic or antiferromagnetic state. Therefore suitable substrate selection is likely to play an important role in controlling the magnetic ordering in monolayers. Interestingly, Mo linear chains are predicted to have a small energy gap at the Fermi energy, a feature not predicted for any other low-dimensional system of studied transition metals. The stable value of magnetic moment in the vicinity of the equilibrium nearest-neighbor separation for monolayers of Y and for linear chains of Zr, Mo, Tc, and Ta suggests potential of these low-dimensional systems as thermally stable nanoscale devices. Comparison with available experimental data on similar systems confirms consistency of our results. We feel that the results predicted here can be helpful guides to experimentalists as well as theorists.
SHG on Transition Metal Oxides
O. Ney
Optical second harmonic generation (SHG), within the electric-dipole approximation, is known to be sensitive to the surface electronic structure of inversion symmetric media. Besides, it allows to distinguish magneto-optically ferromagnetic from antiferromagnetic order of cubic (001) surfaces. Here, we compute the SHG from NiO(001) and CoO(001) surfaces from first principles. To address the notoriously difficult electronic correlations of transition metal
Electronic and geometric structures of Co2Cn and V2Cn: Initial growth mechanisms of late and early 3d transition-metal carbide clusters
Kensuke Tono; Akira Terasaki; Toshiaki Ohta; Tamotsu Kondow
Photoelectron spectra of Co2Cn- (n=2, 3) and V2Cn- (n=2-4) were measured in the energy range below 3 eV. Analyses of these spectra by the density-functional theory deduced their electronic states and geometric structures. The growth mechanisms of the 3d transition-metal carbide clusters were discussed on the basis of the structural models obtained. The geometric structures of Co2Cn- exhibit a tendency
Synthesis of transition metal carbonitrides
Munir, Zuhair A. R. (Davis, CA); Eslamloo-Grami, Maryam (Davis, CA)
Transition metal carbonitrides (in particular, titanium carbonitride, TiC.sub.0.5 N.sub.0.5) are synthesized by a self-propagating reaction between the metal (e.g., titanium) and carbon in a nitrogen atmosphere. Complete conversion to the carbonitride phase is achieved with the addition of TiN as diluent and with a nitrogen pressure .gtoreq.0.6 MPa. Thermodynamic phase-stability calculations and experimental characterizations of quenched samples provided revealed that the mechanism of formation of the carbonitride is a two-step process. The first step involves the formation of the nonstoichiometric carbide, TiC.sub.0.5, and is followed by the formation of the product by the incorporation of nitrogen in the defect-structure carbide.
Piperazine pivoted transition metal dithiocarbamates
NASA Astrophysics Data System (ADS)
Khan, Sadaf; Nami, Shahab A. A.; Siddiqi, K. S.
A quadridentate ligand disodium bis(2,2'-dithiopiperazinato-2,2'-diamino diethylamine) Na 2L 2 and its self assembled transition metal complexes of the type, M 2(L 2) 2 {M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II)} have been reported. The piperazine pivoted homodinuclear complexes have been characterized by a range of spectral, thermal, microanalytical and conductometric techniques. On the basis of IR and 1HNMR data a symmetrical bidentate coordination of the dithiocarbamato moiety has been observed in all the cases. The TGA profile of the ligand exhibits two stage thermolytic pattern although the complexes decompose in three steps, respectively. Metal sulfide is found to be the end product. The formation of homodinuclear complexes has been ascertained on the basis of FAB mass spectral data and a probable fragmentation pattern has been proposed. On the basis of UV-visible spectroscopic results and room temperature magnetic moment data a tetrahedral geometry has been proposed for all the complexes except for the Ni(II) and Cu(II) which are found to be square-planar.
Transition Metal Sulfide Electrocatalysts for PEM Fuel
Transition Metal Sulfide Electrocatalysts for PEM Fuel Cells Hua Zhang1, Ysmael Verde-G¾mez1 Paso, TX 79968 Transition Metal Sulfide Electrocatalysts for PEM Fuel Cells Hua Zhang1, Ysmael-ray diffraction patterns of RuSx RuSx dispersion on carbon (TEM image) and RuSx/C sample: RuSx/C-1A #12
Transition metal fluorides: from superconductors to multiferroics.Ā
Drathen, Christina
Transition metal fluorides represent an important family of complex solids displaying a variety of different properties and interesting phenomena. Despite their remarkable behaviour, these classes of materials have not ...
Quantum Criticality in Transition-Metal Oxides
N. B³ttgen; H.-A. Krug von Nidda; W. Kraetschmer; A. G³nther; S. Widmann; S. Riegg; A. Krimmel; A. Loidl
We report on experiments of the bulk susceptibility ?(T), heat capacity C(T)\\/T, resistivity ?(T) and nuclear resonances (NMR and NQR) in order to review evidence of quantum critical behaviour in some metallic transition-metal\\u000a oxides. In analogy to the conventional 4f- and 5f-electron based heavy-fermion compounds, the prerequisites of quantum criticality, i.e. a magnetic phase transition at T=0 accompanied by non-Fermi
Orbital Physics in Transition-Metal Oxides
Y. Tokura; N. Nagaosa
An electron in a solid, that is, bound to or nearly localized on the specific atomic site, has three attributes: charge, spin, and orbital. The orbital represents the shape of the electron cloud in solid. In transition-metal oxides with anisotropic-shaped d-orbital electrons, the Coulomb interaction between the electrons (strong electron correlation effect) is of importance for understanding their metal-insulator transitions
Mechanisms for Metal-Nonmental Transitions in Transition-Metal Oxides and Sulfides
David Adler
Transition-metal oxides and sulfides can be insulators, semiconductors, or metals, or undergo metal-nonmetal transitions. The experimental situation in these materials is briefly reviewed. Pure Bloch-Wilson theory predicts that many of the insulators should be metallic, so this class cannot be explained in terms of pure band theory. However, it now appears likely that band theory can be adapted to describe
Orbital polarization transition metal oxides (invited) (abstract)
M. R. Norman
One of the most controversial topics in condensed matter physics concerns the nature of the electronic structure of transition metal oxides. Unpolarized band structure calculations predict metallic ground states for these oxides, whereas experimentally they are insulating. This led to the development of the Mott insulator picture for these materials fifty years ago. Later, though, SlaterÆs group showed that by
Technology Transfer Automated Retrieval System (TEKTRAN)
Proteins obtained from agricultural sources were blended with divalent metal ions. Feather keratin, egg albumin, and wheat gluten had low, medium, and high levels of aspartic and glutamic acid, respectively, and FT-IR showed that the divalent transition metal ions Mn2+, Cu2+, and Zn2+ were tightly ...
Phase transitions in the early universe
NASA Astrophysics Data System (ADS)
Wainwright, Carroll L.
I explore the theory and computation of early-Universe finite-temperature phase transitions involving scalar fields. I focus primarily on the electroweak phase transition, but some of the methods I develop are applicable to any scalar-field cosmological phase transition (such as the computation of the lifetime of zero-temperature metastable vacua). I begin by examining phase transition thermodynamics with many extra coupled degrees of freedom, finding that such transitions have the potential to produce large amounts of entropy and can significantly dilute the concentration of thermal relic species (e.g., dark matter). I then detail a novel algorithm for calculating instanton solutions with multiple dynamic scalar fields, and present a computational package which implements the algorithm and computes the finite-temperature phase structure. Next, I discuss theoretical and practical problems of gauge dependence in finite-temperature effective potentials, using the Abelian Higgs and Abelian Higgs plus singlet models to show the severity of the problem. Finally, I apply the aforementioned algorithm to the electroweak phase transition in the next-to-minimal supersymmetric standard model (NMSSM). My collaborators and I find viable regions of the NMSSM which contain a strongly first-order phase transition and large enough CP violation to support electroweak baryogenesis, evade electric dipole moment constraints, and provide a dark matter candidate which could produce the observed 130 GeV gamma-ray line observed in the galactic center by the Fermi Gamma-ray Space Telescope.
Continuous and discontinuous metal-insulator transitions
N. F. Mott
In non-crystalline systems a continuous metal-insulator transition of Andersontype is frequently observed when the Fermi energy moves through a mobility edge. In crystalline systems, on the other hand, the activation energy for conduction should change discontinuously at the transition. The aim of this paper is to enquire under what conditions a discontinuous transition can occur in non-crystalline systems. It clearly
Microwave-assisted synthesis of transition metal phosphide
Viswanathan, Tito
A method of synthesizing transition metal phosphide. In one embodiment, the method has the steps of preparing a transition metal lignosulfonate, mixing the transition metal lignosulfonate with phosphoric acid to form a mixture, and subjecting the mixture to a microwave radiation for a duration of time effective to obtain a transition metal phosphide.
Method of boronizing transition-metal surfaces
Koyama, K.; Shimotake, H.
A method is presented for preparing a boride layer on a transition metal substrate for use in corrosive environments or as a harden surface in machine applications. This method is particularly useful in treating current collectors for use within a high temperature and corrosive electrochemical cell environment. A melt of a alkali metal boride tetrafluoride salt including such as KF to lower its melting point is prepared including a dissolved boron containing material, for instance NiB, MnB/sub 2/, or CrB/sub 2/. A transition metal to be coated is immersed in the melt at a temperature of no more than 700/sup 0/C and a surface boride layer of that transition metal is formed within a period of about 24 hours on the substrate surface.
Method of boronizing transition metal surfaces
Koyama, Koichiro; Shimotake, Hiroshi.
A method is presented for preparing a boride layer on a transition metal substrate for use in corrosive environments or as a harden surface in machine applications. This method is particularly useful in treating current collectors for use within a high temperature and corrosive electrochemical cell environment. A melt of a alkali metal boride tetrafluoride salt including such as KF to lower its melting point is prepared including a dissolved boron containing material, for instance NiB, MnB[sub 2], or CrB[sub 2]. A transition metal to be coated is immersed in the melt at a temperature of no more than 700 C and a surface boride layer of that transition metal is formed within a period of about 24 hours on the substrate surface. 4 figs.
Cascade morphology transition in bcc metals.
Setyawan, Wahyu; Selby, Aaron P; Juslin, Niklas; Stoller, Roger E; Wirth, Brian D; Kurtz, Richard J
Energetic atom collisions in solids induce shockwaves with complex morphologies. In this paper, we establish the existence of a morphological transition in such cascades. The order parameter of the morphology is defined as the exponent, b, in the defect production curve as a function of cascade energy [Formula: see text]. Response of different bcc metals can be compared in a consistent energy domain when the energy is normalized by the transition energy, ?, between the high- and the low-energy regime. Using Cr, Fe, Mo and W data, an empirical formula of ? as a function of displacement threshold energy, Ed, is presented for bcc metals. PMID:25985256
Intermultiplet transitions in rare-earth metals
Stirling, W.G.; McEwen, K.A.; Loong, C.K.
We report here on direct observations of intermultiplet (IM) transitions in the rare-earth metals, Pr, Nd and Tb, made using the chopper spectrometers at the Intense Pulsed Neutron Source of Argonne National Laboratory. Despite hydrogen contamination of the polycrystalline samples, we have observed inelastic peaks at small Q arising from IM transitions in Pr (approx.260 MeV), Nd (approx.242 MeV) and Tb (approx.256 MeV) although the latter has not yet been observed unambiguously. Future work will investigate the energy level structure and dynamical properties of rare-earth ions in stoichiometric metallic systems. 5 refs., 3 figs.
Cascade morphology transition in bcc metals
NASA Astrophysics Data System (ADS)
Setyawan, Wahyu; Selby, Aaron P.; Juslin, Niklas; Stoller, Roger E.; Wirth, Brian D.; Kurtz, Richard J.
Energetic atom collisions in solids induce shockwaves with complex morphologies. In this paper, we establish the existence of a morphological transition in such cascades. The order parameter of the morphology is defined as the exponent, b, in the defect production curve as a function of cascade energy (NF ? E_MD^b) . Response of different bcc metals can be compared in a consistent energy domain when the energy is normalized by the transition energy, ?, between the high- and the low-energy regime. Using Cr, Fe, Mo and W data, an empirical formula of ? as a function of displacement threshold energy, Ed, is presented for bcc metals.
Disorder driven quantum phase transitions in transition metal oxides
Kohjiro Kobayashi; Nandini Trivedi
We investigate the effect of disorder on a class of transition metal oxides described by a single orbital Hubbard model at half filling and away from half filling. The phases are characterized by the nature of the electronic and spin excitations. We calculate the local density of states, frequency and temperature-dependent conductivity and spin susceptibility as functions of disorder and
Wettability of transition metal oxide surfaces
Aiguo Feng; Benjamin J McCoy; Zuhair A Munir; Domenick Cagliostro
The wettability of transition metal oxide surfaces (TiO2, ZrO2 and HfO2) by water was investigated by measuring the contact angles using the sessile drop method. The oxide surfaces were prepared by thermal oxidation of polished metal foils of Ti, Zr and Hf, which had been polished to different levels of surface roughness. The surfaces were also characterized by optical microscopy,
Thermal decomposition of transition metal dithiocarbamates
S. Singhal; A. N. Garg; K. Chandra
Transition metal dithiocarbamate complexes, [M(S2CN(C2H5)(CH2CH2OH)] (M=Co, Ni, Cu, Zn and Cd) have been prepared and characterized by elemental analysis and infrared spectra. Thermal decomposition of all the complexes occurs in two or three stages. The first stage in all the complexes is always fast with 65¢70% mass loss. In all cases the end product is metal oxide except in the
New approaches to supramolecular transition metal catalysis
M. T. Reetz
The combination of molecular recognition, phase transfer catalysis and transition metal catalysis in one and the same water¢soluble\\u000a catalyst leads to new perspectives, including increased activity, novel substrate selectivity and the possibility of simple\\u000a catalyst recovery. For example, Rh¢complexes of ?¢cyclodextrin modified diphosphanes function as supramolecular catalysts\\u000a in aqueous two¢phase hydrogenation and hydroformylation. Metal¢containing dendritic compounds constitute another new class
Upconversion Processes in Transition Metal and Rare Earth Metal Systems
Daniel R. Gamelin; Hans U. G³del
The design and characterization of new luminescent materials is an active area of research. Here we present several current\\u000a topics in the area of upconversion by transition-metal and rare-earth-metal doped halide lattices. Following introduction\\u000a to the necessary background material related to upconversion mechanisms and kinetics, a series of topics are discussed which\\u000a illustrate some key areas of developing interest in
Superconducting rare earth transition metal borocarbides
S.-L. Drechsler; S. V. Shulga; K.-H. M³ller; G. Fuchs; J. Freudenberger; G. Behr; H. Eschrig; L. Schultz; M. S. Golden; H. von Lips; J. Fink; V. N. Narozhnyi; H. Rosner; P. Zahn; A. Gladun; D. Lipp; A. Kreyssig; M. Loewenhaupt; K. Koepernik; K. Winzer; K. Krug
We present an overview of selected properties of quaternary intermetallic rare earth transition metal borocarbides and related boronitride compounds, as well as of theoretical calculations with possible relevance to the mechanism of superconductivity. The interplay of superconductivity and magnetism for compounds with pure and mixed rare earth components is considered. We suggest that the incommensurate magnetic structure modulated along the
High Pressure Synthesis of Transition Metal Carbonyls.
ERIC Educational Resources Information Center
Hagen, A. P.; And Others
Presents an experiment which uses readily available starting materials and inexpensive equipment for synthesis of transition metal carbonyls at 1000 atm and which is intended to give students experience in techniques used in research and industry. Safety precautions are emphasized. (Author/SA)
Transition metal based borohydrides for hydrogen storage
NASA Astrophysics Data System (ADS)
Jayanthi, Chakram; Liu, Jianjun; Wei, Suhuai; Zhao, Yufeng
Using ab-initio studies based on the density-functional theory, we have calculated binding energies per hydrogen molecule for decomposition reactions of transition metal borohydrides MHxB12H12 to MB12 structures, where M corresponds to Sc, Ti, or V. Depending on the valence of the transition metal, x can be 1, 2, or 3. Crystal structures considered for MB12 included both hypothetical and those found in the international crystallographic structural database. On the other hand, the crystal structure considered for MHxB12H12 belongs to C2/c (space group 15) structure as reported in a previous study [V. Ozolins et al. JACS, 131, 230 (2009)]. Among the structures investigated, Titanium-based metal borohydride structure has the lowest binding energy per hydrogen molecule relative to the cubic TiB12 structure (ś0.37 eV/H2). Our finding should be contrasted with the binding energy/H2 for simple metal based borohydrides (e.g., CaB12H12 ), which has a value of ś 1.5 eV/H2, suggesting that transition metals play a significant role in lowering the H2 binding energy in borohydrides.
(Electronic structure and reactivities of transition metal clusters)
Not Available
The following are reported: theoretical calculations (configuration interaction, relativistic effective core potentials, polyatomics, CASSCF); proposed theoretical studies (clusters of Cu, Ag, Au, Ni, Pt, Pd, Rh, Ir, Os, Ru; transition metal cluster ions; transition metal carbide clusters; bimetallic mixed transition metal clusters); reactivity studies on transition metal clusters (reactivity with H{sub 2}, C{sub 2}H{sub 4}, hydrocarbons; NO and CO chemisorption on surfaces). Computer facilities and codes to be used, are described. 192 refs, 13 figs.
Volume Collapse Transitions in Transition Metals and Compounds at High Pressures
Choong-Shik Yoo; B. Maddox; M. Hu; P. Chow; V. Iota; B. Baer; W. Evans; H. Cynn; D. Hausermann
Unusual phase transitions driven by electron correlation effects occur in many transition metals and transition metal compounds at high pressures, some accompanied with large volume collapses (5-17% in rare-earth metals and 10-15% in magnetic 3d transition metal oxides, for example). However, the exact nature of these transitions has not been well understood, including the relationships between the crystal and electronic
MATERIALS THEORY From transition metal oxides to cosmic strings
MATERIALS THEORY From transition metal oxides to cosmic strings (and how electronic structure Transition metal oxides Thursday, September 20, 2012 6D-MATL / Materials Theory Compounds containing and competing interactions! Energy scale ~ 1eV #12;MATERIALS THEORY Where do transition metal oxides fit
Paris-Sud XI, Universitķ de
125 THEORY OF MOTTTRANSITION : APPLICATION S TO TRANSITION METAL OXIDES M. CYROT Institut Laue discuss the relevance of the theory to transition metal oxides. The main qualitative features in transition metal oxides. These oxides form a very interesting class of materials [1]. Their electrical
Transition metal oxides on organic semiconductors Yongbiao Zhao a
Demir, Hilmi Volkan
Transition metal oxides on organic semiconductors Yongbiao Zhao a , Jun Zhang b , Shuwei Liu Accepted 14 January 2014 Available online 1 February 2014 Keywords: p-doping Transition metal oxide Organic semiconductor Diffusion Organic light-emitting diode a b s t r a c t Transition metal oxides (TMOs) on organic
Thermal decomposition of transition metal dithiocarbamates
S. Singhal; A. N. Garg; K. Chandra
Transition metal dithiocarbamate complexes, [M(S2CN(C2H5)(CH2CH2OH)] (M=Co, Ni, Cu, Zn and Cd) have been prepared and characterized by elemental analysis and infrared spectra. Thermal decomposition\\u000a of all the complexes occurs in two or three stages. The first stage in all the complexes is always fast with 65-70% mass loss.\\u000a In all cases the end product is metal oxide except in the
Potential carcinogenicity of some transition metal ions.
Dovinovß, I; Vachßlkovß, A; Novotn², L
Potential carcinogenicity of some transition metal ions was tested using a direct-current polarography method. The measurements were based on the reduction of tested compounds in an anhydrous solution using alpha-lipoic acid as the detection compound. The potential carcinogenicity was expressed in terms of the parameter tg alpha, which is known to directly correlate with the carcinogenicity of tested compounds. For the metal ions tested, tg alpha was found to decrease in the following sequence: Fe(III) > Pb(II) > V(IV) > Fe(II) > Mn(II) > Cu(II). Zero values of tg alpha were found for Cd(II) and Mn(III). PMID:10065599
Smoothing of ultrathin silver films by transition metal seeding
Anders, Andre; Byon, Eungsun; Kim, Dong-Ho; Fukuda, Kentaro; Lim, Sunnie H.N.
the presence of the transition metal (oxide) islands. Thistransition metals as well as noble metals have a thermodynamic tendency to cluster into 3D islands on oxides.transition metals have the highest affinity to oxygen. To consider whether or not they can reduce the oxide
Excitons in monolayer transition metal dichalcogenides.
Li, J; Zhong, Y L; Zhang, Dong
We theoretically investigate the exciton formed with two massive Dirac particles in monolayer [Formula: see text] and other transition metal dichalcogenides as well as two layers separated by a dielectric layer. In the low-energy limit, the separation of the center-of-mass and relative motions is obtained. Analytical solutions for the exciton wave function and energy dispersion are obtained including the Coulomb interaction between electron and hole, the exciton Bohr radius, binding energy and its effective mass are obtained in monolayer transition metal dichalcogenides. In the case of two monolayers separated by a dielectric layer, we find that the exciton effective mass can be continuously tuned by the interlayer separation. PMID:26190703
Topological crystalline insulators in transition metal oxides.
Kargarian, Mehdi; Fiete, Gregory A
Topological crystalline insulators possess electronic states protected by crystal symmetries, rather than time-reversal symmetry. We show that the transition metal oxides with heavy transition metals are able to support nontrivial band topology resulting from mirror symmetry of the lattice. As an example, we consider pyrochlore oxides of the form A2M2O7. As a function of spin-orbit coupling strength, we find two Z2 topological insulator phases can be distinguished from each other by their mirror Chern numbers, indicating a different topological crystalline insulators. We also derive an effective kĘp Hamiltonian, similar to the model introduced for Pb(1-x)Sn(x)Te, and discuss the effect of an on-site Hubbard interaction on the topological crystalline insulator phase using slave-rotor mean-field theory, which predicts new classes of topological quantum spin liquids. PMID:25167290
Catalytic decomposition of N 2O over monolithic supported noble metal-transition metal oxides
Viviane Boissel; Saad Tahir; Carolyn Ann Koh
The decomposition of nitrous oxide to nitrogen and oxygen using a series of monolithic (ceria-alumina washcoated cordierite) supported transition metal (Cu, Fe, Co, Ni, Mn) and noble metal (Ir, Rh) oxide catalysts has been studied using gas chromatography. The effect of combining a transition metal with a noble metal has also been investigated. A synergetic effect was observed between transition
Muon Sites in Transition Metal Oxides
Kwaichow Benjamin Chan
Muon behavior in a selected series of transition -metal oxides has been investigated by the Muon Spin Rotation (muSR) technique. The materials studied are the corundum structured oxides (M_2 O_3: M = Fe, Cr, V, Ti) and the high-Tc superconducting oxides in Y-Ba-Cu-O system. The muon is first implanted into the oxide crystalline and its subsequent behavior in the presence
The gel route to transition metal oxides
J. Livage
The so-called ``sol-gel'' process offers new approaches to the synthesis of transition metal oxides. Based on inorganic polymerization from molecular precursors, it leads to highly condensed species or colloids. These colloids are actually two-phase systems in which small oxide particles are dispersed in a liquid medium. A very large interface separates both phases and interfacial phenomena, at the oxide-water interface,
Magnetic Interactions in Transition-Metal Oxides
I. V. Solovyev
This a review article, which presents a general framework for the analysis of interatomic magnetic interactions in the spin-density-functional theory, which is based on the magnetic force theorem, make a link with the models for transition-metal oxides, and gives several examples of how this strategy can be used for the analysis of magnetic properties of colossal-magnetoresistive perovskite manganites, double perovskite
Electroluminescent devices from ionic transition metal complexes
Jason D. Slinker; Jonathan Rivnay; Joshua S. Moskowitz; Jeffrey B. Parker; Stefan Bernhard; George G. Malliaras
Ionic transition metal complexes (iTMCs) are receiving increased attention as materials capable of yielding efficient electroluminescent devices with air-stable electrodes. The operational characteristics of these devices are dominated by the presence of mobile ions that redistribute under an applied bias and assist in electronic charge injection. This article reviews recent efforts in the field of iTMC devices: i) to understand
Metal-insulator transition in vanadium dioxide
A. Zylbersztejn; N. F. Mott
The basic physical parameters which govern the metal-insulator transition in vanadium dioxide are determined through a review of the properties of this material. The major importance of the Hubbard intra-atomic correlation energy in determining the insulating phase, which was already evidenced by studies of the magnetic properties of V1-xCrxO2 alloys, is further demonstrated from an analysis of their electrical properties.
Single-layer transition metal sulfide catalysts
Thoma, Steven G. (Albuquerque, NM)
Transition Metal Sulfides (TMS), such as molybdenum disulfide (MoS.sub.2), are the petroleum industry's "workhorse" catalysts for upgrading heavy petroleum feedstocks and removing sulfur, nitrogen and other pollutants from fuels. We have developed an improved synthesis technique to produce SLTMS catalysts, such as molybdenum disulfide, with potentially greater activity and specificity than those currently available. Applications for this technology include heavy feed upgrading, in-situ catalysis, bio-fuel conversion and coal liquefaction.
Electron Dynamics in Transition Metal Granular Films
NASA Astrophysics Data System (ADS)
Massa, N. E.; Denardin, J. C.; Socolovsky, L. M.; Knobel, M.; Zhang, X. X.
Near normal incidence reflectivity spectra of transition metal ś500 nm thick cosputtered granular films on SiO2 subtracts were measured from 30 to 11000 cm-1 and at temperatures from 30 to 490 K. The reflectivity for Co0.85(SO2)0.15 has a frequency and temperature behavior according to conducting metal oxides. The electron scattering rate denotes an unique relaxation time characteristic of a single type of carriers and has a very strong temperature dependence due to strong electron-phonon interactions. Using small polaron fits we individualize these as related to glass stretching vibrational modes. The optical conductivity of Ni0.61(SO2)0.39, undergoing a metal-insulator transition at ś77 K, has a Drude mode (freer carriers) and a mid-IR band (mid-infrared ``carriers''). This last disorder related strong resonance drives the phase transition by localization decreasing in magnitude as the temperature is lowered and points to a double relaxation process (two different scattering mechanisms). On the other hand, Co0.51(SO2)0.49 has an insulator reflectivity in which a distinctive band at ś1450cm-1 originates in electron promotion, localization, and defect induced quasiparticle formation.
Catabolism of hyaluronan: involvement of transition metals.
Soltķs, Ladislav; Kogan, Grigorij
One of the very complex structures in the vertebrates is the joint. The main component of the joint is the synovial fluid with its high-molar-mass glycosaminoglycan hyaluronan, which turnover is approximately twelve hours. Since the synovial fluid does not contain any hyaluronidases, the fast hyaluronan catabolism is caused primarily by reductive-oxidative processes.Eight transition metals - V(23), Mn(25), Fe(26), Co(27), Ni(28), Cu(29), Zn(30), and Mo(42) - naturally occurring in living organism are essential for the control of various metabolic and signaling pathways. They are also the key elements in catabolism of hyaluronan in the joint.In this overview, the role of these metals in physiological and pathophysiological catabolism of hyaluronan is described. The participation of these metals in the initiation and propagation of the radical degradation hyaluronan is critically reviewed. PMID:21217859
Catabolism of hyaluronan: involvement of transition metals
Ŗoltķs, Ladislav; Kogan, Grigorij
One of the very complex structures in the vertebrates is the joint. The main component of the joint is the synovial fluid with its high-molar-mass glycosaminoglycan hyaluronan, which turnover is approximately twelve hours. Since the synovial fluid does not contain any hyaluronidases, the fast hyaluronan catabolism is caused primarily by reductive-oxidative processes. Eight transition metals ¢ V23, Mn25, Fe26, Co27, Ni28, Cu29, Zn30, and Mo42 ¢ naturally occurring in living organism are essential for the control of various metabolic and signaling pathways. They are also the key elements in catabolism of hyaluronan in the joint. In this overview, the role of these metals in physiological and pathophysiological catabolism of hyaluronan is described. The participation of these metals in the initiation and propagation of the radical degradation hyaluronan is critically reviewed. PMID:21217859
Synthesis of arsenic transition metal sulfides and metal arsenides
Singhal, G.H.; Brown, L.D.; Ryan, D.F. [Exxon Research & Development Labs, Baton Rouge, LA (United States)
One of the chief problems in upgrading shale oil is the presence of inherent arsenic which is known to poison downstream catalysts. Highly dispersed transition metal sulfides formed in situ from the decomposition of dithiocarbamate (DTC) complexes of transition metals show excellent potential as dearsenation agents. The authors have studied the reaction of these sulfides with various arsenic compositions and characterized the metal arsenides and arsenic metal sulfides formed as well as the ease of their formation. Thus, the reaction of bis(butyldithiocarbamato)Ni, (NiBuDTC) with model compounds was very facile and gave NiAs, NiAsS, and NiAs2=xSx. In general the effectiveness of the sulfides for dearsenation followed the sequence Ni>Mo{much_gt}Co, while iron sulfides were totally ineffective. Based upon these results, tests were run in autoclaves (as well as a fixed-bed flow-through unit) with NiBuDTC and shale oil having 73 ppm inherent As. Under optimum conditions, dearsenation down to les than 1 ppm was obtained.
The Early History of Chondritic Metal
NASA Astrophysics Data System (ADS)
Kim, J. S.
Recently several related studies of chondritic metal were performed in order to obtain information on its origin and history. Most chondritic metal grains contain inclusions such as silica, chromite, and phosphate. Some inclusions in metal of low petrographic type chondrites contain chain-like structures which suggest that metal agglomerated from relic grains (Perron et al., 1989). Rb-Sr studies on chondritic metal show evolved initial ^87Sr (Podosek et al., 1991). Chronometric information can be obtained from studies of fission Xe from extinct ^244Pu, radiogenic ^129Xe from extinct ^129I, and radiogenic ^40Ar from long-lived ^40K. Therefore, the study of noble gas in chondritic metal can provide independent information on origin and thermal history. Some constraints on the origin and thermal history of metal which are based on detailed noble gas studies are discussed. High-purity (>99.5% by microscopic inspection) metal separates from H3.8 Dhajala (Dh), H4 Ste Marguerite (SM), H4 Forest Vale (FV), and H6 Estacado (Es), were obtained and studied for Ar and Xe isotopic abundances. They contain several noble gas components: fission Xe due to ^244Pu recoils and ^244Pu in inclusions, FVM-Xe (Marti et al., 1989), radiogenic ^129Xe, and radiogenic ^40Ar, together with in situ spallogenic products. 1). The ^244Pu fission Xe record: ^244Pu fission Xe which recoiled from adjacent phosphates is observed in decreasing amounts as Dh > FV=SM >> Es. The release of substantial amounts of ^244Pu-derived fission Xe at low temperatures (600 degrees C) in H4 metal implies that these metal grains were never heated to 600 degrees C after the decay of ^244Pu. 2). FVM-Xe: The metal of low petrographic type chondrites (H3 and 4) contains the novel component FVM-Xe (Marti et al., 1989). The most plausible source of FVM-Xe is a mixture of a ^235U neutron- induced fission Xe component with solar Xe (Kim and Marti, 1992). The phosphate separate from Forest Vale that contains most uranium does not show a neutron irradiation effect (Lavielle et al., 1992). Therefore the neutron irradiation occurred before the Xe closure time of phosphates. Possible sources of neutrons include: secondary neutrons produced by proto-solar activity and neutrons produced by fission of transuranic elements. 3). Radiogenic ^129Xe: All chondritic metals show ^129Xe(sub)r excesses, but the amounts of retained radiogenic ^129Xe(sub)r decrease with increasing petrographic type. The ratios ^129Xe/^132Xe in metal phases are not higher than those of bulk samples, indicating that metamorphic events may have taken place after decay of much of the ^129I. However, Estacado metal was not totally melted during metamorphism because its ^129Xe(sub)r is associated with inclusions. 4). Radiogenic ^40Ar: The amounts of radiogenic ^40Ar are very similar in the different petrographic types as expected for a parent with long half life. Also, the metamorphic event did not strongly fractionate potassium in the metal. In conclusion, acceptable models for the origin of chondritic metal need to consider the following constraints: Before or during accretion in the solar system, nebular materials including metal were exposed to a neutron fluence (>10^16 n/cm^2). After accretion of these early metal grains, secondary processing and metamorphism occurred, and the high petrographic types (H6) lost most of their fission Xe together with FVM-Xe and trapped gases. However, H4 metal was not heated to 600 degrees C after the decay of ^244Pu. References Kim J. S. and Marti K. (1992) Lunar Planet. Sci. (abstract) 23, 689. Lavielle B., Marti K., Pellas P., and Perron C. (1992) Search for 248Cm in the early solar system. Meteoritics (in Press). Marti K., Kim J. S., Lavielle B., Pellas P., and Perron C. (1989) Z. Naturforsch. 44a, 963-967. Perron C., Bourot-Denise M., Pellas P., Marti K., Kim J. S., and Lavielle B. (1989) Lunar Planet. Sci. (abstract) 20, 838. Podosek F. A., Brannon J. C., Perron C., and Pellas P. (1991) Lunar Planet. Sci. (abstract) 22, 1081.
Pseudopotentials for quantum Monte Carlo calculations of transition metal oxides
NASA Astrophysics Data System (ADS)
Krogel, Jaron; Santana, Juan; Kent, Paul; Reboredo, Fernando
Quantum Monte Carlo calculations of transition metal oxides are partially limited by the availability of high quality pseudopotentials that are both accurate in QMC and compatible with major electronic structure codes, e.g. by not being overly hard in the standard planewave basis. Following insight gained from recent GW calculations, a set of neon core pseudopotentials with small cutoff radii have been created for the early transition metal elements Sc to Zn within the local density approximation of DFT. The pseudopotentials have been tested for energy consistency within QMC by calculating the first through fourth ionization potentials of the isolated transition metal (TM) atoms and the binding curve of each TM-O dimer. The vast majority of the ionization potentials fall within 0.3 eV of the experimental values, with exceptions occurring mainly for atoms with multiple unpaired d electrons where multireference effects are the strongest. The equilibrium bond lengths of the dimers are within 1% of experimental values and the binding energy errors are typically less than 0.3 eV. Given the uniform treatment of the core, the larger deviations occasionally observed may primarily reflect the limitations of a Slater-Jastrow trial wavefunction. This work is supported by the Materials Sciences & Engineering Division of the Office of Basic Energy Sciences, U.S. DOE. Research by PRCK was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.
Simple and perovskite oxides of transition-metals: Why some are metallic, while most are insulating
NASA Astrophysics Data System (ADS)
Torrance, Jerry B.; Lacorro, Philippe; Asavaroengchai, Chinnarong; Metzger, Robert M.
Some 76 simple and perovskite transition-metal oxides are classified as "metals," "insulators," and those exhibiting metal-insulator transitions. Using the framework of Zaanen, Sawatzky, and Allen and a simple ionic model to estimate the two relevant energies ( ?0 and U' 0), we can find boundaries which separate the insulating oxides from two types of metals: low ?0 metals and low U' 0 metals. In addition, compounds with metal-insulator transitions are found to be on (or near) these boundaries. It is concluded that the large differences in conductivity behavior of oxides are largely due to differences in the ionization potentials of the transition metal cations.
Bridging Early Services Transition Project--Outreach. Final Report.
ERIC Educational Resources Information Center
Rosenkoetter, Sharon; Shotts, Cynthia
The Bridging Early Services Transition (BEST) Project--Outreach was designed to help families, administrators, and service providers facilitate the transition of young children, birth through age 5, as they move between services and service systems including: from early intervention to special preschool services; from Head Start or preschool toģ
High-Pressure Thermodynamic Properties of f-electron Metals, Transition Metal Oxides, and Half-Metallic Magnets
Richard T. Scalettar; Warren E. Pickett
This project involves research into the thermodynamic properties of f-electron metals, transition metal oxides, and half-metallic magnets at high pressure. These materials are ones in which the changing importance of electron-electron interactions as the distance between atoms is varied can tune the system through phase transitions from localized to delocalized electrons, from screened to unscreened magnetic moments, and from normal metal to one in which only a single spin specie can conduct. Three main thrusts are being pursued: (i) Mott transitions in transition metal oxides, (ii) magnetism in half-metallic compounds, and (iii) large volume-collapse transitions in f-band metals.
High-Pressure Thermodynamic Properties of f-electron Metals, Transition Metal Oxides, and Half-Metallic Magnets
Scalettar, Richard T.; Pickett, Warren E.
This project involves research into the thermodynamic properties of f-electron metals, transition metal oxides, and half-metallic magnets at high pressure. These materials are ones in which the changing importance of electron-electron interactions as the distance between atoms is varied can tune the system through phase transitions from localized to delocalized electrons, from screened to unscreened magnetic moments, and from normal metal to one in which only a single spin specie can conduct. Three main thrusts are being pursued: (1) Mott transitions in transition metal oxides, (2) magnetism in half-metallic compounds, and (3) large volume-collapse transitions in f-band metals.
Ultrafast photophysics of transition metal complexes.
Chergui, Majed
The properties of transition metal complexes are interesting not only for their potential applications in solar energy conversion, OLEDs, molecular electronics, biology, photochemistry, etc. but also for their fascinating photophysical properties that call for a rethinking of fundamental concepts. With the advent of ultrafast spectroscopy over 25 years ago and, more particularly, with improvements in the past 10-15 years, a new area of study was opened that has led to insightful observations of the intramolecular relaxation processes such as internal conversion (IC), intersystem crossing (ISC), and intramolecular vibrational redistribution (IVR). Indeed, ultrafast optical spectroscopic tools, such as fluorescence up-conversion, show that in many cases, intramolecular relaxation processes can be extremely fast and even shorter than time scales of vibrations. In addition, more and more examples are appearing showing that ultrafast ISC rates do not scale with the magnitude of the metal spin-orbit coupling constant, that is, that there is no heavy-atom effect on ultrafast time scales. It appears that the structural dynamics of the system and the density of states play a crucial role therein. While optical spectroscopy delivers an insightful picture of electronic relaxation processes involving valence orbitals, the photophysics of metal complexes involves excitations that may be centered on the metal (called metal-centered or MC) or the ligand (called ligand-centered or LC) or involve a transition from one to the other or vice versa (called MLCT or LMCT). These excitations call for an element-specific probe of the photophysics, which is achieved by X-ray absorption spectroscopy. In this case, transitions from core orbitals to valence orbitals or higher allow probing the electronic structure changes induced by the optical excitation of the valence orbitals, while also delivering information about the geometrical rearrangement of the neighbor atoms around the atom of interest. With the emergence of new instruments such as X-ray free electron lasers (XFELs), it is now possible to perform ultrafast laser pump/X-ray emission probe experiments. In this case, one probes the density of occupied states. These core-level spectroscopies and other emerging ones, such as photoelectron spectroscopy of solutions, are delivering a hitherto unseen degree of detail into the photophysics of metal-based molecular complexes. In this Account, we will give examples of applications of the various methods listed above to address specific photophysical processes. PMID:25646968
Thermoelectric Properties of Early Transition Metal Antimonides
Enkhthsetseg Dashjav; Yulia Zhu; Holger Kleinke
\\u000a Thermoelectric materials may be classified based on their thermoelectric figure-of-merit, namely ZT, which is defined as ZT = TS2?\\/?. Therein, T is the actual temperature, S the Seebeck coefficient (thermopower), and ? and ? are the electrical and the\\u000a thermal conductivities, respectively. The commercially used materials such as Bi2Te3 may exhibit ZT values around 1 at the ideal operating temperature;
Diagnostic Transitions from Childhood to Adolescence to Early Adulthood
ERIC Educational Resources Information Center
Copeland, William E.; Adair, Carol E.; Smetanin, Paul; Stiff, David; Briante, Carla; Colman, Ian; Fergusson, David; Horwood, John; Poulton, Richie; Costello, E. Jane; Angold, Adrian
Background: Quantifying diagnostic transitions across development is needed to estimate the long-term burden of mental illness. This study estimated patterns of diagnostic transitions from childhood to adolescence and from adolescence to early adulthood. Methods: Patterns of diagnostic transitions were estimated using data from three prospective,ģ
Nonpolar resistance switching of metal\\/binary-transition-metal oxides\\/metal sandwiches: Homogeneous\\/inhomogeneous transition of current distribution
I. H. Inoue; S. Yasuda; H. Akinaga; H. Takagi
Exotic features of a metal\\/oxide\\/metal sandwich, which will be the basis for a drastically innovative nonvolatile memory device, is brought to light from a physical point of view. Here the insulator is one of the ubiquitous and classic binary-transition-metal oxides (TMO), such as Fe2O3 , NiO , and CoO . The sandwich exhibits a resistance that reversibly switches between two
Compensation Mechanisms and Functionality of Transition Metal Oxide Surfaces and
Compensation Mechanisms and Functionality of Transition Metal Oxide Surfaces and Interfaces metal oxide surfaces and interfaces can lead to properties and functionality that are not observed-code on HLRBI and HLRBII. 1 Introduction The surfaces and interfaces of transition metal oxides represent
Cavity Ringdown Laser Absorption Spectroscopy of Transition Metal Cluster Systems
James Joseph Scherer
The cavity ringdown technique has been developed for the spectroscopic investigation of transition metal cluster systems, which are produced in a supersonic laser vaporization source. Rovibronic absorption spectra of both bare metal clusters and transition metal silicides have been measured and analyzed to yield information on the specific bonding characteristics of the associated species. Included in this work are the
Reentrant Metal-Insulator Transitions in Silicon -
John William M. Campbell
This thesis describes a study of reentrant metal -insulator transitions observed in the inversion layer of extremely high mobility Si-MOSFETs. Magneto-transport measurements were carried out in the temperature range 20mK-4.2 K in a ^3He\\/^4 He dilution refrigerator which was surrounded by a 15 Tesla superconducting magnet. Below a melting temperature (T_{M}~500 mK) and a critical electron density (n_{s }~9times10^{10} cm^{-2}),
Strain induced fragility transition in metallic glass
Yu, Hai-Bin; Richert, Ranko; Maa▀, Robert; Samwer, Konrad
Relaxation dynamics are the central topic in glassy physics. Recently, there is an emerging view that mechanical strain plays a similar role as temperature in altering the relaxation dynamics. Here, we report that mechanical strain in a model metallic glass modulates the relaxation dynamics in unexpected ways. We find that a large strain amplitude makes a fragile liquid become stronger, reduces dynamical heterogeneity at the glass transition and broadens the loss spectra asymmetrically, in addition to speeding up the relaxation dynamics. These findings demonstrate the distinctive roles of strain compared with temperature on the relaxation dynamics and indicate that dynamical heterogeneity inherently relates to the fragility of glass-forming materials. PMID:25981888
Magnetic correlations in doped transition metal oxides
Aeppli, G. [AT and T Bell Labs., Murray Hill, NJ (United States); Bao, W.; Broholm, C. [Johns Hopkins Univ., Baltimore, MD (United States)] [and others
The authors review recent reactor- and spallation-source-based neutron scattering experiments on the magnetic fluctuations and order in a variety of doped transition metal oxides. In particular, data are shown for the NiO chain compound, Y{sub 2{minus}x}Ca{sub x}BaNiO{sub 5}, the two-dimensional cuprate superconductors La{sub 2{minus}x}Sr{sub x}CuO{sub 4} and YBa{sub 2}Cu{sub 3} O{sub 6+x}, and the classical three-dimensional ``Mott-Hubbard`` system V{sub 2{minus}y}O{sub 3}.
Cosmological Consequences of QCD Phase Transition(s) in Early Universe
Tawfik, A. [ECTP, Egyptian Center for Theoretical Physics, MTI Modern University, Mukattam Cairo (Egypt)
We discuss the cosmological consequences of QCD phase transition(s) on the early universe. We argue that our recent knowledge about the transport properties of quark-gluon plasma (QGP) should thraw additional lights on the actual time evolution of our universe. Understanding the nature of QCD phase transition(s), which can be studied in lattice gauge theory and verified in heavy ion experiments, provides an explanation for cosmological phenomenon stem from early universe.
Precursors in the preparation of transition metal nitrides and transition metal carbonitrides and their reaction intermediates
Maya, L.
This patent describes a composition comprising: a transition metal bound to; a first ligand selected from the group inorganic amide and imide ligands; and a second ligand being acetylide ligands. This patent also describes a process for making ceramics comprising: pyrolyzing the precursor in an inert atmosphere.
Metal Insulator transition in Vanadium Dioxide
NASA Astrophysics Data System (ADS)
Jovaini, Azita; Fujita, Shigeji; Suzuki, Akira; Godoy, Salvador
MAR12-2011-000262 Abstract Submitted for the MAR12 Meeting of The American Physical Society Sorting Category: 03.9 (T) On the metal-insulator-transition in vanadium dioxide AZITA JOVAINI, SHIGEJI FUJITA, University at Buffalo, SALVADOR GODOY, UNAM, AKIRA SUZUKI, Tokyo University of Science --- Vanadium dioxide (VO2) undergoes a metal-insulator transition (MIT) at 340 K with the structural change from tetragonal to monoclinic crystal. The conductivity _/ drops at MIT by four orders of magnitude. The low temperature monoclinic phase is known to have a lower ground-state energy. The existence of the k-vector k is prerequisite for the conduction since the k appears in the semiclassical equation of motion for the conduction electron (wave packet). The tetragonal (VO2)3 unit is periodic along the crystal's x-, y-, and z-axes, and hence there is a three-dimensional k-vector. There is a one-dimensional k for a monoclinic crystal. We believe this difference in the dimensionality of the k-vector is the cause of the conductivity drop. Prefer Oral Session X Prefer .
Transition Metal Oxides: Extra Thermodynamic Stability as Thin Films
Charles T. Campbell
Introduction.ŚTransition metals are used as industrial catalysts for a wide range of oxidation reactions of great importance in fuel processing, chemical production, and pollution cleanup. A key fundamental question is whether the surface under catalytic reaction conditions is metallic or is, instead, the metal's oxide. We show here that since these oxides often wet their metals, an? 1n mthick film
Transition-metal-mediated thiosulfinate ester synthesis
Raseta, M.E.; Cawood, S.A.; Welker, M.E. (Wake Forest Univ., Winston-Salem, NC (USA)); Rheingold, A.L. (Univ. of Delaware, Newark (USA))
Unlike sulfur dioxide (SO{sub 2}), the coordination and organic reaction chemistry of disulfur monoxide (S{sub 2}O) has received little attention. A few Diels-Alder reactions of simple dienes with S{sub 2}O have been reported. However, there were no reports of direct S{sub 2}O complex synthesis prior to our initial work. The authors recently published a synthesis of 4,5-diphenyl-3,6-dihydro-1,2-dithiin 1-oxide (2) which liberates S{sub 2}O via a transition-metal-assisted retro-Diels-Alder reaction. Here we report further on the unusual reactivity of 2 and its utilization in the synthesis of cyclic thiosulfinate esters.
Radiation damage of transition metal carbides
Dixon, G.
In this grant period we have investigated electrical properties of transition metal carbides and radiation-induced defects produced by low-temperature electron irradiation in them. Special attention has been given to the composition VC[sub 0.88] in which the vacancies on the carbon sublattice of this fcc crystal order to produce a V[sub 8]C[sub 7] superlattice. The existence of this superlattice structure was found to make the crystal somewhat resistant to radiation damage at low doses and/or at ambient temperature. At larger doses significant changes in the resistivity are produced. Annealing effects were observed which we believe to be connected with the reconstitution of the superlattice structure.
Combustion Synthesis of Transition Metal Carbosulfides
NASA Astrophysics Data System (ADS)
Wally, Pablo; Ueki, Masanori
The combustion synthesis of ternary and quaternary carbosulfides of the transition metals was investigated to obtain high-purity materials to be tested as solid lubricants in high-temperature environments. The main carbosulfide formation mechanisms have been clarified in the case of the technologically interesting tantalum carbosulfide, enabling the preparation of reaction products with controlled purity. The formation of molten tantalum sulfide as an intermediate product was found to play a key role for obtaining a homogeneous reaction product, and the presence of excess carbon reduced the oxygen content of the carbosulfide. However, the higher the overall carbon contents, the higher is the amount of carbide impurities in the reaction products. Use of iodine or hexachlorobenzene as a catalyzer minimized the need of excess sulfur in the raw powder mixture, reduced the amount of carbide impurities, and contributed to the homogenization of the reaction products.
Electronic entanglement in late transition metal oxides
Thunstr÷m, Patrik; Eriksson, Olle
Here we present a study of the entanglement in the electronic structure of the late transition metal monoxides - MnO, FeO, CoO, and NiO - obtained by means of density-functional theory in the local density approximation combined with dynamical mean-field theory (LDA+DMFT). The impurity problem is solved through Exact Diagonalization (ED), which grants full access to the thermally mixed many-body ground state density operator. The quality of the electronic structure is affirmed through a direct comparison between the calculated electronic excitation spectrum and photoemission experiments. Our treatment allows for a quantitative investigation of the entanglement in the electronic structure. Two main sources of entanglement are explicitly resolved through the use of a fidelity based geometrical entanglement measure, and additional information is gained from a complementary entropic entanglement measure. We show that the interplay of crystal field effects and Coulomb interaction causes the entanglement in CoO to ...
Transition to School from Pacific Islands Early Childhood Services.
ERIC Educational Resources Information Center
Sauvao, Le'autuli'ilagi M.; Mapa, Lia; Podmore, Valerie N.
Noting the need for additional information on the transition of children from Pacific Islands early childhood services to primary school, this exploratory study was designed to provide an account of the experiences of children, parents, and teachers, focusing on language and other aspects of children's move from Pacific Islands early childhoodģ
Polytypism in superhard transition-metal triborides
NASA Astrophysics Data System (ADS)
Liang, Yongcheng; Yang, Jiong; Yuan, Xun; Qiu, Wujie; Zhong, Zheng; Yang, Jihui; Zhang, Wenqing
The quest of novel compounds with special structures and unusual functionalities continues to be a central challenge to modern materials science. Even though their exact structures have puzzled scientists for decades, superhard transition-metal borides (TMBs) have long been believed to exist only in simple crystal structures. Here, we report on a polytypic phenomenon in superhard WB3 and MoB3 with a series of energetically degenerate structures due to the random stacking of metal layers amongst the interlocking boron layers. Such polytypism can create a multiphase solid-solution compound with a large number of interfaces amongst different polytypes, and these interfaces will strongly hinder the interlayer sliding movement within each polytype, thereby further increase the hardness of this particular material. Furthermore, in contrast to the conventional knowledge that intrinsically strong chemical bonds in superhard materials should lead to high lattice thermal conductivity, the polytypic TMB3 manifest anomalously low lattice thermal conductivity due to structural disorders and phonon folding. These findings promise to open a new avenue to searching for novel superhard materials with additional functionalities.
Polytypism in superhard transition-metal triborides.
Liang, Yongcheng; Yang, Jiong; Yuan, Xun; Qiu, Wujie; Zhong, Zheng; Yang, Jihui; Zhang, Wenqing
The quest of novel compounds with special structures and unusual functionalities continues to be a central challenge to modern materials science. Even though their exact structures have puzzled scientists for decades, superhard transition-metal borides (TMBs) have long been believed to exist only in simple crystal structures. Here, we report on a polytypic phenomenon in superhard WB3 and MoB3 with a series of energetically degenerate structures due to the random stacking of metal layers amongst the interlocking boron layers. Such polytypism can create a multiphase solid-solution compound with a large number of interfaces amongst different polytypes, and these interfaces will strongly hinder the interlayer sliding movement within each polytype, thereby further increase the hardness of this particular material. Furthermore, in contrast to the conventional knowledge that intrinsically strong chemical bonds in superhard materials should lead to high lattice thermal conductivity, the polytypic TMB3 manifest anomalously low lattice thermal conductivity due to structural disorders and phonon folding. These findings promise to open a new avenue to searching for novel superhard materials with additional functionalities. PMID:24863493
Transition-metal substitutions in iron chalcogenides
NASA Astrophysics Data System (ADS)
Bezusyy, V. L.; Gawryluk, D. J.; Malinowski, A.; Cieplak, Marta Z.
The a b -plane resistivity and Hall effect are studied in Fe1 -yMyTe0.65Se0.35 single crystals doped with two transition-metal elements, M = Co or Ni, over a wide doping range, 0 ?y ?0.2 . The superconducting transition temperature, Tc, reaches zero for Co at y ?0.14 and for Ni at y ?0.032 , while the resistivity at the Tc onset increases weakly with Co doping, and strongly with Ni doping. The Hall coefficient RH, positive for y =0 , remains so at high temperatures for all y , while it changes sign to negative at low T for y >0.135 (Co) and y >0.06 (Ni). The analysis based on a two-band model suggests that at high T residual hole pockets survive the doping, but holes get localized upon the lowering of T , so that the effect of the electron doping on the transport becomes evident. The suppression of the Tc by Co impurity is related to electron doping, while in the case of the Ni impurity strong electron localization most likely contributes to fast decrease of the Tc.
Nanostructured transition metal oxides useful for water oxidation catalysis
Frei, Heinz M; Jiao, Feng
The present invention provides for a composition comprising a nanostructured transition metal oxide capable of oxidizing two H.sub.2O molecules to obtain four protons. In some embodiments of the invention, the composition further comprises a porous matrix wherein the nanocluster of the transition metal oxide is embedded on and/or in the porous matrix.
Quantum superconductor metal transition in a proximity array
Skvortsov, Mikhail
Quantum superconductor ± metal transition in a proximity array M V Fe|¯gel'man, A I Larkin, M A Skvortsov Abstract. A theory of the zero-temperature superconductor- metal transition is developed [1] that the superconductor ± insulator transi- tion happens at the universal quantum value RQ ha4e2
Copyright A. J. Millis 2013 Columbia University Transition Metal Oxides
Millis, Andrew
Copyright A. J. Millis 2013 Columbia University Transition Metal Oxides A. J. Millis Department and Prospects #12;Copyright A. J. Millis 2013 Columbia University Transition metal ``oxides and Electronic Structure 4. Theory--General 5. Dynamical Mean Field Theory and Oxides 6. Applications
Technology Transfer Automated Retrieval System (TEKTRAN)
The carbohydrate trehalose was a preliminary test model to be used as a replacement conjugate in an alternate transition-metal complex. Using an oxidant, we hypothesized that if the primary alcohols formed aldehyde functional groups upon oxidation then the transition metal could complex with the tr...
Characterization of electrically conductive transition metal dichalcogenide lubricant films
Harish C. Waghray
Groups VB and VIB transition metal dichalcogenides with layered structures, are intrinsic solid lubricants, and constitute a class of materials with unique and unusual properties based on their extreme anisotropy. The primary objective of this investigation was to conduct a comprehensive study on the tribological and electrical properties of burnished and sputtered transition metal dichalcogenide films, and characterize the performance
Pristine and intercalated transition metal dichalcogenide superconductors
NASA Astrophysics Data System (ADS)
Klemm, Richard A.
Transition metal dichalcogenides (TMDs) are quasi-two-dimensional layered compounds that exhibit strongly competing effects of charge-density wave (CDW) formation and superconductivity (SC). The weak van der Waals interlayer bonding between hexagonal layers of octahedral or trigonal prismatic TMD building blocks allows many polytypes to form. In the single layer 1 T polytype materials, one or more CDW states can form, but the pristine TMDs are not superconducting. The 2 H polytypes have two or more Fermi surfaces and saddle bands, allowing for dual orderings, which can be coexisting CDW and SC orderings, two SC gaps as in MgB2, two CDW gaps, and possibly even pseudogaps above the onset TCDW s of CDW orderings. Higher order polytypes allow for multiple CDW gaps and at least one superconducting gap. The CDW transitions TCDW s usually greatly exceed the superconducting transitions at their low Tc values, their orbital order parameters (OPs) are generally highly anisotropic and can even contain nodes, and the SC OPs can be greatly affected by their simultaneous presence. The properties of the CDWs ubiquitously seen in TMDs are remarkably similar to those of the pseudogaps seen in the high-Tc cuprates. In 2H-NbSe2, for example, the CDW renders its general s-wave SC OP orbital symmetry to be highly anisotropic and strongly reduces its Josephson coupling strength (IcRn) with the conventional SC, Pb. Hence, the pristine TMDs are highly "unconventional" in comparison with Pb, but are much more "conventional" than are the ferromagnetic superconductors such as URhGe. Applied pressure and intercalation generally suppress the TMD CDWs, allowing for enhanced SC formation, even in the 1 T polytype materials. The misfit intercalation compound (LaSe)1.14(NbSe2) and many 2 H -TMDs intercalated with organic Lewis base molecules, such as TaS2(pyridine)1/2, have completely incoherent c-axis transport, dimensional-crossover effects, and behave as stacks of intrinsic Josephson junctions. Except for the anomalously large apparent violation of the Pauli limit of the upper critical field of (LaSe)1.14(NbSe2), these normal state and superconducting properties of these intercalation compounds are very similar to those seen in the high-Tc superconductor, Bi2Sr2CaCu2O8+? and in the organic layered superconductor, ?-(ET)2Cu[N(CN)2]Br, where ET is bis(ethylenedithio) tetrathiafulvalene. Electrolytic intercalation of TMDs with water and metallic ions leads to compounds with very similar properties to cobaltates such as NaxCoO2 Ę y H2O.
Route to transition metal carbide nanoparticles through cyanamide and metal oxides
P. G. Li; M. Lei; W. H. Tang
We have designed an efficient route to the synthesis of transition metal carbide nanoparticles starting from an organic reagent cyanamide and transition metal oxides. Four technologically important metal carbide nanoparticles such as tungsten carbide, niobium carbide, tantalum carbide and vanadium carbide were synthesized successfully at moderate temperatures. It is found that cyanamide is an efficient carburization reagent and that the
Nutritional immunity: transition metals at the pathogen-host interface
Hood, M. Indriati; Skaar, Eric P.
Transition metals occupy an essential niche in biological systems. Their electrostatic properties stabilize substrates or reaction intermediates in the active sites of enzymes, while their heightened reactivity is harnessed for catalysis. However, the latter property renders transition metals toxic at high concentrations. Bacteria, like all living organisms, must regulate the levels of these elements to satisfy their physiological needs while avoiding harm. It is therefore not surprising that the host capitalizes on both the essentiality and toxicity of transition metals to defend against bacterial invaders. This review will discuss established and emerging paradigms in nutrient metal homeostasis at the pathogen-host interface. PMID:22796883
Steam Reforming on Transition-metal Carbides from Density-functional Theory
Vojvodic, Aleksandra
A screening study of the steam reforming reaction on clean and oxygen covered early transition-metal carbides surfaces is performed by means of density-functional theory calculations. It is found that carbides provide a wide spectrum of reactivities, from too reactive via suitable to too inert. Several molybdenum-based systems are identified as possible steam reforming catalysts. The findings suggest that carbides provide a playground for reactivity tuning, comparable to the one for pure metals.
Electric quadrupole transitions in x-ray spectra: 3d transition-metal oxides
N. V. Dobrodey; Yu. V. Luniakov
The intensities of the electric quadrupole transitions in the x-ray spectra of the 3d-transition-metal monoxide diatomic molecules were calculated using the discrete variational method of the local-density approximation. The quadrupole transition intensities have been found to be negligible for the left-hand-side 3d metal oxides. It was shown that the electric quadrupole transitions can appreciably contribute to the MKbeta5 emission spectra
Electric quadrupole transitions in x-ray spectra: 3d transition-metal oxides
NASA Astrophysics Data System (ADS)
Dobrodey, N. V.; Luniakov, Yu. V.
The intensities of the electric quadrupole transitions in the x-ray spectra of the 3d-transition-metal monoxide diatomic molecules were calculated using the discrete variational method of the local-density approximation. The quadrupole transition intensities have been found to be negligible for the left-hand-side 3d metal oxides. It was shown that the electric quadrupole transitions can appreciably contribute to the MK?5 emission spectra of the right-hand-side 3d metal oxides, along with the dipole transitions.
F. R. Schwartzberg; H. R. Ogden; R. I. Jaffee
Ductile-brittle transition behavior occurs in the bodycentered cubic ; refractory metals niobium, molybdenum. tungsten, and chromium. The available ; data for vanadium suggest that brittleness at low temperatures is attributable a ; low-stress-rate hydogen embrittlement. However, the occurrence of ductile-; brittle transition behavior is not precluded. No brittle transition has yet been ; found in tantalum. Trsnsition temperature can be
Reentrant Metal-Insulator Transitions in Silicon -
NASA Astrophysics Data System (ADS)
Campbell, John William M.
This thesis describes a study of reentrant metal -insulator transitions observed in the inversion layer of extremely high mobility Si-MOSFETs. Magneto-transport measurements were carried out in the temperature range 20mK-4.2 K in a ^3He/^4 He dilution refrigerator which was surrounded by a 15 Tesla superconducting magnet. Below a melting temperature (T_{M}~500 mK) and a critical electron density (n_{s }~9times10^{10} cm^{-2}), the Shubnikov -de Haas oscillations in the diagonal resistivity enormous maximum values at the half filled Landau levels while maintaining deep minima corresponding to the quantum Hall effect at filled Landau levels. At even lower electron densities the insulating regions began to spread and eventually a metal-insulator transition could be induced at zero magnetic field. The measurement of extremely large resistances in the milliKelvin temperature range required the use of very low currents (typically in the 10^ {-12} A range) and in certain measurements minimizing the noise was also a consideration. The improvements achieved in these areas through the use of shielding, optical decouplers and battery operated instruments are described. The transport signatures of the insulating state are considered in terms of two basic mechanisms: single particle localization with transport by variable range hopping and the formation of a collective state such as a pinned Wigner crystal or electron solid with transport through the motion of bound dislocation pairs. The experimental data is best described by the latter model. Thus the two dimensional electron system in these high mobility Si-MOSFETs provides the first and only experimental demonstration to date of the formation of an electron solid at zero and low magnetic fields in the quantum limit where the Coulomb interaction energy dominates over the zero point oscillation energy. The role of disorder in favouring either single particle localization or the formation of a Wigner crystal is explored by considering a variety of samples with a wide range of mobilities and by varying the ratio of the carrier density (controlled by the applied gate voltage) to the impurity density (fixed during sample growth). A phase diagram showing the boundaries between the two dimensional electron gas, the Wigner solid, and the single particle localization induced insulator is established in terms of carrier density and sample mobility.
Paris-Sud XI, Universitķ de
METAL-NON METAL TRANSITIONS /N RARE EARTH COMPOUNDS. EXPERIMENT AND THEORK /. VALENCE INSTABILITIES, superconductivity, electron-phonon and band theory, to name a few. 2. Properties of normal rare earth metals. - Before discussing rare earth valence instabilities, three relevant general features of rare earth metals
Chemistry of polynuclear transition-metal complexes in ionic liquids.
Ahmed, Ejaz; Ruck, Michael
Transition-metal chemistry in ionic liquids (IL) has achieved intrinsic fascination in the last few years. The use of an IL as environmental friendly solvent, offers many advantages over traditional materials synthesis methods. The change from molecular to ionic reaction media leads to new types of materials being accessible. Room-temperature IL have been found to be excellent media for stabilising transition-metal clusters in solution and to crystallise homo- and heteronuclear transition-metal complexes and clusters. Furthermore, the use of IL as solvent provides the option to replace high-temperature routes, such as crystallisation from the melt or gas-phase deposition, by convenient room- or low-temperature syntheses. Inorganic IL composed of alkali metal cations and polynuclear transition-metal cluster anions are also known. Each of these areas will be discussed briefly in this contribution. PMID:21743925
A ferroelectric-like structural transition in a metal
NASA Astrophysics Data System (ADS)
Shi, Youguo; Guo, Yanfeng; Wang, Xia; Princep, Andrew J.; Khalyavin, Dmitry; Manuel, Pascal; Michiue, Yuichi; Sato, Akira; Tsuda, Kenji; Yu, Shan; Arai, Masao; Shirako, Yuichi; Akaogi, Masaki; Wang, Nanlin; Yamaura, Kazunari; Boothroyd, Andrew T.
Metals cannot exhibit ferroelectricity because static internal electric fields are screened by conduction electrons, but in 1965, Anderson and Blount predicted the possibility of a ferroelectric metal, in which a ferroelectric-like structural transition occurs in the metallic state. Up to now, no clear example of such a material has been identified. Here we report on a centrosymmetric () to non-centrosymmetric (R3c) transition in metallic LiOsO3 that is structurally equivalent to the ferroelectric transition of LiNbO3 (ref. ). The transition involves a continuous shift in the mean position of Li+ ions on cooling below 140?K. Its discovery realizes the scenario described in ref. , and establishes a new class of materials whose properties may differ from those of normal metals.
A ferroelectric-like structural transition in a metal.
Shi, Youguo; Guo, Yanfeng; Wang, Xia; Princep, Andrew J; Khalyavin, Dmitry; Manuel, Pascal; Michiue, Yuichi; Sato, Akira; Tsuda, Kenji; Yu, Shan; Arai, Masao; Shirako, Yuichi; Akaogi, Masaki; Wang, Nanlin; Yamaura, Kazunari; Boothroyd, Andrew T
Metals cannot exhibit ferroelectricity because static internal electric fields are screened by conduction electrons, but in 1965, Anderson and Blount predicted the possibility of a ferroelectric metal, in which a ferroelectric-like structural transition occurs in the metallic state. Up to now, no clear example of such a material has been identified. Here we report on a centrosymmetric (R3c) to non-centrosymmetric (R3c) transition in metallic LiOsO3 that is structurally equivalent to the ferroelectric transition of LiNbO3 (ref. 3). The transition involves a continuous shift in the mean position of Li(+) ions on cooling below 140 K. Its discovery realizes the scenario described in ref. 2, and establishes a new class of materials whose properties may differ from those of normal metals. PMID:24056805
Electronic entanglement in late transition metal oxides
Patrik Thunstr÷m; Igor Di Marco; Olle Eriksson
Here we present a study of the entanglement in the electronic structure of the late transition metal monoxides - MnO, FeO, CoO, and NiO - obtained by means of density-functional theory in the local density approximation combined with dynamical mean-field theory (LDA+DMFT). The impurity problem is solved through Exact Diagonalization (ED), which grants full access to the thermally mixed many-body ground state density operator. The quality of the electronic structure is affirmed through a direct comparison between the calculated electronic excitation spectrum and photoemission experiments. Our treatment allows for a quantitative investigation of the entanglement in the electronic structure. Two main sources of entanglement are explicitly resolved through the use of a fidelity based geometrical entanglement measure, and additional information is gained from a complementary entropic entanglement measure. We show that the interplay of crystal field effects and Coulomb interaction causes the entanglement in CoO to take a particularly intricate form.
Photoelectron spectroscopy of transition metal oxide interfaces
NASA Astrophysics Data System (ADS)
Zegenhagen, J÷rg
In the present paper we review applications of the photoelectron spectroscopy (PES) technique to the investigation of transition metal oxide (TMO) interfaces. We summarize very briefly some of the principle, specific characteristics of TMOs. Because of the buried nature of the interfaces, the photoelectrons must penetrate certain thicknesses of material, which is easier with higher kinetic energies in the keV range. Thus, we also briefly summarize some of the hallmarks of hard X-ray photoelectron spectroscopy (HAXPES), before presenting four explicit samples of the analysis of TMO interfaces: The LaAlO3/SrTiO3 (0 0 1) interface, which had attracted attention because of the discovery of a sheet of high mobility electrons below the thin layer of LaAlO3; superlattices of the two insulators CaCuO2 and SrTiO3, which can be prepared to become superconducting at about 40 K; epitaxial films of the 90 K superconductor GdBa2Cu3O7-? on NdGaO3; and finally the analysis of the nucleation of the 90 K superconductor YBa2Cu3O7-? on SrTiO3 (0 0 1). The latter two cases of the investigation of the superconducting films are examples of photoelectron spectroscopy by X-ray standing wave (XSW) excitation. Because of this, the bare essential features of the XSW technique are also briefly reviewed.
Environmentally relevant metal and transition metal ions enhance Fc epsilon RI-mediated mast cell activation.
Walczak-Drzewiecka, Aurelia; Wycz¾lkowska, Janina; Dastych, Jaroslaw
Upon contact with allergen, sensitized mast cells release highly active proinflammatory mediators. Allergen-mediated mast cell activation is an important mechanism in the pathogenesis of atopic asthma. Asthmatic patients are especially susceptible to air pollution. Epidemiologic studies found a positive correlation between severity of symptoms among asthmatic patients and the level of particulate matter (PM) in the air. Among the constituents of PM are metals and transition metals, which could mediate some of its adverse effects on human health. We sought to determine the effect of metal and transition metal ions on allergen-mediated mast cell activation. We observed that several metal and transition metal ions activated mast cells and enhanced allergen-mediated mast cell activation. Thus, Al(3+), Cd(2+), and Sr(2+) induced release of granule-associated N-acetyl-ss-d-hexosaminidase, and Al(3+) and Ni(2+) enhanced antigen-mediated release. Metal and transition metal ions also induced significant secretion of interleukin (IL)-4 and increased antigen-mediated IL-4 secretion in mast cells. These effects of metal and transition metal ions on mast cells were observed at concentrations that do not result in direct cytotoxicity and might be relevant for environmental exposure. Thus, metals and transition metals could increase the level of allergen-mediated mast cell activation, which might be one of the mechanisms mediating exacerbation of allergen-driven asthma symptoms by air pollution. PMID:12727598
Critical Elements and Practices of Transition Programs Linking Early Childhood Education and Early Elementary School.
ERIC Educational Resources Information Center
Jang, Younghee
This paper examines how various elements of five early- childhood-to-elementary school transition programs are being developed and carried out. Five transition programs were identified and investigated in regard to shared leadership and decisionmaking among stakeholders, comprehensive and integrated services, family involvement and empowerment,ģ
Transition Program Practices: Improving Linkages between Early Childhood Education and Early Elementary School.
ERIC Educational Resources Information Center
Jang, Younghee; Mangione, Peter L.
Whether the transition to primary school is smooth or abrupt for children depends on whether early childhood and elementary school programs work together to build bridges between their services. This report presents several examples of collaborative efforts to smooth the transition to school. Specifically, it describes how various elements of fiveģ
The interactions between transition metal nanoparticles and their metal-oxide supports are often critical for heterogeneous metal nanoparticle
Napp, Nils
Reports The interactions between transition metal nanoparticles and their metal- oxide supports at the metal-support interface is widely regarded as the key step in room-temperature CO oxidation (13 water, surface hydroxyls, and the metal-support inter- face interact during CO oxidation over Au/TiO2
Supporting the Child in Transition Packet. Bridging Early Services Transition Project.
ERIC Educational Resources Information Center
Rosenkoetter, Sharon; Shotts, Cynthia
This packet provides a variety of information pieces to help early childhood educational programs prepare young children with disabilities for transition out of one program and into another. The booklet opens with sample thoughts children may have when anticipating transition. Next, 20 possible differences among programs are specificallyģ
Family Partnership in Transition Planning Packet. Bridging Early Services Transition Project.
ERIC Educational Resources Information Center
Rosenkoetter, Sharon; Shotts, Cynthia
This packet of handouts focuses on the role of the family in easing their child with disabilities from one early childhood educational program to another. Typical items include: reasons for involving families in transition; questions parents ask about "appropriate" placement; planned conversations with parents during the transition process; parentģ
Process for making transition metal nitride whiskers
Bamberger, C.E.
A process for making metal nitrides, particularly titanium nitride whiskers, using a cyanide salt as a reducing agent for a metal compound in the presence of an alkali metal oxide. Sodium cyanide, various titanates and titanium oxide mixed with sodium oxide react to provide titanium nitride whiskers that can be used as reinforcement to ceramic composites. 1 fig., 1 tab.
Electrolytic separation of crystals of transition-metal oxides
NASA Technical Reports Server (NTRS)
Arnott, R. J.; Feretti, A.; Kunnamann, W.
Versatile flux system grows large, well-formed, stoichiometric single crystals of mixed oxides of the transition-metal elements. These crystals have important uses in the microwave field, and applications as lasers and masers in communications.
Charge, orbital and magnetic ordering in transition metal oxidesĀ
Senn, Mark Stephen
Neutron and x-ray diffraction has been used to study charge, orbital and magnetic ordering in some transition metal oxides. The long standing controversy regarding the nature of the ground state (Verwey structure) of the ...
Recent Advances in Transition Metal¢Catalyzed O-Glycosylations
Xiaohua Li; Jianglong Zhu
This review summarizes recent accomplishments involving transition metal catalysis in efficient and stereoselective O-glycosylation reactions. In addition, proposed mechanisms of each catalytic O-glycosylation are discussed respectively.
Electronic structure of double-perovskite transition-metal oxides
Y. Moritomo; Sh. Xu; A. Machida; T. Akimoto; E. Nishibori; M. Takata; M. Sakata
Electronic structure has been investigated by reflectivity measurement for ordered-perovskite transition-metal oxides, Sr2MMoO6 (M=Cr, Mn, Fe, and Co). The ground states of the Co and Mn compounds are paramagnetic insulators, making a sharp contrast with the ferrimagnetic metallic Sr2FeMoO6. For all the compounds, we have observed intense optical transition at ~2 eV, which has been ascribed to the charge-transfer excitation
Upper Critical Fields of Transition Metal Alloy Superconductors
T. G. Berlincourt; R. R. Hake
An interpretation is presented of the persistence of superconductivity ; of transition metal alloys at very high magnetic fields. The interpretation is ; based on pulsed-magneticfield data on the low-current-density resistive critical ; fields of concentrated transition metal alloys, the GinsbergLandau-Abrikosov-; Gor'kov theory, and a paramagnetically limited critical-field criterion. The ; arguments add weight to previous evidence of a negative-surface-energy
Nonpercolative metal-insulator transition in VO2 single crystals
Bongjin Simon Mun; Kai Chen; Joonseok Yoon; Catherine Dejoie; Nobumichi Tamura; Martin Kunz; Zhi Liu; Michael E. Grass; Sung-Kwan Mo; Changwoo Park; Y. Yvette Lee; Honglyoul Ju
Detailed temperature-dependent transport, optical microscopy, and synchrotron-based polychromatic x-ray microdiffraction measurements have been carried out in the vicinity of the metal-insulator transition (MIT) temperature of VO2 single crystals. The formation and propagation of a real-space phase boundary along the rutile c axis is monitored during the transition. Pure metallic rutile R, as well as insulating monoclinic M1 phases, is observed
Search for transition zone lines in early dwarf A stars
R. Freire Ferrero
The presence of hot external atmospheric layers in some A stars, demonstrated by the X-ray detections with the Einstein satellite, stimulated the search for optical counterparts, in particular, the search for the UV transition zone indicators like Si IV, C IV and He II lines. The observations of a sample of early dwarf A stars with IUE allows one to
Mortality transition and differential incentives for early retirement
Paris-Sud XI, Universitķ de
Mortality transition and differential incentives for early retirement Hippolyte d'Albis Sau-Him Paul Lau , ¦ Miguel Sßnchez-RomeroČ Abstract Many studies specify human mortality patterns parametrically, with a parameter change affecting mortality rates at different ages si- multaneously. Motivated
Be Prepared: Tips for Transitioning into Early Childhood Education
ERIC Educational Resources Information Center
Oliver, Laura Ann
The first days of school can be scary for parents, too. As a child reaches an age where she is ready to begin school, the very thought can overwhelm any parent. As a parent of a child with a disability there are added considerations. Nonetheless, transitions through the early years of school can become relatively easier for both the parents andģ
Facilitating Family Involvement in Early Intervention to Preschool Transition
ERIC Educational Resources Information Center
Pang, Yanhui
Active family involvement and important family roles in the early intervention to preschool transition have been mandated by laws, recognized by the position statements of professional organizations, and validated through evidence-based research. In order to involve families in this process, reduce stresses, and conquer the challenges families mayģ
Early Warning Signals for Critical Transitions: A Generalized Modeling Approach
Lade, Steven J.; Gross, Thilo
Critical transitions are sudden, often irreversible, changes that can occur in a large variety of complex systems; signals that warn of critical transitions are therefore highly desirable. We propose a new method for early warning signals that integrates multiple sources of information and data about the system through the framework of a generalized model. We demonstrate our proposed approach through several examples, including a previously published fisheries model. We regard our method as complementary to existing early warning signals, taking an approach of intermediate complexity between model-free approaches and fully parameterized simulations. One potential advantage of our approach is that, under appropriate conditions, it may reduce the amount of time series data required for a robust early warning signal. PMID:22319432
The discontinuous volume change in transition-metal compounds due to the pressure-induced spin-state transition
F. Ogata; T. Kambara; N. Sasaki; K. I. Gondaira
Several transition-metal compounds change their volume abruptly under high pressure through their electronic spin-state transition. The pressure-induced spin-state transitions in transition-metal compounds are studied on the basis of the ligand-field theory by using the model in which the coupling of transition-metal ions with crystal distortions is taken into account. The various types of transitions occur due to the crystal distortion
Hydrothermal synthesis of transition metal oxides under mild conditions
M Stanley Whittingham
A notable development in mild (? 200░C) hydrothermal synthesis in the past few years has been the extension of the synthesis technique from zeolites and metal phosphates to binary and ternary transition metal oxides. This low-temperature technique is critical to the formation of metastable phases not obtainable using traditional high-temperature methods. New phases, as well as known materials, of the
[Non-empirical interatomic potentials for transition metals
Not Available
The report is divided into the following sections: potential-energy functions for d-band metals, potential-energy functions for aluminides and quasicrystals, electronic structure of complex structures and quasicrystals, potential-energy functions in transition-metal oxides, applications to defect structure and mechanical properties, and basic theory of interatomic potentials.
[Non-empirical interatomic potentials for transition metals]. Progress report
Not Available
The report is divided into the following sections: potential-energy functions for d-band metals, potential-energy functions for aluminides and quasicrystals, electronic structure of complex structures and quasicrystals, potential-energy functions in transition-metal oxides, applications to defect structure and mechanical properties, and basic theory of interatomic potentials.
Metal-Insulator Transitions at Surfaces Michael Potthoff
Potthoff, Michael
of a solid binary alloy (surface segregation) [2]. Typical exam- ples for surface phase transitions are also that for the correlation-driven Mott metal-insulator transition the surface phase diagram should be expected to have the same topology as the phase diagram for magnetic order at surfaces: The corresponding linearized mean
Magnetic Collapse in Transition Metals Oxides at High Pressures
R. E. Cohen; I. I. Mazin; D. G. Isaak
We predict magnetic collapse (i.e. high-spin low-spin transitions) in transition metal ions using LMTO-ASA and LAPW methods and the GGA. Magnetic collapse under pressure results from band widening, rather than from changes in crystal field splitting. We find that the extended Stoner model is accurate in predicting the magnetic moments and transition pressures compared with our self-consistent calculations. The GGA
Transit times for electromagnetic waves in metallic layered systems
NASA Astrophysics Data System (ADS)
Palomino-Ovando, M.; Coyotķcatl, H. A.; Esquivel-Sirvent, R.; Cocoletzi, Gregorio H.
Metallic multilayered arrays are considered, in the local Drude theory, to investigate transit times of electromagnetic waves and the fast response mediated by plasmon polaritons. The transit times are calculated for frequencies corresponding to band gaps in the dispersion relation of periodic layered media. At these frequencies, and depending on the thickness of the structure, a fast response or even a superluminal effect is predicted. This effect is more evident near the plasmon polariton resonances. Moreover, the time delay is also affected by the surface plasmon coupling between the metallic layers. The metallic superlattices are described according to the Drude theory.
Melting of bcc Transition Metals and Icosahedral Clustering
Ross, M; Boehler, R; Japel, S
In contrast to polyvalent metals, transition metals have low melting slopes(dT/dP) that are due to partially filled d-bands that allow for a lowering of liquid phase energy through s-d electron transfer and the formation of local structures. In the case of bcc transition metals we show the apparent discrepancy of DAC melting measurements with shock melting of Mo can be understood by reexamining the shock data for V and Ta and introducing the presence of an icosahedral short range order (ISRO) melt phase.
Theory of metal insulator transition in strongly correlated electron systems
Gulacsi, M.; Bedell, K.S.
A comprehensive theory of the correlation driven metal insulator transition in 1D and 2D strongly correlated electron systems is given. In both the 1D and 2D Hubbard model the metal insulator transition encountered close to half filling is of Pokrovsky-Talapov type. An important consequence of this in the 2D Hubbard model is the break down of the Fermi liquid theory. We also describe in detail the properties of the Pokrovsky-Talapov transition in 2D ferroelectrics.
Application of Two Dimensional Flourescence Spectroscopy to Transition Metal Clusters.
NASA Astrophysics Data System (ADS)
Kokkin, Damian L.; Steimle, Timothy
Determining the physical properties (bond lengths, angles, dipole moments, etc) of transition metal oxides and dioxides is relevant to catalysis, high temperature chemistry, materials science and astrophysics. Analysis of optical spectra is a convenient method for extraction of physical properties, but can be difficult because of the density of electronic states and in the case of the dioxides, presence of both the oxide and superoxide forms. Here we demonstrate the application of two dimensional fluorescence spectroscopy for aiding in the assignment and analysis. Particular attention will be paid to the spectroscopy of first row transition metal monoxides and dioxides of Nickel, NiO and NiO_2, and Manganese, MnO. Furthermore, the application of this technique to discovering the spectrum of other transition metal systems such as Metal-dicarbides will be outlined. N.J. Reilly, T.W. Schmidt, S.H. Kable, J. Phys. Chem. A., 110(45), 12355-12359, 2006
Insulator-to-Metal Transition in Sulfur-Doped Silicon
NASA Astrophysics Data System (ADS)
Winkler, Mark T.; Recht, Daniel; Sher, Meng-Ju; Said, Aurore J.; Mazur, Eric; Aziz, Michael J.
We observe an insulator-to-metal transition in crystalline silicon doped with sulfur to nonequilibrium concentrations using ion implantation followed by pulsed-laser melting and rapid resolidification. This insulator-to-metal transition is due to a dopant known to produce only deep levels at equilibrium concentrations. Temperature-dependent conductivity and Hall effect measurements for temperatures T>1.7K both indicate that a transition from insulating to metallic conduction occurs at a sulfur concentration between 1.8 and 4.3ū1020cm-3. Conduction in insulating samples is consistent with variable-range hopping with a Coulomb gap. The capacity for deep states to effect metallic conduction by delocalization is the only known route to bulk intermediate band photovoltaics in silicon.
Binding and catalytic reduction of NO by transition metal aluminosilicates
Klier, K.; Herman, R.G.; Hou, Shaolie.
The objective of this research is to provide the scientific understanding of processes that actively and selectively reduce NO in dilute exhaust streams, as well as in concentrated streams, to N{sub 2}. Experimental studies of NO chemistry in transition metal-containing aluminosilicate catalysts are being carried out with the aim of determining the chemical rules for NO reduction on non-precious metals. The catalyst supports chosen for this investigation are A and Y zeolites, mordenite, and monoliths based on cordierite. The supported transition metal cations that were examined are principally the first row redox metals, e.g. Cr(2), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Cu(I). The reactions of interest are the reductions of NO by H{sub 2}, CO, and CH{sub 4}, as well as the disproportionation of NO. Rare earth cations that possess redox properties were placed in the more shielded sites, e.g. Site I in Y zeolite, prior to or simultaneously with the exchange procedure with the transition metal cations. Theoretical calculations of the electronic structure of the transition metal cations in zeolitic sites were carried out by ab initio methods. The aim of this part of the research is to find the best match between the metal-based antibonding orbitals and the antibonding orbitals of the NO molecule such that the N-O bond is weakened and is readily broken. 9 refs., 4 figs., 3 tabs.
Early warning of atmospheric regime transitions using transfer operators
NASA Astrophysics Data System (ADS)
Tantet, Alexis; Dijkstra, Henk
The existence of persistent midlatitude atmospheric regimes, such as blocking events, with time scales larger than 5-10 days and indications of preferred transition paths between them motivates the development of early-warning indicators of regime transitions. Here, we use a barotropic model of the northern midlatitudes winter flow to study such meta-stable regimes. We look at estimates of transfer operators acting on densities evolving on a reduced phase space spanned by the first Empirical Orthogonal Functions of the streamfunction and develop an early-warning indicator of zonal to blocked flow transition. The study of the spectra of transfer operators estimated for different lags reveals a multi-level structure in the flow as well as the effect of memory on the reduced dynamics due to past interactions between the resolved and unresolved variables. The slowest motions in the reduced phase space are thereby found to have time scales larger than 8 days and to behave as Markovian for larger lags. These motions are associated with meta-stable regimes and their transitions and can be detected as almost-invariant sets of the transfer operator. The early-warning indicator is based on the action on an initial density of products of the transfer operators estimated for sufficiently long lags, making use of the semi-group property of these operators and shows relatively good Peirce skill score. From the energy budget of the model, we are able to explain the meta-stability of the regimes and the existence of preferred transition paths as the manifestation of barotropic instability. Finally, even though the model is highly simplified, the skill of the early warning indicator is promising, suggesting that the transfer operator approach can be used in parallel to an operational deterministic model for stochastic prediction or to assess forecast uncertainty.
High-Pressure Thermodynamic Properties of f-electron Metals, Transition Metal Oxides, and Half-Metallic Magnets
Richard T. Scalettar; Warren E. Pickett
This project involves research into the thermodynamic properties of f-electron metals, transition metal oxides, and half-metallic magnets at high pressure. These materials are ones in which the changing importance of electron-electron interactions as the distance between atoms is varied can tune the system through phase transitions from localized to delocalized electrons, from screened to unscreened magnetic moments, and from normal
(S)TEM analysis of functional transition metal oxides
NASA Astrophysics Data System (ADS)
Chi, Miaofang
Perovskite vanadates (AVO3) form an ideal family to study the structure-property relationships in transition metal oxides because their physical properties can easily be tailored by varying the A-site cations. (S)TEM is an ideal tool for this type of study due to its capacity for simultaneous imaging and chemical analysis. Determination of the oxidation state of vanadium in complex oxides have been carried out by electron energy loss spectroscopy. SrVO3/LaAlO3 is then studied both experimentally and theoretically as a prototype system. Extra electrons have been detected on the interface layer, and further proven to originate mainly from a change in the local bonding configuration of V at the La-O terminated substrate surface. Cr-containing stainless steel deposited with a LaCrO3 thin-film layer is a promising interconnect material of Solid Oxide Fuel Cells (SOFC). Our investigation on its microstructural evolution reveals that the LaCrO 3 thin film plays a role in inhibiting the growth of an oxide layer on the metal surface and thus protects the surface of the stainless steel. Ca-doped LaCoO3 is a promising SOFC cathode material. The domain structures and the oxidation state of Co in Ca-doped LaCoO3, which are directly related to its mechanical properties and electronic conductivity, are investigated by in-situ TEM and EELS. The formation of microcracks is observed during thermal cycles. Ca-doping in LaCoO3 is shown to not only improve the electronic conductivity of the material, but is also likely to strengthen the grain boundaries. The realization of its application in SOFCs depends on depressing the ferroelastisity to reduce strain formation during thermal cycles. The application of the (S)TEM techniques used for studying the perovskite systems are further extended to other compounds containing transition metal elements. The refractory minerals from Comet 81 P/Wild-2 are studied to investigate the formation of the early solar system. A relatively high Ti3+/Ti 4+ ratio in fassaite and the presence of osbornite indicate that the Comet refractory minerals formed in the inner solar nebula and were later transported to the outer solar system where the comet formed. This implies a much more dynamic and perhaps more violent solar nebula than was previously suspected.
Quark-Hadron Phase Transitions in Viscous Early Universe
A. Tawfik; T. Harko
Based on hot big bang theory, the cosmological matter is conjectured to undergo QCD phase transition(s) to hadrons, when the universe was about $1-10 \\mu$s old. In the present work, we study the quark-hadron phase transition, by taking into account the effect of the bulk viscosity. We analyze the evolution of the quantities relevant for the physical description of the early universe, namely, the energy density $\\rho$, temperature $T$, Hubble parameter $H$ and scale factor $a$ before, during and after the phase transition. To study the cosmological dynamics and the time evolution we use both analytical and numerical methods. By assuming that the phase transition may be described by an effective nucleation theory (prompt {\\it first-order} phase transition), we also consider the case where the universe evolved through a mixed phase with a small initial supercooling and monotonically growing hadronic bubbles. The numerical estimation of the cosmological parameters, $a$ and $H$ for instance, makes it clear that the time evolution varies from phase to phase. As the QCD era turns to be fairly accessible in the high-energy experiments and the lattice QCD simulations, the QCD equation of state is very well defined. In light of this, we introduce a systematic study of the {\\it cross-over} quark-hadron phase transition and an estimation for the time evolution of Hubble parameter.
Transition metal complexes with (NO) 3 ? anions
Itai Panas; Anders Snis; Filip Acke; Tobias Johnson
Density functional theory calculations are performed to determine structures, stabilities, and characteristic vibrational frequencies for metal complexes with nitric oxide trimer ligands. The study emphasizes the possible crucial importance of the 3.0 eV first ionization energy of the (NO)3? anion. Metal complexes with the composition M[(NO)3]2 are considered, where M is Mg, Ni, Cu, and Zn. Properties of the Cu(NO)3
Metal Selectivity Determinants in a Family of Transition Metal Transporters*
Podar, Dorina; Scherer, Judith; Noordally, Zeenat; Herzyk, Pawel; Nies, Dietrich; Sanders, Dale
Metal tolerance proteins (MTPs) are plant members of the cation diffusion facilitator (CDF) transporter family involved in cellular metal homeostasis. Members of the CDF family are ubiquitously found in all living entities and show principal selectivity for Zn2+, Mn2+, and Fe2+. Little is known regarding metal selectivity determinants of CDFs. We identified a novel cereal member of CDFs in barley, termed HvMTP1, that localizes to the vacuolar membrane. Unlike its close relative AtMTP1, which is highly selective for Zn2+, HvMTP1 exhibits selectivity for both Zn2+ and Co2+ as assessed by its ability to suppress yeast mutant phenotypes for both metals. Expression of HvMTP1/AtMTP1 chimeras in yeast revealed a five-residue sequence within the AtMTP1 N-segment of the His-rich intracytoplasmic loop that confines specificity to Zn2+. Furthermore, mutants of AtMTP1 generated through random mutagenesis revealed residues embedded within transmembrane domain 3 that additionally specify the high degree of Zn2+ selectivity. We propose that the His-rich loop, which might play a role as a zinc chaperone, determines the identity of the metal ions that are transported. The residues within transmembrane domain 3 can also influence metal selectivity, possibly through conformational changes induced at the cation transport site located within the membrane or at the cytoplasmic C-terminal domain. PMID:22139846
Theoretical analysis of structural stability of TM5Si3 transition metal silicides
Yue Chen; A. N. Kolmogorov; D. G. Pettifor; Jia-Xiang Shang; Yue Zhang
A combination of electronic-structure calculations from density-functional theory (DFT) through a tight-binding (TB) model to analytic bond-order potentials (BOPs) has been used to investigate the structural trend of the TM5Si3 compounds across the early transition metals (TM). First of all, the formation energies of TM5Si3 , whose ground states adopt the competing D88 , D8l , or D8m structure types,
Flexible metallic seal for transition duct in turbine system
Flanagan, James Scott; LeBegue, Jeffrey Scott; McMahan, Kevin Weston; Dillard, Daniel Jackson; Pentecost, Ronnie Ray
A turbine system is disclosed. In one embodiment, the turbine system includes a transition duct. The transition duct includes an inlet, an outlet, and a passage extending between the inlet and the outlet and defining a longitudinal axis, a radial axis, and a tangential axis. The outlet of the transition duct is offset from the inlet along the longitudinal axis and the tangential axis. The transition duct further includes an interface member for interfacing with a turbine section. The turbine system further includes a flexible metallic seal contacting the interface member to provide a seal between the interface member and the turbine section.
Transition Metals in Control of Gene Expression
NASA Astrophysics Data System (ADS)
O'Halloran, Thomas V.
Metalloproteins play structural and catalytic roles in gene expression. The metalloregulatory proteins are a subclass that exerts metal-responsive control of genes involved in respiration, metabolism, and metal-specific homeostasis or stress-response systems, such as iron uptake and storage, copper efflux, and mercury detoxification. Two allosteric mechanisms for control of gene expression were first discovered in metalloregulatory systems: an iron-responsive translational control mechanism for ferritin production and a mercury-responsive DNA-distortion mechanism for transcriptional control of detoxification genes. These otherwise unrelated mechanisms give rise to a rapid physiological response when metal ion concentrations exceed a dangerous threshold. Molecular recognition in these allosteric metal ion receptors is achieved through atypical coordination geometries, cluster formation, or complexes with prosthetic groups, such as sulfide and heme. Thus, many of the inorganic assemblies that otherwise buttress the structure of biopolymers or catalyze substrate transformation in active sites of enzymes have also been adapted to serve sensor functions in the metalloregulatory proteins. Mechanistic studies of these metal-sensor protein interactions are providing new insights into fundamental aspects of inorganic chemistry, molecular biology, and cellular physiology.
Transition metal-substituted cobalt ferrite nanoparticles for biomedical applications.
Sanpo, Noppakun; Berndt, Christopher C; Wen, Cuie; Wang, James
Transition metals of copper, zinc, chromium and nickel were substituted into cobalt ferrite nanoparticles via a sol-gel route using citric acid as a chelating agent. The microstructure and elemental composition were characterized using scanning electron microscopy combined with energy-dispersive X-ray spectroscopy. Phase analysis of transition metal-substituted cobalt ferrite nanoparticles was performed via X-ray diffraction. Surface wettability was measured using the water contact angle technique. The surface roughness of all nanoparticles was measured using profilometry. Moreover, thermogravimetric analysis and differential scanning calorimetry were performed to determine the temperature at which the decomposition and oxidation of the chelating agents took place. Results indicated that the substitution of transition metals influences strongly the microstructure, crystal structure and antibacterial property of the cobalt ferrite nanoparticles. PMID:23137676
Hybrid uranium-transition-metal oxide cage clusters.
Ling, Jie; Hobbs, Franklin; Prendergast, Steven; Adelani, Pius O; Babo, Jean-Marie; Qiu, Jie; Weng, Zhehui; Burns, Peter C
Transition-metal based polyoxometalate clusters have been known for decades, whereas those built from uranyl peroxide polyhedra have more recently emerged as a family of complex clusters. Here we report the synthesis and structures of six nanoscale uranyl peroxide cage clusters that contain either tungstate or molybdate polyhedra as part of the cage, as well as phosphate tetrahedra. These transition-metal-uranium hybrid clusters exhibit unique polyhedral connectivities and topologies that include 6-, 7-, 8-, 10-, and 12-membered rings of uranyl polyhedra and uranyl ions coordinated by bidentate peroxide in both trans and cis configurations. The transition-metal polyhedra appear to stabilize unusual units built of uranyl polyhedra, rather than templating their formation. PMID:25434424
Metallic state and the metal-insulator transition of NdNiO3
NASA Astrophysics Data System (ADS)
Granados, X.; Fontcuberta, J.; Obradors, X.; Ma±osa, Ll.; Torrance, J. B.
We report detailed measurements of the electrical resistivity, Seebeck coefficient, and differential scanning calorimetry across the metal-insulator (MI) transition (TMI~=205 K) of NdNiO3. As in the isostructural oxide PrNiO3, the transition is extremely hysteretic, consistent with the first-order character of the transition. Analysis of the data shows that metallic and nonmetallic phases coexist in a broad temperature interval (~=70 K). The electrical resistivity and Seebeck coefficients of the metallic state display a clear linear temperature dependence. These properties can be rationalized in terms of a Fermi gas picture of rather heavy electrons (effective mass of about 6m0). Some physical parameters of the gas are predicted. The entropy change across the MI transition has been measured and it has been used to predict a strong pressure dependence of the MI transition temperature: dTMI/dP~=-4.8 K/kbar.
Covalent functionalization of monolayered transition metal dichalcogenides by phase engineering.
Voiry, Damien; Goswami, Anandarup; Kappera, Rajesh; e Silva, Cecilia de Carvalho Castro; Kaplan, Daniel; Fujita, Takeshi; Chen, Mingwei; Asefa, Tewodros; Chhowalla, Manish
Chemical functionalization of low-dimensional materials such as nanotubes, nanowires and graphene leads to profound changes in their properties and is essential for solubilizing them in common solvents. Covalent attachment of functional groups is generally achieved at defect sites, which facilitate electron transfer. Here, we describe a simple and general method for covalent functionalization of two-dimensional transition metal dichalcogenide nanosheets (MoS?, WS? and MoSe?), which does not rely on defect engineering. The functionalization reaction is instead facilitated by electron transfer between the electron-rich metallic 1T phase and an organohalide reactant, resulting in functional groups that are covalently attached to the chalcogen atoms of the transition metal dichalcogenide. The attachment of functional groups leads to dramatic changes in the optoelectronic properties of the material. For example, we show that it renders the metallic 1T phase semiconducting, and gives it strong and tunable photoluminescence and gate modulation in field-effect transistors. PMID:25515889
Role of phonons in the metal-insulator phase transition.
NASA Technical Reports Server (NTRS)
Langer, W. D.
Review, for the transition series oxides, of the Mattis and Lander model, which is one of electrons interacting with lattice vibrations (electron and phonon interaction). The model displays superconducting, insulating, and metallic phases. Its basic properties evolve from a finite crystallographic distortion associated with a dominant phonon mode and the splitting of the Brillouin zone into two subzones, a property of simple cubic and body centered cubic lattices. The order of the metal-insulator phase transition is examined. The basic model has a second-order phase transition and the effects of additional mechanisms on the model are calculated. The way in which these mechanisms affect the magnetically ordered transition series oxides as described by the Hubbard model is discussed.
Transition-metal acceptor complexes in zinc oxide
NASA Astrophysics Data System (ADS)
Gluba, M. A.; Nickel, N. H.
The incorporation of hydrogen shallow donors gives rise to Mn2+ fine and hyperfine lines in the electron paramagnetic resonance of ZnO. This cannot be explained by recharging of isolated Mn atoms. First-principles density functional calculations reveal that transition metals readily form complexes with acceptors by a charge-transfer process or by bond formation. This mechanism has implications on both complex partners. Acceptors are neutralized and uncommon charge states of the transition metals are stabilized. Implications for dilute magnetic ZnO are discussed.
Efficient photocarrier injection in a transition metal oxide heterostructure
Y Muraoka; T Yamauchi; Y Ueda; Z Hiroi
An efficient method for doping a transition metal oxide (TMO) with hole carriers is presented: photocarrier injection (PCI) in an oxide heterostructure. It is shown that an insulating vanadium dioxide (VO2) film is rendered metallic under light irradiation by PCI from an n-type titanium dioxide (TiO2) substrate doped with Nb. Consequently, a large photoconductivity, which is exceptional for TMOs, is
Nanostructured transition metal oxides for aqueous hybrid electrochemical supercapacitors
T. Cottineau; M. Toupin; T. Delahaye; T. Brousse; D. Bķlanger
In this paper, we wish to present an overview of the research carried out in our laboratories with low-cost transition metal\\u000a oxides (manganese dioxide, iron oxide and vanadium oxide) as active electrode materials for aqueous electrochemical supercapacitors.\\u000a More specifically, the paper focuses on the approaches that have been used to increase the capacitance of the metal oxides\\u000a and the cell
Mechanism for bipolar resistive switching in transition metal oxides
Marcelo Rozenberg; Maria Jose Sanchez; Ruben Weht; Carlos Acha; Fernando Gomez-Marlasca; Pablo Levy
Resistive andom access memories (RRAM) composed of a transtition metal oxide dielectric in a capacitor-like structure is a candidate technology for next generation non-volatile memory devices. We introduce a model that accounts for the bipolar resistive switching phenomenom observed in many perovskite transition metal oxides. It qualitatively describes the electric field-enhanced migration of oxygen vacancies at the nano-scale. The numerical
Electronic structure systematics of 3d transition metal oxides
R. Zimmermann; P. Steiner; R. Claessen; F. Reinert; S. H³fner
The electronic structure of a series of 3d transition metal oxides is investigated by core-level and valence band photoemission spectroscopy. The results are analyzed in terms of metal¢ligand interactions and on-site correlation energies, using a simple cluster model approach. For some oxides we also present a comparison with band structure calculations. It is shown that the electronic structure of the
Transition metal-promoted biomimetic steps in total syntheses.
Li, Xu-Wen; Nay, Bastien
This review summarizes the state-of-the-art on the use of transition metals in synthetic steps inspired by biosynthesis. After an introduction showing the importance of metals in life processes, with special emphasis on biosynthetic processes, their place in biomimetic synthesis will be described. Topics include oxidative cyclizations for polyether synthesis, electrophilic and radical cyclizations of polyprenoids, the role of palladium in electrocyclizations, oxidative biaryl couplings and other rearrangement steps. PMID:24481377
Terahertz transitions in quasi-metallic carbon nanotubes
NASA Astrophysics Data System (ADS)
Hartmann, R. R.; Portnoi, M. E.
We study interbanddipole transitions across curvature-induced narrow gaps in quasi-metallic single-walled carbon nanotubes. The curvature effects not only open a gap in the nanotube energy spectrum but also allow optical transitions, which happen to be in the highly- desired terahertz frequency range. Applying a magnetic field along the nanotube axis allows one to tune the frequency peaks in the spectral density of absorption.
Multi-atom resonant photoemission in transition metal chlorides
NASA Astrophysics Data System (ADS)
Kikas, A.; N§mmiste, E.; Ruus, R.; Saar, A.; Martinson, I.
Recently multi-atom resonances in photoemission of transition metal oxides were observed by Kay et al. [A. Kay, E. Arenholz, S. Mun, F.J. Garcia de Abajo, C.S. Fadley, R. Denecke, Z. Hussain, M.A. Van Hove, Science 281 (1998) 679]. We present the results of a systematic study of multi-atom resonant photoemission in transition metal chlorides. We measured the intensity of the chlorine 2p photoelectron line excited around the metal 2p photoabsorption edge of the transition metal compounds MnCl 2, CrCl 2, and VCl 2 as well as of the ionic solid KCl. Our experimental results show that the intensity of Cl 2p photoelectron lines enhance by about 29% for MnCl 2, 21% for CrCl 2, 17% for VCl 2 and 11% in the case of KCl at the metal 2p photoabsorption edge. We ascribe the systematic decrease of the magnitude of this effect, going from MnCl 2 to KCl, to the extension of the excited metal 3d wavefunction and, as a consequence, decrease of the overlap between the 3d and 2p atomic orbitals.
Factors that affect Li mobility in layered lithium transition metal oxides Kisuk Kang and Gerbrand Ceder*
Ceder, Gerbrand
Factors that affect Li mobility in layered lithium transition metal oxides Kisuk Kang and Gerbrand in layered lithium transition metal oxides are systematically studied in this paper by means of first, and the nature of the metal ion in the transition metal layer have all been proposed to influence the Li mobility
Photofragment spectroscopy of covalently bound transition metal complexes: a window into CH and CC bond
Metz, Ricardo B.
of Massachusetts, Amherst, MA 01003, USA Transition metal cations M■ and metal oxide cations MO■ can activate CŁ4]. In many cases, the transition metal oxides MO■ are even more reactive, able to perform difficultPhotofragment spectroscopy of covalently bound transition metal complexes: a window into CŁH and C
Anharmonic Vibrational Spectroscopy on Metal Transition Complexes
NASA Astrophysics Data System (ADS)
Latouche, Camille; Bloino, Julien; Barone, Vincenzo
Advances in hardware performance and the availability of efficient and reliable computational models have made possible the application of computational spectroscopy to ever larger molecular systems. The systematic interpretation of experimental data and the full characterization of complex molecules can then be facilitated. Focusing on vibrational spectroscopy, several approaches have been proposed to simulate spectra beyond the double harmonic approximation, so that more details become available. However, a routine use of such tools requires the preliminary definition of a valid protocol with the most appropriate combination of electronic structure and nuclear calculation models. Several benchmark of anharmonic calculations frequency have been realized on organic molecules. Nevertheless, benchmarks of organometallics or inorganic metal complexes at this level are strongly lacking despite the interest of these systems due to their strong emission and vibrational properties. Herein we report the benchmark study realized with anharmonic calculations on simple metal complexes, along with some pilot applications on systems of direct technological or biological interest.
Synthesis and characterization of transition metal and metal oxide nanoparticles inside mesoporous carbon CMK-3
Holger Huwe; Michael Fr÷ba
Transition metal oxides were synthesized inside the pore system of mesoporous carbon CMK-3. By a wet impregnation, drying and calcination procedure iron, copper, nickel, cobalt, manganese and zinc oxides were formed almost exclusively within the mesopores. A reduction into the metal forms with regard to structural array in the case of zinc oxide and a reduction to manganese(II) oxide for
Spectroscopic studies of metal high-k dielectrics: transition metal oxides and silicates, and complex rare earth\\/transition metal oxides
G. Lucovsky; J. G. Hong; C. C. Fulton; Y. Zou; R. J. Nemanich; H. Ade; D. G. Scholm; J. L. Freeouf
This paper uses X-ray absorption spectroscopy to the study of electronic structure of the transition metal oxides TiO2, ZrO2 and HfO2, Zr and Hf silicate alloys, and the complex oxides, GdScO3, DyScO3 and HfTiO4. Qualitative and quantitative differences are identified between dipole allowed intra-atomic transitions from core p-states to empty d*- and s*-states, and inter-atomic transitions from transition metal and
Preparation of nanoporous metal foam from high nitrogen transition metal complexes
Tappan, Bryce C.; Huynh, My Hang V.; Hiskey, Michael A.; Son, Steven F.; Oschwald, David M.; Chavez, David E.; Naud, Darren L.
Nanoporous metal foams are prepared by ignition of high nitrogen transition metal complexes. The ammonium salts of iron(III) tris[bi(tetrazolato)-amine], cobalt(III) tris(bi(tetrazolato)amine), and high nitrogen compounds of copper and silver were prepared as loose powders, pressed into pellets and wafers, and ignited under an inert atmosphere to form nanoporous metal foam monoliths having very high surface area and very low density.
Metal-insulator transition in epitaxial vanadium sesquioxide thin films
NASA Astrophysics Data System (ADS)
Allimi, Bamidele S.
Of all the transition metal oxides which exhibit metal-insulator transitions (MIT), one of the most extensively studied in recent years is the vanadium sesquioxide (V2O3), both from experimental and theoretical point of view. At a transition temperature of about 160 K at an ambient pressure of 1 atm, pure V2O3 transforms from a rhombohedral paramagnetic metallic (PM) to a monoclinic antiferromagnetic insulating (AFI) phase upon cooling, with a jump in the resistivity of about seven orders of magnitude. Experimental studies have focused more on bulk V2O3 and recently there have been significant interest in thin film fabrication of this material due to potential applications as thermal sensors, current limiters, Positive Temperature Coefficient (PTC) thermistors, and optical switches. This study addresses the deposition, characterization, and properties of high-quality epitaxial V2O3 thin films grown on a-, c-Al2O3 and c-LiTaO 3 substrates by a straightforward method of pulsed laser deposition (PLD). Various characterization techniques including X-ray diffraction, atomic force microscopy, scanning electron microscopy, and X-ray photoemission spectroscopy were used to examine the structural, crystallographic, and surface properties, while four point probe resistivity measurements were used to examine the electrical properties of the films. V2O3 thin films of different thicknesses ranging from 10-450 nm were deposited on c-Al 2O3 and c-LiTaO3 substrates by PLD to understand also the role of epitaxial strains. Resistivity measurements showed that depending on the thicknesses of films, different electrical transitions were exhibited by the samples. While some of the samples displayed the expected metal-insulator transition typical of bulk V2O3, some showed insulating behavior only and others exhibited metallic characteristics only over the whole temperature range. For example, for films on c-LiTaO3 with increasing film thickness, first an insulator-insulator, then a metal-insulator, followed by a metal-metal transition is observed. Thicker films (>202 nm) remain metallic in the temperature range of the measurements.
Pressure-Induced Phase Transitions, Amorphization and Metallization in Sulfur.
NASA Astrophysics Data System (ADS)
Luo, Huan
Physical and chemical properties of molecular system of sulfur have been studied under high pressures up to 212 GPa using a variety of modern experimental techniques, including energy-dispersive x-ray diffraction (EDXD) using a synchrotron source, micro-Raman spectroscopy, optical absorption and optical reflection in combination with the diamond anvil cell techniques. This work studies pressure -induced crystallographic structural transitions, electronic band structure changes, amorphization, metallization and molecular dissociation, as well as the physical reasons for these changes. This thesis brings the following results on sulfur (initially S_8) to the public: an orthorhombic to monoclinic phase transition at 5.3 GPa, a crystal to amorphous transition at 25 GPa, a recrystallization process starting at 37 GPa and completing at 78 GPa, an insulator to metal transition at 95 GPa accompanied by a first-order structural transition to a base-centered orthorhombic structure with 4 atom per unit cell, and finally, a transition to a 6-fold coordinated monatomic beta-Po phase at 162 GPa with one atom per unit cell. The mechanisms of pressure-induced amorphization and metallization have been proposed. A puzzling problem with sulfur in which a weak conducting behavior was observed in the resistivity vs temperature experiments around 50 GPa, although the present optical data showed no metallic behavior until 95 GPa, has been explained. The micro-Raman spectroscopic experiments on sulfur using very low laser power have demonstrated the laser heating and photo-reaction problems in sulfur and their relation to the previously reported phase transitions. The physical reason for the phase transition in sulfur caused by laser heating was first proposed in this thesis work. The phase transition to the beta-Po structure in sulfur at 162 GPa is the first experimental result in which a structural determination has been made for the low-atomic -number materials above 100 GPa. With the observation of the beta-Po phase in sulfur, the similarity in the transition sequence among the Group VIb elements S, Se and Te in their metallic phases is confirmed up to the beta-Po phase. Also included in the thesis work are the theoretical calculations on the mechanical properties of diamond, diamond anvil and carbon-carbon bond under compression. Through calculations the states of stress and strain in a loaded diamond anvil have been approximately obtained which are used to study the ultimate yield strength of diamond at ultra high pressures. Using finite elastic strain energy theory, potentials for stretching and bending of the aliphatic carbon bonds are obtained and compared with that of the molecular mechanics force field theory.
Transition Metal Complexes of Expanded Porphyrins
Sessler, Jonathan L.; Tomat, Elisa
Over the last two decades, the rapid development of new synthetic routes for the preparation of expanded porphyrin macrocycles has allowed exploration of a new frontier consisting of ōporphyrin-likeö coordination chemistry. In this Account, we summarize our exploratory forays into the still relatively poorly explored area of oligopyrrolic macrocycle metalation chemistry. Specifically, we describe our successful formation of both mono- and binuclear complexes and in doing so highlight the diversity of coordination modes available to expanded porphyrin-type ligands. The nature of the inserted cation, the emerging role of tautomeric equilibria, and the importance of hydrogen-bonding interactions in regulating this chemistry are also discussed. PMID:17397134
Nanosize Powders of Transition Metals Binary Systems
NASA Astrophysics Data System (ADS)
Zaharov, Yu A.; Pugachev, V. M.; Dodonov, V. G.; Popova, A. N.; Kolmykov, R. P.; Rostovtsev, G. A.; Vasiljeva, O. V.; Zyuzyukina, E. N.; Ivanov, A. V.; Prosvirin, I. P.
The review of the results obtained by the authors in the field of synthesis of Fe-Co, Fe-Ni, Co-Ni and Cu-Ni nano-size binary systems (NBS) by the liquid-phase reducing of metal chlorides by hydrazine (pH>10) and sodium tetrahydroborate (NaBH4) (pH = 7-8) by the process conditions variation (temperature of reaction medium, concentration and input order of reagents) as well as NBS properties investigation is presented.
Phase Stability of Lithiated Transition Metal Oxides
Alexander I. Landa; Chun-Chieh Chang; Prashant N. Kumta; Levente Vitos; Igor A. Abrikosov
There is significant interest in LiCoO2 and LiMnO2 primarily for use in Li-ion batteries. However LiMnO2 is unstable and a prediction of the structural stability and the phase transitions in the LiMnO_2-LiCoO2 system would be very useful. In this presentation we report the results of the full charge density exact muffin-tin orbital studies conducted on the orthorhombic, monoclinic and rhombohedral
Pressure-driven "molecular metal" to "atomic metal" transition in crystalline Ga.
Gromnitskaya, E L; Yagafarov, O F; Stalgorova, O V; Brazhkin, V V; Lyapin, A G
We present the ultrasonic study of gallium (Ga I) under high pressure up to 1.7 GPa, including the measurements of the density and elastic properties during phase transitions to Ga II and to a liquid state. The observed large drop of both bulk and shear moduli (by 30% and 55%, correspondingly) during the phase transition to Ga II, as well as the increase of the Poisson's ratio from typically "covalent" ( approximately 0.22) to "metallic" ( approximately 0.32) values, experimentally testifies to the coexistence of a molecular and metallic behavior in Ga I and to the disappearance of the "covalency" during the transition to Ga II. A high value of the pressure derivative of the bulk modulus for Ga I and the increase in the Poisson's ratio can be associated with the weakening of the covalency in compressed Ga I and considered as a precursor of the transition to normal metal. PMID:17501429
Magnetism and transport properties of transition metal oxides and nanoparticles
Dane Thomas Gillaspie
This dissertation is devoted to the study of the properties of transition metal oxides in both thin film and nanocrystalline forms. The first section is devoted to the transport properties of manganese oxide thin film samples. The colossal magnetoresistance in these materials is usually explained using double-exchange, but this explanation is only partially correct. Recent theoretical and experimental work has
Quadrupole terms in defect energies in transition metal oxides
A. M. Stoneham; M. J. L. Sangster; J. H. Harding
In a previous paper it was shown that, for defects in transition metal oxides, there was evidence for an additional term in the potential energy beyond the usual monopole and dipole terms of the shell model. This appeared to be a quadrupole term in which the open 3d shells of the host ions, as well as any impurities, responded to
Novel Extended Solids Composed of Transition Metal Oxide Clusters
M. Ishaque Khan
Transition metal oxide clusters and their derivatives offer an unmatched variety of structural motifs that are potential building blocks for the design and development of extended solids. In view of the proven applications of these clusters in catalysis, molecular recognition, environmental decontamination, clinical and analytical processes, materials science, nanotechnology, and medicine, their usage as building blocks promises the development of
Jones, William D.
TRANSITION METAL ACTIVATION AND FUNCTIONALIZATION OF CARBON-HYDROGEN BONDS William D. Jones-H and C-C bond functionalization, and (4) carbon-fluorine bond activation. We have made progress in each in our proposal where we have had success. These include: (1) carbon-carbon bond cleavage reactions, (2
Luminescent molecular rods - transition-metal alkynyl complexes.
Yam, Vivian Wing-Wah; Wong, Keith Man-Chung
A number of transition-metal complexes have been reported to exhibit rich luminescence, usually originating from phosphorescence. Such luminescence properties of the triplet excited state with a large Stoke's shift, long lifetime, high luminescence quantum yield as well as lower excitation energy, are envisaged to serve as an ideal candidate in the area of potential applications for chemosensors, dye-sensitized solar cells, flat panel displays, optics, new materials and biological sciences. Organic alkynes (poly-ynes), with extended or conjugated?-systems and rigid structure with linear geometry, have become a significant research area due to their novel electronic and physical properties and their potential applications in nanotechnology. Owing to the presence of unsaturated sp-hybridized carbon atoms, the alkynyl unit can serve as a versatile building block in the construction of alkynyl transition-metal complexes, not only through?-bonding but also via?-bonding interactions. By incorporation of linear alkynyl groups into luminescent transition-metal complexes, the alkynyl moiety with good?-donor,?-donor and?-acceptor abilities is envisaged to tune or perturb the emission behaviors, including emission energy (color), intensity and lifetime by its role as an auxiliary ligand as well as to govern the emission origin from its direct involvement. This review summarizes recent efforts on the synthesis of luminescent rod-like alkynyl complexes with different classes of transition metals and details the effects of the introduction of alkynyl groups on the luminescence properties of the complexes. PMID:22179333
Freericks, Jim
INELASTIC LIGHT SCATTERING AND THE CORRELATED METAL-INSULATOR TRANSITION J.K. FREERICKS (freericks©at Augsburg, D-86135, Augsburg, Germany Abstract. Inelastic light scattering is an important probe of the two to inelastic light scattering exactly in the limit of large spatial dimensions by employing dynamical mean
Inelastic light scattering near the Mott metal-insulator transition
Freericks, Jim
Inelastic light scattering near the Mott metal-insulator transition Jim Freericks (Georgetown scattering of light with electron-hole excitations of the correlated many-body system. Ę Use of polarizers, one finds large enhancements to the inelastic scattering. Ę Advanced light sources have linearly
Transport properties of transition metal impurities on gold nanowires
NASA Astrophysics Data System (ADS)
Pontes, Renato B.; da Silva, Edison Z.; Fazzio, Adalberto; da Silva, Ant¶nio J. R.
Performing first principles density functional theory (DFT) we calculated the electronic and transport properties of a Au thin nanowire with transition metal atoms (Mn, Fe, Ni or Co) bridging the two sides of the Au nanowire. We will show that these systems have strong spin dependent transport properties and that the local symmetry can dramatically change them, leading to a significant spin polarized conductance. This spin dependent transport is also associated with the transition metal in the nanowire, in particular with the d-level positioning. Using Co, for example [1], when the symmetry permits the mixing between the wire s-orbitals with the transition metal d-states, there are interference effects that resemble Fano-like resonances with an anisotropy of 0.07 at the Fermi level. On the other hand, if this symmetry decouples such states, we simply have a sum of independent transmission channels and the calculated anisotropy was 0.23. The anisotropies for the other transition metals, as well as calculated transmittances for two Co impurities will also be presented [1] R. B. Pontes, E. Z. da Silva, A. Fazzio and Ant¶nio J. R. da Silva, J. Am. Chem. Soc. 130 (30), 9897-903, 2008
Electronic and geometric structure of transition-metal nanoclusters
D. R. Jennison; P. A. Schultz; M. P. Sears; T. Klitsner
A massively-parallel ab initio computer code, which uses Gaussian bases, pseudopotentials, and the local density approximation, permits the study of transition-metal systems with literally hundreds of atoms. We present total energies and relaxed geometries for Ru, Pd, and Ag clusters with N = 55, 135, and 140 atoms; we also used the DMOL code to study 13-atom Pd and Cu
Rare earth transition metal magnesium compoundsŚAn overview
NASA Astrophysics Data System (ADS)
Rodewald, Ute Ch.; Chevalier, Bernard; P÷ttgen, Rainer
Intermetallic rare earth-transition metal-magnesium compounds play an important role as precipitations in modern light weight alloys and as host materials for hydrogen storage applications. Recent results on the crystal chemistry, the chemical bonding peculiarities, physical properties, and hydrogenation behavior of these materials are reviewed.
Onset of magnetism in supported transition metal encapsulated silicon cages
Roberto Robles; Shiv N. Khanna
In the past few years, silicon based clusters have attracted a lot of attention as building blocks of nanomaterials. Some of the most promising candidates are the transition metal encapsulated silicon cages, which have been shown to be specially stable, both experimentally and theoretically. However, for the use of these materials in fields like spintronics, it is not only necessary
The Existence of Pure?Phase Transition Metal Hydroxy Apatites
Claude H. Yoder; Nathan Fedors; Natalie J. Flora; Heather Brown; Keith Hamilton; Charles D. Schaeffer Jr
The existence of pure?phase transition metal hydroxy apatites of zinc, copper, manganese, and cobalt could not be verified by repeated attempts to duplicate the few literature procedures reported for their preparation in aqueous solution. Variations of temperature, time of reactions, pH, and use of decomplexation also did not produce apatitic compounds. In the case of zinc, the product resulting from
Vapor growth of electrochromic thin films of transition metal oxides
K. A. Gesheva; T. Ivanova; B. Marsen; G. Zollo; M. Kalitzova
Mixed oxide films of transition metals gain considerable much attention due to their interesting optoelectronic properties. The low temperature chemical vapor growth processing of thin films of mixed W and Mo oxides is presented. The investigation is related to optimization of films structure and the related optoelectronic properties in dependence on the chemical vapor deposition (CVD) process parameters. Their electrochromic
Synthesis, characterization and reactivity of transition-metal-containing zeolites
Rossin, J.A.
Transition metal containing zeolites (zeolite A and ZSM-5) were prepared by addition of various transition metal containing substrates to zeolite synthesis gels. Crystal growth data were recorded in order to determine the influence of the transition metal species on the rate of crystal growth. X-ray diffraction, oxygen adsorption, FTIR and SEM were utilized to evaluate crystal purity. X-ray photoelectron spectroscopy (XPS), chemical analysis and electron microprobe analysis were performed in order to ascertain the position (intrazeolitic versus surface) and homogeneity of the transition metal. It was concluded that intrazeolitic transition metals are produced by the novel procedure presented in this work. 1-Hexane hydroformylation by rhodium zeolite A showed intrazeolitic rhodium to migrate to the external surface of the zeolite. However, in the presence of a solution and surface rhodium poison, intrazeolitic rhodium was found to hydroformylate 1-hexene exclusively to heptanal. Ruthenium containing zeolite A was evaluated under CO-hydrogenation conditions. No migration of intrazeolitic ruthenium to the external surface of the zeolite was observed over the course of the reaction. The product distribution obtained for this catalyst did not follow a log normal behavior. Also, loss of zeolite crystallinity was observed following the reaction. Cobalt ZSM-5 was evaluated under CO-hydrogenation conditions. No migration of cobalt to the external surface of the zeolite occurred. XPS analysis of the catalyst following various stages of the reaction indicated that intrazeolitic cobalt was not reduced to the zero valent state. Consequently, the non-zero valent cobalt was not capable of hydrogenating carbon monoxide.
High-Pressure Thermodynamic Properties of f-electron Metals,; Transition Metal Oxides, and Half-Metallic Magnets
Richard T. Scalettar; Warren E. Pickett
This project involves research into the thermodynamic properties;\\u000aof f-electron metals, transition metal oxides, and half-metallic;\\u000amagnets at high pressure. These materials are ones in which the;\\u000achanging importance of electron-electron interactions as;\\u000athe distance between atoms is varied;\\u000acan tune the system through phase transitions;\\u000afrom localized to delocalized electrons,;\\u000afrom screened to unscreened magnetic moments,;\\u000aand from normal
Early and Late Stage Metals and Sulfides in Diogenites
NASA Technical Reports Server (NTRS)
Sideras, L. C.; Domanik, K. J.; Lauretta, D. S.
Diogenites are typically highly brecciated orthopyroxenites that contain 84-100 vol.% orthopyroxene. Common accessory minerals include olivine, chromite, Ca-pyroxene, plagioclase, silica, troilite and Fe-Ni metal. Metal and sulfides are minor phases in diogenites with an average abundance of < 1 vol.% and 0-2 vol.% respectively. However their presence is important, as they could provide information on T-fO2-fS2 conditions and the evolution of the diogenite parent magma during crystallization and/or later metamorphism. We have examined the occurrence of Fe-Ni metal and sulfides in thin sections of several diogenites including, Johnstown, Manegaon, Roda, Shalka, Bilanga, and Tatahouine using optical microscopy and the electron microprobe. Here, we describe three features of metals and sulfides that are common in most of these diogenites. These are: i) The widespread occurrence of pentlandite associated with copper and copper sulfide minerals; ii) Textural evidence that at least some of the metal and sulfide occurring interstitially between, and as inclusions within, orthopyroxene formed from an early immiscible sulfide-oxide liquid; iii) That this sulfide- oxide liquid subsequently fractionated into assemblages containing either Fe-Ni metal, troilite, and chromite or pentlandite, troilite, and copper-bearing sulfide.
Pressure induced quantum phase transitions in metallic oxides and pnictides
NASA Astrophysics Data System (ADS)
Fallah Tafti, Fazel
Quantum phase transitions occur as a result of competing ground states. The focus of the present work is to understand quantum criticality and its consequences when the competition is between insulating and metallic ground states. Metal-insulator transitions are studied by means of electronic transport measurements and quantum critical points are approached by applying hydrostatic pressure in two different compounds namely Eu2Ir22O 7 and FeCrAs. The former is a ternary metal oxide and the latter is a ternary metal pnictide. A major component of this work was the development of the ultra-high pressure measurements by means of Anvil cells. A novel design is introduced which minimizes the alignment accessory components hence, making the cell more robust and easier to use. Eu2Ir22O7 is a ternary metal oxide and a member of the pyrochlore iridate family. Resistivity measurements under pressure in moissanite anvil cells show the evolution of the ground state of the system from insulating to metallic. The quantum phase transition at Pc ś 6 GPa appears to be continuous. A remarkable correspondence is revealed between the effect of the hydrostatic pressure on Eu2Ir22O7 and the effect of chemical pressure by changing the R size in the R2Ir2O7 series. This suggests that in both cases the tuning parameter controls the t2g bandwidth of the iridium 5d electrons. Moreover, hydrostatic pressure unveils a curious cross-over from incoherent to conventional metallic behaviour at a T* > 150 K in the neighbourhood of Pc, suggesting a connection between the high and low temperature phases. The possibility of a topological semi-metallic ground state, predicted in recent theoretical studies, is explained. FeCrAs is a ternary metal pnictide with Fermi liquid specific heat and susceptibility behaviour but non-metallic non-Fermi liquid resistivity behaviour. Characteristic properties of the compound are explained and compared to those of superconducting pnictides. Antiferromagnetic (AFM) order sets in at ś125 K with the magnetic moments residing on the Cr site. Pressure measurements are carried out in moissanite and diamond anvil cells in order to suppress the AFM order and resolve the underlying electronic transport properties. While AFM order is destroyed by pressure, the non-metallic non-Fermi liquid behaviour is shown to be robust against pressure.
Syntheses and applications of soluble polyisobutylene (PIB)-supported transition metal catalystsĀ
Tian, Jianhua
Soluble polymer supports facilitate the recovery and recycling of expensive transition metal complexes. Recently, polyisobutylene (PIB) oligomers have been found to be suitable polymer supports for the recovery of a variety of transition metal...
Cross-linking proteins with bimetallic tetracarboxylate compounds of transition metals
Kostic, Nenad M. (Ames, IA); Chen, Jian (Ames, IA)
Stable cross-linked complexes of transition-metal tetracarboxylates and proteins are formed. The preferred transition-metal is rhodium. The protein may be collagen or an enzyme such as a proteolytic enzyme.
Stable Isotopes of Transition and Post-Transition Metals as Tracers in Environmental Studies
Thomas D. Bullen
\\u000a The transition and post-transition metals, which include the elements in Groups 3¢12 of the Periodic Table, have a broad range\\u000a of geological and biological roles as well as industrial applications and thus are widespread in the environment. Interdisciplinary\\u000a research over the past decade has resulted in a broad understanding of the isotope systematics of this important group of\\u000a elements and
Buckley Prize Talk: The Suprerconductor-(Metal)-Insulator Transition
NASA Astrophysics Data System (ADS)
Kapitulnik, Aharon
While the classical theory of phase transitions has been extraordinarily successful, there are several reasons to exercise caution when applying this approach to the zero temperature superconducting transition. First, experimental identification of the relevant phases requires extrapolation to zero temperature, which becomes complicated, especially when one needs to identify sources of dissipation. In addition, since superconductivity may be highly inhomogeneous as appreciable superconducting order parameter may be concentrated in ``superconducting puddles'' due to disorder and/or spontaneous phase separation, the nature of the quantum phase transition to a superconducting state may be highly anomalous, where the system attempts to optimizes the formation of puddles with the Josephson coupling among them to obtain global superconductivity. In this talk we will review some of the consequences of these considerations, emphasizing the possible emergence of anomalous metallic phases close to the superconductor-insulator transition.
Changes in Paratethyan marine molluscs at the Early/Middle Miocene transition: diversity, palaeo-
Zuschin, Martin
Changes in Paratethyan marine molluscs at the Early/Middle Miocene transition: diversity, palaeo/Middle Miocene transition: diversity, palaeogeography and palaeoclimate. Acta Geologica Polonica, 53 (4), 323-339. Warszawa. The transition from the Early Miocene to the Middle Miocene is a crucial point
Family structure and the transition to early parenthood.
Hofferth, Sandra L; Goldscheider, Frances
With the rise in out-of-wedlock childbearing and divorce in the last quarter of the twentieth century, an increasing proportion of children have been exposed to a variety of new family forms. Little research has focused on the consequences of childhood family structure for men's transition to fatherhood or on the family processes that account for the effects of family structure on the likelihood that young women and men become first-time unmarried parents, what we now call "fragile families." The data come from the linked Children and Young Adult samples of the 1979 National Longitudinal Survey of Youth (NLSY79), which provide information on the children of the women of the NLSY79 from birth until they enter young adulthood. Females growing up with a single parent and males experiencing an unstable family transition to parenthood early, particularly to nonresidential fatherhood for males. For males, the effects are strongly mediated by parenting processes and adolescent behaviors and are shaped by economic circumstances. Having experienced multiple transitions as a child is associated with a reduced likelihood that males father their first child within marriage and an increased likelihood that they become fathers within cohabitation, demonstrating how changes in family structure alter family structure patterns over time and generations. PMID:20608104
Quantifying limits to detection of early warning for critical transitions
Boettiger, Carl; Hastings, Alan
Catastrophic regime shifts in complex natural systems may be averted through advanced detection. Recent work has provided a proof-of-principle that many systems approaching a catastrophic transition may be identified through the lens of early warning indicators such as rising variance or increased return times. Despite widespread appreciation of the difficulties and uncertainty involved in such forecasts, proposed methods hardly ever characterize their expected error rates. Without the benefits of replicates, controls or hindsight, applications of these approaches must quantify how reliable different indicators are in avoiding false alarms, and how sensitive they are to missing subtle warning signs. We propose a model-based approach to quantify this trade-off between reliability and sensitivity and allow comparisons between different indicators. We show these error rates can be quite severe for common indicators even under favourable assumptions, and also illustrate how a model-based indicator can improve this performance. We demonstrate how the performance of an early warning indicator varies in different datasets, and suggest that uncertainty quantification become a more central part of early warning predictions. PMID:22593100
Templated Assembly of Metal-Anion Arrays within Layered Hosts; Synthesis and Characterization of New Transition-Metal Oxyhalide Perovskites
Spinu, Leonard
of New Transition-Metal Oxyhalide Perovskites Gabriel Caruntu, Liliana Viciu, Leonard Spinu, Weilie Zhou-layered perovskites with divalent transition-metal halides leads to the templated assembly of metal-anion layers4+ , Mn4+ , Nb4+ , Mo4+ ), have been prepared by a direct route. INTRODUCTION Layered perovskites
Thermal properties of zirconium diboride -- transition metal boride solid solutions
NASA Astrophysics Data System (ADS)
McClane, Devon Lee
This research focuses on the thermal properties of zirconium diboride (ZrB2) based ceramics. The overall goal was to improve the understanding of how different transition metal (TM) additives influence thermal transport in ZrB2. To achieve this, ZrB2 with 0.5 wt% carbon, and 3 mol% of individual transition metal borides, was densified by hot-press sintering. The transition metals that were investigated were: Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, and Re. The room temperature thermal diffusivities of the compositions ranged from 0.331 cm2/s for nominally pure ZrB2 to 0.105 cm2/s for (Zr,Cr)B2 and converged around 0.155cm2/s at higher temperatures for all compositions. Thermal conductivities were calculated from the diffusivities, using temperature-dependent values for density and heat capacity. The electron contribution to thermal conductivity was calculated from measured electrical resistivity according to the Wiedemann-Franz law. The phonon contribution to thermal conductivity was calculated by subtracting the electron contribution from the total thermal conductivity. Rietveld refinement of x-ray diffraction data was used to determine the lattice parameters of the compositions. The decrease in thermal conductivity for individual additives correlated directly to the metallic radius of the additive. Additional strain appeared to exist for additives when the stable TM boride for that metal had different crystal symmetries than ZrB2. This research provided insight into how additives and impurities affect thermal transport in ZrB2. The research potentially offers a basis for future modeling of thermal conductivity in ultra-high temperature ceramics based on the correlation between metallic radius and the decrease in thermal conductivity.
Mapping the Valence States of Transition-Metal Elements Using Energy-Filtered Transmission Electron Microscopy
Wang, Zhong L.
The properties of transition-metal oxides are related to the presence of elements with mixed valences, such as MnMapping the Valence States of Transition-Metal Elements Using Energy-Filtered Transmission Electron and Co. Spatial mapping of the valence-state distribution of transition-metal elements is a challenge
Role of Electronic Structure in the Susceptibility of Metastable Transition-Metal Oxide Structures to Transformation
Ceder, Gerbrand
Role of Electronic Structure in the Susceptibility of Metastable Transition-Metal Oxide Structures transition-metal ions in an oxide and how these factors in turn affect the resistance of metastable 3d transition-metal oxides against transformation. This is a relevant topic to the Li rechargeable battery field
Electronic structure of high-k transition metal oxides and their silicate and aluminate alloys
Electronic structure of high-k transition metal oxides and their silicate and aluminate alloys G alloys of: i group IIIB, IVB, and VB transition metal TM oxides and ii first row RE oxides with SiO2 addresses differences between the electronic structure of: i alternative high-k transition metal TM rare
Transition metal-catalyzed oxidation of atmospheric sulfur: Global implications for the sulfur budget
Alexander, Becky
Transition metal-catalyzed oxidation of atmospheric sulfur: Global implications for the sulfur(IV)) oxidation by O2 catalyzed by transition metals. Due to the lack of photochemically produced OH and H2O2), Transition metal-catalyzed oxidation of atmospheric sulfur: Global implications for the sulfur budget, J
NEXAFS investigations of transition metal oxides, nitrides, carbides, sulfides and other interstitial compounds
J. G. Chen
Owing to their unique physical and chemical properties, transition metal compounds, especially transition metal oxides, nitrides, carbides and sulfides, have been the subject of many surface science investigations. In this article we will review applications of the near-edge X-ray absorption fine structure (NEXAFS) technique in the investigations of electronic and structural properties of transition metal compounds. This review covers NEXAFS
Methane-to-Methanol Conversion by Gas-Phase Transition Metal Oxide Cations: Experiment and Theory
Metz, Ricardo B.
Methane-to-Methanol Conversion by Gas-Phase Transition Metal Oxide Cations: Experiment and Theory-phase transition metal oxide cations can convert methane to methanol. Methane activation by MO+ is discussed-phase transition metal oxides with methane are thus a simple model system for the direct conversion of methane
MAGNETIC COLLAPSE AND THE BEHAVIOR OF TRANSITION METAL OXIDES: FeO AT HIGH PRESSURES
Downs, Robert T.
MAGNETIC COLLAPSE AND THE BEHAVIOR OF TRANSITION METAL OXIDES: FeO AT HIGH PRESSURES ** *** ****R- putations using the GGA (PW91) [8] and the LMTO-ASA method [9] were performed on the transition metal oxides insulator. Implications for geophysics are discussed. At low pressures transition metal ions such as ferrous
Understanding the NMR shifts in paramagnetic transition metal oxides using density functional theory calculations
Ceder, Gerbrand
Understanding the NMR shifts in paramagnetic transition metal oxides using density functionalŁ14 however, no such studies, to our knowledge, have been performed in transition metal oxides or other of the lithium local environment and electronic configuration of the transition metal ions. We focus
Physica B 403 (2008) 14011403 Probing nanoscale inhomogeneities in transition metal oxides with
Physica B 403 (2008) 1401Ł1403 Probing nanoscale inhomogeneities in transition metal oxides the universality of phase coexistence in complex transition metal oxides. r 2007 Elsevier B.V. All rights reserved-temperature superconductivity and colossal magnetoresistance in transition metal oxides is an intense area of contemporary
Chemistry of Sulfur Oxides on Transition Metals. II. Thermodynamics of Sulfur Oxides on Platinum(111)
Lin, Xi
Chemistry of Sulfur Oxides on Transition Metals. II. Thermodynamics of Sulfur Oxides on Platinum-containing molecules on transition metal catalysts has been of interest to scientists for more than 2 decades.1. How- ever, sulfur chemistry on transition metal surfaces is so complicated that it remains far from
Structural electronic and phonon properties of some transition metal aluminides
NASA Astrophysics Data System (ADS)
Fatima, Bushra; Pandit, Premlata; Sanyal, Sankar P.
The structural and electronic properties of some Transition metal Aluminides (TMAl) namely ruthenium aluminide (RuAl), nickel aluminide (NiAl) and cobalt aluminide (CoAl) have been studied using plane wave pseudopotential method (PWSCF) within the local density approximation (LDA). The three TMAl's crystallizes in the CsCl-type structure (B2 phase). From the analysis of band structure and density of state, we found that these TMAl's are metallic in nature. The vibrational properties in terms of phonon dispersion curves and density of state have also been reported for RuAl using density functional perturbation theory (DFPT).
Synthesis and characterization of reduced transition metal oxides and nanophase metals with hydrazine in aqueous solution
Zhou Gui; Rong Fan; Weiqin Mo; Xianhui Chen; Ling Yang; Yuan Hu
Reduction of the first row (3d) transition metal complexes in aqueous solution using hydrazine as the reducing agent has been investigated systematically. Nanoscale reduced metal oxides such as VO2(B), Cr2O3 were obtained through hydrazine reduction followed by proper thermal treatments. Nanocrystalline ?-Mn2O3 was synthesized directly through aqueous phase reduction at room temperature. Nanoclusters of cobalt, copper and the one-dimensional single
Transition Metal Exchanged Zeolite Layers for Selectivity Enhancement of Metal-Oxide Semiconductor Gas Sensors
Dominic P. Mann; Keith F. E. Pratt; Themis Paraskeva; Ivan P. Parkin; David E. Williams
A novel method of improving the selectivity of metal oxide gas sensors has been developed by using catalytically active molecular sieve materials. They have been successfully introduced into a proprietary sensor array. The cracking patterns of linear alkanes over transition metal exchanged zeolite Y have been measured using a zeolite bed\\/GC\\/MS experimental set-up within a temperature range of 300degC to
Theoretical study on the alloying behavior of ?-uranium metal: ?-uranium alloy with 3d transition metals
Masayoshi Kurihara; Masaru Hirata; Rika Sekine; Jun Onoe; Hirohide Nakamatsu
We have investigated the alloying behavior of ?-uranium with 3d transition metals (TMs) using the relativistic discrete-variational Dirac¢Fock¢Slater (DV-DFS) method. The d-orbital energy (Md) as an alloying parameter well reproduces the alloying behavior of ?-uranium metal with TMs: (1) in the case of a large Md value (Ti, V, Cr), the solubility of these TM elements in ?-uranium becomes large;
Quantum phase transition from an antiferromagnet to a spin liquid in a metal
Grover, Tarun
We study quantum phase transitions from easy-plane antiferromagnetic metals to paramagnetic metals in Kondo-Heisenberg lattice systems. If the paramagnetic metal is a fractionalized Fermi liquid then the universal critical ...
Development of Metallodithiolates as a New Class of Versatile Ligands to Transition Metals
Pinder, Tiffany
transition metals, a library of well-characterized synthetic analogues has been established. These metal-bound cis-dithiolates, display a wide range of reactivity, including S-based metallation, oxygenation, and alkylation. SulfurÆs affinity for gold...
Extended household transitions, race/ethnicity, and early childhood cognitive outcomes.
Mollborn, Stefanie; Fomby, Paula; Dennis, Jeff A
Beyond mothers' union status transitions, other adults' transitions into and out of the household contribute to family instability, particularly in early childhood. Using the Early Childhood Longitudinal Study-Birth Cohort (N?8550), this study examines associations between extended household transitions and age 2 cognitive development. A substantial minority of toddlers experiences these transitions, and their consequences vary by household member type, entry versus exit, and race/ethnicity. Extended household transitions predict lower cognitive scores for white children, but the selection of low-socioeconomic status families into extended households explains these disparities. Grandparent transitions predict significantly higher cognitive scores for African American and Latino children than whites, and some "other adult" transitions predict higher scores for Latinos than African Americans and whites. Extended household transitions' consequences are independent of co-occurring residential moves and partner transitions. Findings suggest that studying extended household transitions is useful for understanding children's early development, and their consequences vary by race/ethnicity. PMID:23017924
Structure of liquid transition and rare earth metals S. N. Khanna and F. Cyrot-Lackmann
Boyer, Edmond
L-45 Structure of liquid transition and rare earth metals S. N. Khanna and F. Cyrot-Lackmann Groupe It is shown that the observed structure factors of transition and rare earth liquid metals can be reaso- nably. The difference is particularly large for V, Ti, and rare earth metals which are precisely the metals where
Exciton complexes in low dimensional transition metal dichalcogenides
Thilagam, A., E-mail: email@example.com [Information Technology, Engineering and Environment, University of South Australia, Adelaide 5095 (Australia)
We examine the excitonic properties of layered configurations of low dimensional transition metal dichalcogenides (LTMDCs) using the fractional dimensional space approach. The binding energies of the exciton, trion, and biexciton in LTMDCs of varying layers are analyzed, and linked to the dimensionality parameter ?, which provides insight into critical electro-optical properties (relative oscillator strength, absorption spectrum, exciton-exciton interaction) of the material systems. The usefulness of ? is highlighted by its independence of the physical mechanisms underlying the confinement effects of geometrical structures. Our estimates of the binding energies of exciton complexes for the monolayer configuration of transition metal dichalcogenides suggest a non-collinear structure for the trion and a positronium-molecule-like square structure for the biexciton.
Ductile-to-brittle transition in spallation of metallic glasses
NASA Astrophysics Data System (ADS)
Huang, X.; Ling, Z.; Dai, L. H.
In this paper, the spallation behavior of a binary metallic glass Cu50Zr50 is investigated with molecular dynamics simulations. With increasing the impact velocity, micro-voids induced by tensile pulses become smaller and more concentrated. The phenomenon suggests a ductile-to-brittle transition during the spallation process. Further investigation indicates that the transition is controlled by the interaction between void nucleation and growth, which can be regarded as a competition between tension transformation zones (TTZs) and shear transformation zones (STZs) at atomic scale. As impact velocities become higher, the stress amplitude and temperature rise in the spall region increase and micro-structures of the material become more unstable. Therefore, TTZs are prone to activation in metallic glasses, leading to a brittle behavior during the spallation process.
Chemical Trends for Transition Metal Compound Bonding to Graphene
NASA Astrophysics Data System (ADS)
Lange, Bjoern; Blum, Volker
Transition metal compounds are of interest as catalysts for the hydrogen evolution reaction (HER). However, a perfect candidate to replace expensive platinum has not yet been identified. To tailor a specific compound, several properties come into play. One is the bonding to the underlying substrate, for which ?-bonded carbon nanostructures are promising candidates. Here we analyze the bonding of small transition metal compound nanoclusters to a graphene layer for a range of chemical compositions: MxAy (M = Mo, Ti; A = S, O, B, N, C). The clusters are generated by an unbiased random search algorithm. We perform total energy calculations based on density functional theory to identify lowest energy clusters. We calculate binding energies using the PBE and HSE functionals with explicit van der Waals treatment and benchmark those against RPA cluster calculations. Our results indicate that molybdenum-carbides and -nitrides tend to bond tightly to graphene. Mo-oxides and -sulfides show small binding energies, indicating van der Waals bonding.
Recent Advances in Transition Metal-Catalyzed Glycosylation
McKay, Matthew J.; Nguyen, Hien M.
Having access to mild and operationally simple techniques for attaining carbohydrate targets will be necessary to facilitate advancement in biological, medicinal, and pharmacological research. Even with the abundance of elegant reports for generating glycosidic linkages, stereoselective construction of ?- and ?-oligosaccharides and glycoconjugates is by no means trivial. In an era where expanded awareness of the impact we are having on the environment drives the state-of-the-art, synthetic chemists are tasked with developing cleaner and more efficient reactions for achieving their transformations. This movement imparts the value that prevention of waste is always superior to its treatment or cleanup. This review will highlight recent advancement in this regard by examining strategies that employ transition metal catalysis in the synthesis of oligosaccharides and glycoconjugates. These methods are mild and effective for constructing glycosidic bonds with reduced levels of waste through utilization of sub-stoichiometric amounts of transition metals to promote the glycosylation. PMID:22924154
Theoretical Study on Hydrazine Chemisorption on Transition Metal Surfaces
NASA Astrophysics Data System (ADS)
Agusta, Mohammad Kemal; Kasai, Hideaki
Theoretical study on hydrazine adsorption on transition metal surfaces is conducted using the density functional theory. The study is focused on the trend of stability of hydrazine on several transition metal surfaces. The mechanism of the stabilization of conformations is elucidated based on the interaction between frontier orbitals and the d-band which is also used to explain the mechanism of the overall adsorption stability. A model based on perturbative interaction is used to describe the adsorption trend in terms of d-band center and coupling matrix elements. It is found that the interactions are characterized by repulsive interaction of the d-states with adsorbate states. The sp-states are found to be important in overcoming the repulsion and establishing bonding with the surface. Vibrational analysis is used to investigate the nature of adsorption configurations.
The metal-insulator transition in dimerized H³ckel chains.
Monari, Antonio; Bendazzoli, Gian Luigi; Evangelisti, Stefano
The metal-insulator transition is investigated in the case of linear chains described by a one-electron H³ckel Hamiltonian. In these systems, the transition is a consequence of a dimerization of the chain bond length, which induces a similar dimerization of the hopping integral. Three indicators of the chain character are considered: The highest occupied molecular orbital-lowest unoccupied molecular orbital gap, the polarizability, and the localization tensor. In the case of even open chains, the behavior of the large chains depends in a crucial way on the alternating structure of the hopping integrals. If the ending atoms of the chain are weakly bonded to their neighbors, the energy spectrum of the Hamiltonian shows two quasidegenerated eigenvalues, and all the indicators would predict a (spurious) metallic behavior. It is shown that if the corresponding eigenvectors are removed from the Hamiltonian, the ordinary insulating behavior of alternating chains is recovered. PMID:19045075
Ductile-to-brittle transition in spallation of metallic glasses
Huang, X. [State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190 (China); Institute of Systems Engineering, China Academy of Engineering Physics, Mianyang, Sichuan 621999 (China); Ling, Z. [State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190 (China); Dai, L. H., E-mail: firstname.lastname@example.com [State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190 (China); State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 10081 (China)
In this paper, the spallation behavior of a binary metallic glass Cu{sub 50}Zr{sub 50} is investigated with molecular dynamics simulations. With increasing the impact velocity, micro-voids induced by tensile pulses become smaller and more concentrated. The phenomenon suggests a ductile-to-brittle transition during the spallation process. Further investigation indicates that the transition is controlled by the interaction between void nucleation and growth, which can be regarded as a competition between tension transformation zones (TTZs) and shear transformation zones (STZs) at atomic scale. As impact velocities become higher, the stress amplitude and temperature rise in the spall region increase and micro-structures of the material become more unstable. Therefore, TTZs are prone to activation in metallic glasses, leading to a brittle behavior during the spallation process.
Excited-State Mixed Valence in Transition Metal Complexes
Edward A. Plummer; Jeffrey I. Zink
Mixed valence in the lowest-energy metal-to-ligand charge-transfer excited state of di-(4-acetylpyridine)tetraammineruthenium(II) com- plexes is defined and analyzed. The excited state has two interchangeably equivalent ligands with different oxidation states. The electronic absorption band energies, selection rules, and bandwidths are analyzed quantitatively in terms of the signs and orientations of the transition dipole moments, sign and magnitude of the coupling, and
Performance of density functionals for first row transition metal systems
Kasper P. Jensen; Bj÷rn O. Roos; Ulf Ryde
This article investigates the performance of five commonly used density functionals, B3LYP, BP86, PBE0, PBE, and BLYP, for studying diatomic molecules consisting of a first row transition metal bonded to H, F, Cl, Br, N, C, O, or S. Results have been compared with experiment wherever possible. Open-shell configurations are found more often in the order PBE0>B3LYP>PBE~BP86>BLYP. However, on average,
Performance of density functionals for first row transition metal systems
Kasper P. Jensen; Ulf Ryde
This article investigates the performance of five commonly used density functionals, B3LYP, BP86, PBE0, PBE, and BLYP, for studying diatomic molecules consisting of a first row transition metal bonded to H, F, Cl, Br, N, C, O, or S. Results have been compared with experiment wherever possible. Open-shell configurations are found more often in the order PBE0>B3LYP>PBE?BP86>BLYP. However, on average,
Kinetics of Catalytic Combustion of Propane on Transition Metal Oxides
Experiments have been conducted to determine the global kinetics for lean-fuel propane oxidation on a transition metal oxide catalyst. The catalyst used was composed of chromium and cobalt oxides (Cr2O3, CO3O4) deposited on alumina washcoated honeycomb ceramic substrates.The heterogeneous propane oxidation was modelled by a global mechanism consisting of the following three first order reactions;To determine the rate parameters it
Colossal dielectric constants in transition-metal oxides
P. Lunkenheimer; S. Krohns; S. Riegg; S. G. Ebbinghaus; A. Reller; A. Loidl
.ĀĀ\\u000a Many transition-metal oxides show very large (ōcolossalö) magnitudes of the dielectric constant and thus have immense potential\\u000a for applications in modern microelectronics and for the development of new capacitance-based energy-storage devices. In the\\u000a present work, we thoroughly discuss the mechanisms that can lead to colossal values of the dielectric constant, especially\\u000a emphasising effects generated by external and internal interfaces,
Methanol oxidation over nonprecious transition metal oxide catalysts
Umit S. Ozkan; Richard F. Kueller; Edgar Moctezuma
This paper reports methanol oxidation over nonprecious transition metal oxide catalysts studied in a gradient less external recycle reactor. The catalysts (oxides of Cr, Mn, Fe, Co, Ni, Cu), which were prepared by using the incipient wetness technique, were supported on 1\\/8 in.╬│-AlŌOŌ tablets. The catalysts were characterized by using BET surface area measurement, X-ray diffraction, laser Raman spectroscopy, scanning
Electronic Hamiltonian for transition-metal oxide compounds
Erwin M³ller-Hartmann; Elbio Dagotto
An effective electronic Hamiltonian for transition-metal oxide compounds is presented. For Mn oxides, the Hamiltonian contains spin-2 ``spins'' and spin-3\\/2 ``holes'' as degrees of freedom. The model is constructed from the Kondo-lattice Hamiltonian for mobile eg electrons and localized t2g spins, in the limit of a large Hund's coupling. The effective electron-bond-hopping amplitude fluctuates in sign as the total spin
Structural and electronic properties of isostructural transition metal nitrides
NASA Astrophysics Data System (ADS)
Sahnoun, M.; Parlebas, J. C.; Driz, M.; Daul, C.
The structural and electronic properties of three isostructural transition metal nitrides VN, NbN and TaN have been calculated using the full-potential linearized augmented plane-wave method within a generalized gradient approximation scheme for the exchange-correlation potential. Perfect NaCl structures as well as lattices containing nitrogen vacancies (M 4N 3), and an hexagonal ?-M 2N structure are considered. The trends are discussed and compared with available experimental and other theoretical results.
Ab initio molecular dynamics for open-shell transition metals
G. Kresse; J. Hafner
We show that quantum-mechanical molecular-dynamics simulations in a finite-temperature local-density approximation based on the calculation of the electronic ground state and of the Hellmann-Feynman forces after each time step are feasible for liquid noble and transition metals. This is possible with the use of Vanderbilt-type ``ultrasoft'' pseudopotentials and efficient conjugate-gradient techniques for the determination of the electronic ground state. Results
Theory of bonding in transition-metal carbides and nitrides
J. Hõglund; A. Fernßndez Guillermet; G. Grimvall; M. K÷rling
This paper deals with the bonding properties of 3d-, 4d-, and 5d-transition-metal carbides and nitrides. We consider NaCl-structure compounds, MC and MN, as well as carbides and nitrides with more complex crystal structures. The enthalpies of formation at zero temperature Delta0H(0) of the MC and MN compounds are taken from previous ab initio linear-muffin-tin-oribtals calculations. We describe the structure as
Transition-metal dihalide MX2 as magnetoelectric multiferroics
Shinichiro Seki; Takashi Kurumaji; Shintaro Ishiwata; Hiroyuki Matsui; Yoshinori Tokura; Hiroshi Murakawa; Yusuke Tokunaga; Yoshio Kaneko; Tatsuo Hasegawa
Magnetoelectric properties were investigated for transition-metal dihalide MX2, which turns out to be the first example of non-chalcogen based spiral-spin induced multiferroics. We discovered the emergence of ferroelectric polarization (P) in helimagnetic state for several compounds such as CuCl2 with edge-shared S=1\\/2 chain and MnI2 with stacked triangular lattice. In the latter material, in-plane magnetic field (H) was found to
Bifunctional transition metal-based molecular catalysts for asymmetric syntheses
Takao Ikariya; Kunihiko Murata; Ryoji Noyori
The discovery and development of conceptually new chiral bifunctional transition metal-based catalysts for asymmetric reactions is described. The chiral bifunctional Ru catalyst was originally developed for asymmetric transfer hydrogenation of ketones and imines and is now successfully applicable to enantioselective C-C bond formation reaction with a wide scope and high practicability. The deprotonation of 1,3-dicarbonyl compounds with the chiral amido
Magnetic ordering in transition metal doped ZnO
Hitoshi Tabata
3d-transition metal doped ZnO films (n-type (Zn,M): M= Co, Mn, Cr, Ni O (0.05-0.25)) are formed on sapphire substrates using a pulsed laser deposition (PLD) technique, and their magnetic and electric properties are examined. The Co doped ZnO films showed the maximum solubility limit. Some of the Co doped ZnO films exhibit ferromagnetic behaviors with the Curie temperature higher than
ZnO Doped With Transition Metal Ions
Stephen J. Pearton; David P. Norton; M. P. Mil; Art F. Hebard; John M. Zavada; Weimin M. Chen; Irina A. Buyanova
Spin-dependent phenomena in ZnO may lead to devices with new or enhanced functionality, such as polarized solid-state light sources and sensitive biological and chemical sensors. In this paper, we review the experimental results on transition metal doping of ZnO and show that the material can be made with a single phase at high levels of Co incorporation (~15 at.%) and
The energetics of ordered intermetallic alloys (of the transition metals)
Watson, R.E.; Weinert, M.; Davenport, J.W. (Brookhaven National Lab., Upton, NY (United States)); Fernando, G.W. (Connecticut Univ., Storrs, CT (United States). Dept. of Physics); Bennett, L.H. (National Inst. of Standards and Technology, Gaithersburg, MD (United States). Metallurgy Div.)
The atomically ordered phases in ordered transition metal alloys are discussed. This chapter is divided into: physical parameters controlling phase stability (Hume-Rothery, structural maps, Miedema Hamiltonian), wave functions band theory, comment on entropy terms, cohesive energies (electron promotion energies, Hund's rule on orbital effects), structural energies/stabilities of elemental solids, total energies and atomic positions, charge transfer (Au alloys, charge tailing), heats of formation of ordered compounds.
The energetics of ordered intermetallic alloys (of the transition metals)
Watson, R.E.; Weinert, M.; Davenport, J.W. [Brookhaven National Lab., Upton, NY (United States); Fernando, G.W. [Connecticut Univ., Storrs, CT (United States). Dept. of Physics; Bennett, L.H. [National Inst. of Standards and Technology, Gaithersburg, MD (United States). Metallurgy Div.
The atomically ordered phases in ordered transition metal alloys are discussed. This chapter is divided into: physical parameters controlling phase stability (Hume-Rothery, structural maps, Miedema Hamiltonian), wave functions & band theory, comment on entropy terms, cohesive energies (electron promotion energies, Hund`s rule on orbital effects), structural energies/stabilities of elemental solids, total energies and atomic positions, charge transfer (Au alloys, charge tailing), heats of formation of ordered compounds.
Valence-bond theory of compounds of transition metals
Pauling, Linus
An equation relating the strength (bondforming power) of an spd hybrid bond orbital to the angles it makes with other bond orbitals is formulated and applied in the discussion of the structures of transition-metal carbonyls and other substances by the valence-bond method. The rather simple theory gives results that agree well with those obtained by the complicated and laborious calculation of sets of orthogonal hybrid bond orbitals with maximum strength. PMID:16592279
Novel Transition Metal Compounds with Promising Thermoelectric Properties
NASA Technical Reports Server (NTRS)
Caillat, T.; Borshchevsky, A.; Fleurial, J. -P.
Progress in the search for new high temperature thermoelectric materials at the Jet Propulsion Laboratory is reviewed. Novel transition metal compounds were selected as potential new high performance thermoelectric materials and criteria of selection are presented and discussed. Samples of these new compounds were prepared at JPL by a variety of techniques. Encouraging experimental results obtained on several of these compounds are reported and show that they have the potential to be the next generation of thermoelectric materials.
Recent Progress towards Transition-Metal-Catalyzed Synthesis of Fluorenes.
Zhou, Ai-Hua; Pan, Fei; Zhu, Chunyin; Ye, Long-Wu
Fluorenes are a commonly encountered structural motif in materials science, pharmaceutical chemistry, and organic synthesis. Among various strategies towards the synthesis of this unique structure, transition metal-catalyzed functionalization has emerged as one of the most efficient methods. This Minireview presents an overview of the recent advances in this emerging area by highlighting the reactions' specificity and applicability and, where possible, provides a mechanistic rationale. PMID:25858775
Vapor growth of electrochromic thin films of transition metal oxides
NASA Astrophysics Data System (ADS)
Gesheva, K. A.; Ivanova, T.; Marsen, B.; Zollo, G.; Kalitzova, M.
Mixed oxide films of transition metals gain considerable much attention due to their interesting optoelectronic properties. The low temperature chemical vapor growth processing of thin films of mixed W and Mo oxides is presented. The investigation is related to optimization of films structure and the related optoelectronic properties in dependence on the chemical vapor deposition (CVD) process parameters. Their electrochromic behavior and photoelectrode properties were studied.
Tight-binding simulation of transition-metal alloys.
McEniry, Eunan J; Madsen, Georg K H; Drain, John F; Drautz, Ralf
In order to perform atomistic simulations of steel, it is necessary to have a detailed understanding of the complex interatomic interactions in transition metals and their alloys. The tight-binding approximation provides a computationally efficient, yet accurate, method to investigate such interactions. In the present work, an orthogonal tight-binding model for Fe, Mn and Cr, with the explicit inclusion of magnetism, has been parameterized from ab initio density-functional calculations. PMID:21690660
The development of a biological interface for transition metal implants
NASA Astrophysics Data System (ADS)
Melton, Kim R.
The specific goal of this research was to develop an in vitro model for a root-form endosseous dental implant that contains a periodontal ligament and that is biologically integratable into alveolar bone. This objective was based on the following two hypotheses. (1) The chemical attachment of extracellular matrix proteins to the surface of transition metals increases the number of fibroblast cells attached to the surface of the metal. (2) The chemical attachment of extracellular matrix proteins to the surface of transition metals increases the strength of the fibroblast cell attachment to the surface of the metal. The model needed to have a well-controlled surface that was reproducible. Thus, a layer of Au was deposited over a Ti base, and dithiobis(succinimidylpropionate) (DSP) a chemical containing disulfide groups was adsorbed to the Au. Next, extracellular matrix proteins which are periodontal ligament components were attached to the free end group of the chemical that was adsorbed to the Au. This surface served as an attachment substrate on which additional periodontal ligament components such as fibroblast cells could grow. From this model a new implant interface may be developed. This model was tested using the following polypeptides; collagen type I, collagen type IV, fibronectin, and poly-D-lysine. L929 cells were grown on Ti, Ti + Au, Ti + Au + polypeptide, and Ti + Au + DSP + polypeptide. After 72 hours, the live cells were stained with neutral red. The substrates were then subjected to increasing centrifugal forces. The viable stained cells were fixed onto the substrates and cells were counted. The hypotheses were proven for three polypeptides: fibronectin, collagen type I, and poly-D-lysine. The strongest attachment was found with collagen type I. Collagen type IV did not provide any advantage for attachment over uncoated transition metals.
Performance of density functionals for first row transition metal systems.
Jensen, Kasper P; Roos, Bj÷rn O; Ryde, Ulf
This article investigates the performance of five commonly used density functionals, B3LYP, BP86, PBE0, PBE, and BLYP, for studying diatomic molecules consisting of a first row transition metal bonded to H, F, Cl, Br, N, C, O, or S. Results have been compared with experiment wherever possible. Open-shell configurations are found more often in the order PBE0>B3LYP>PBE approximately BP86>BLYP. However, on average, 58 of 63 spins are correctly predicted by any functional, with only small differences. BP86 and PBE are slightly better for obtaining geometries, with errors of only 0.020 A. Hybrid functionals tend to overestimate bond lengths by a few picometers and underestimate bond strengths by favoring open shells. Nonhybrid functionals usually overestimate bond energies. All functionals exhibit similar errors in bond energies, between 42 and 53 kJmol. Late transition metals are found to be better modeled by hybrid functionals, whereas nonhybrid functionals tend to have less of a preference. There are systematic errors in predicting certain properties that could be remedied. BLYP performs the best for ionization potentials studied here, PBE0 the worst. In other cases, errors are similar. Finally, there is a clear tendency for hybrid functionals to give larger dipole moments than nonhybrid functionals. These observations may be helpful in choosing and improving existing functionals for tasks involving transition metals, and for designing new, improved functionals. PMID:17212486
Novel Extended Solids Composed of Transition Metal Oxide Clusters
NASA Astrophysics Data System (ADS)
Ishaque Khan, M.
Transition metal oxide clusters and their derivatives offer an unmatched variety of structural motifs that are potential building blocks for the design and development of extended solids. In view of the proven applications of these clusters in catalysis, molecular recognition, environmental decontamination, clinical and analytical processes, materials science, nanotechnology, and medicine, their usage as building blocks promises the development of new materials whose properties could be rationalized in terms of their constituents. Although the technique of assembling appropriate transition metal oxide clusters to generate new solids is still underdeveloped, recent progress made in this direction is promising. During our ongoing research program focused on this aspect, we have been able to prepare and characterize a series of novel framework materials, composed of well defined vanadium oxide clusters {V18O42(XO4)} (X=V, S, Cl). We have also prepared three-dimensional composite solids by incorporating organic ligands into the transition metal oxide framework. Some of the results of this work are reviewed with reference to the synthesis, structure, and physicochemical properties of the newly prepared extended solids.
Speed limit of the insulator-metal transition in magnetite
NASA Astrophysics Data System (ADS)
de Jong, S.; Kukreja, R.; Trabant, C.; Pontius, N.; Chang, C. F.; Kachel, T.; Beye, M.; Sorgenfrei, F.; Back, C. H.; Brõuer, B.; Schlotter, W. F.; Turner, J. J.; Krupin, O.; Doehler, M.; Zhu, D.; Hossain, M. A.; Scherz, A. O.; Fausti, D.; Novelli, F.; Esposito, M.; Lee, W. S.; Chuang, Y. D.; Lu, D. H.; Moore, R. G.; Yi, M.; Trigo, M.; Kirchmann, P.; Pathey, L.; Golden, M. S.; Buchholz, M.; Metcalf, P.; Parmigiani, F.; Wurth, W.; F÷hlisch, A.; Sch³▀ler-Langeheine, C.; D³rr, H. A.
As the oldest known magnetic material, magnetite (Fe3O4) has fascinated mankind for millennia. As the first oxide in which a relationship between electrical conductivity and fluctuating/localized electronic order was shown, magnetite represents a model system for understanding correlated oxides in general. Nevertheless, the exact mechanism of the insulator-metal, or Verwey, transition has long remained inaccessible. Recently, three-Fe-site lattice distortions called trimerons were identified as the characteristic building blocks of the low-temperature insulating electronically ordered phase. Here we investigate the Verwey transition with pump-probe X-ray diffraction and optical reflectivity techniques, and show how trimerons become mobile across the insulator-metal transition. We find this to be a two-step process. After an initial 300?fs destruction of individual trimerons, phase separation occurs on a 1.5▒0.2?ps timescale to yield residual insulating and metallic regions. This work establishes the speed limit for switching in future oxide electronics.
Probing early-universe phase transitions with CMB spectral distortions
NASA Astrophysics Data System (ADS)
Amin, Mustafa A.; Grin, Daniel
Global, symmetry-breaking phase transitions in the early universe can generate scaling seed networks which lead to metric perturbations. The acoustic waves in the photon-baryon plasma sourced by these metric perturbations, when Silk damped, generate spectral distortions of the cosmic microwave background (CMB). In this work, the chemical potential distortion (? ) due to scaling seed networks is computed and the accompanying Compton y -type distortion is estimated. The specific model of choice is the O (N ) nonlinear ? -model for N ?1 , but the results remain the same order of magnitude for other scaling seeds. If CMB anisotropy constraints to the O (N ) model are saturated, the resulting chemical potential distortion ? ?2 ū1 0-9 .
Method for treating rare earth-transition metal scrap
Schmidt, F.A.; Peterson, D.T.; Wheelock, J.T.; Jones, L.L.
Rare earth-transition metal (e.g., iron) scrap (e.g., Nd-Fe-B scrap) is flux (slag) remelted to reduce tramp non-metallic impurities, such as oxygen and nitrogen, and metallic impurities, such as Li, Na, Al, etc., picked up by the scrap from previous fabrication operations. The tramp impurities are reduced to concentrations acceptable for reuse of the treated alloy in the manufacture of end-use articles, such as permanent magnets. The scrap is electroslag or inductoslag melted using a rare earth fluoride-bearing flux of CaF[sub 2], CaCl[sub 2] or mixtures thereof or the slag resulting from practice of the thermite reduction process to make a rare earth-iron alloy. 3 figs.
Method for treating rare earth-transition metal scrap
Schmidt, Frederick A. (Ames, IA); Peterson, David T. (Ames, IA); Wheelock, John T. (Nevada, IA); Jones, Lawrence L. (Des Moines, IA)
Rare earth-transition metal (e.g., iron) scrap (e.g., Nd-Fe-B scrap) is flux (slag) remelted to reduce tramp non-metallic impurities, such as oxygen and nitrogen, and metallic impurities, such as Li, Na, Al, etc., picked up by the scrap from previous fabrication operations. The tramp impurities are reduced to concentrations acceptable for reuse of the treated alloy in the manufacture of end-use articles, such as permanent magnets. The scrap is electroslag or inductoslag melted using a prefused, rare earth fluoride-bearing flux of CaF.sub.2, CaCl.sub.2 or mixtures thereof or the slag resulting from practice of the thermite reduction process to make a rare earth-iron alloy.
Covalent functionalization of monolayered transition metal dichalcogenides by phase engineering
NASA Astrophysics Data System (ADS)
Voiry, Damien; Goswami, Anandarup; Kappera, Rajesh; Silva, Cecilia De Carvalho Castro E.; Kaplan, Daniel; Fujita, Takeshi; Chen, Mingwei; Asefa, Tewodros; Chhowalla, Manish
Chemical functionalization of low-dimensional materials such as nanotubes, nanowires and graphene leads to profound changes in their properties and is essential for solubilizing them in common solvents. Covalent attachment of functional groups is generally achieved at defect sites, which facilitate electron transfer. Here, we describe a simple and general method for covalent functionalization of two-dimensional transition metal dichalcogenide nanosheets (MoS2, WS2 and MoSe2), which does not rely on defect engineering. The functionalization reaction is instead facilitated by electron transfer between the electron-rich metallic 1T phase and an organohalide reactant, resulting in functional groups that are covalently attached to the chalcogen atoms of the transition metal dichalcogenide. The attachment of functional groups leads to dramatic changes in the optoelectronic properties of the material. For example, we show that it renders the metallic 1T phase semiconducting, and gives it strong and tunable photoluminescence and gate modulation in field-effect transistors.
Environmental Effects on the Metallicities of Early-Type Galaxies
NASA Technical Reports Server (NTRS)
Oliversen, Ronald J. (Technical Monitor); Jones, Christine
In this multi-year project to investigate the metal enrichment of early-type galaxies, we have used ROSAT, ASCA and now Chandra observations to study samples of galaxies. We have published two papers and a third paper that incorporates Chandra archival observations is nearing completion. Below, we briefly describe our findings. Our first paper "SN IA Enrichment in Virgo Early-type Galaxies from ROSAT and ASCA Observations" was published in the Astrophysical Journal (vol 539, 603) reported on the properties of nine X-ray bright elliptical galaxies in the Virgo cluster observed by ROSAT and ASCA. We measured iron abundance gradients as a function of radius in three galaxies. We found that the magnesium and silicon abundance gradients were in general flatter than those of iron. We suggest this is due to a metallicity dependence in the metal production rates of SN Ia's. We calculate SN Ia rates in the center of these galaxies that are comparable to those measured optically. Our second paper "ASCA Observations of Groups at Radii of Low Overdensity: Implications for Cosmic Preheating" also was published in the Astrophysical Journal (vol 578, 74). This paper reported on the ASCA spectroscopy of nine groups of galaxies. We found that the entropy profile in groups is driven by nongravitational heating processes, and could be explained by a short period of preheating by galactic winds. The third paper (in preparation) uses a sample of about 200 galaxies from both ROSAT and Chandra observations. In this paper we characterize both the nuclear and the extended X-ray emission for this sample. We will use these observations to determine the "on-time" of the X-ray emitting AGN and the fraction of "fossil groups" as well as to investigate how large AGN outbursts can sweep the galaxy of its hot ISM, thus leading to changes in the ISM metal enrichment.
Energy transitions in the early 21st Century
Paul Meakin
We are in the early stages of a long and complex transition from a global economy based on fossil energy to an economy based on low carbon renewable energy. However, fossil fuel resources are abundant and widely distributed, and they will remain the dominant source of primary energy for at least the next quarter century. In the United States, displacement of coal by natural gas for electric power generation has done more to reduce CO2 emissions than all new renewables combined, and this may occur globally for the next decade or two, even if the European Union does not take advantage of its large unconventional natural gas resources. Greater energy efficiency (not including the efficiencies associated with displacement of coal by gas) will also be more important than new renewables. Cost/benefit ratios are important for sustainability of the transition, and some energy efficiency technologies and displacement of coal by natural gas have lower cost/benefit ratios than wind power, solar power or biofuels. Money spent on the large scale deployment of wind, solar and especially biofuels would be better spent on research, development and demonstration of a broader suite of technologies that would support the energy transition, with a focus on improving the cost benefit ratios of already deployed technologies and developing alternatives. Advanced nuclear reactors, engineered geothermal systems, fossil fuel recovery coupled with CO2 sequestration and pre-combustion or post-combustion decarbonation of fossil fuels with geological CO2 sequestration are among the technologies that might be more cost effective than wind, solar or biofuels, and biofuels have serious adverse societal and environment consequences.
N-heterocyclic carbene (NHC) complexes of group 4 transition metals.
Zhang, Dao; Zi, Guofu
Since the discovery of a stable N-heterocyclic carbene (NHC), the use of NHCs in chemistry has developed rapidly over the past two decades. These interesting compounds are predominantly employed in organometallic chemistry as ligands for various metal centers, and as organocatalysts for a variety of transformations. In particular, the NHC transition metal complexes have received widespread attention, and significant progress has been made in the development of group 4 NHC-complexes in the last few years. These group 4 NHC-complexes are of interest because of their unique structural properties, and their potential application in organic transformations and catalysis. This review covers the superior design strategies for NHC ligands to stabilize early transition metals and well-defined group 4 metal complexes with mono- and multi-dentate NHC ligands. In this context, four types of NHC-complexes, i.e., carbon-functionalized NHCs, nitrogen-functionalized NHCs, oxygen-functionalized NHCs and nitrogen/oxygen-functionalized unsymmetric NHCs, are described. In addition, the use of group 4 NHC-complexes as catalysts in olefin (co)polymerization, ring-opening polymerization of rac-lactide, copolymerization of epoxides and CO2, as well as hydroamination/cyclization of aminoalkenes, is presented. Furthermore, limitations and challenges are discussed. PMID:25608933
Open volume defects and magnetic phase transition in Fe60Al40 transition metal aluminide
NASA Astrophysics Data System (ADS)
Liedke, M. O.; Anwand, W.; Bali, R.; Cornelius, S.; Butterling, M.; Trinh, T. T.; Wagner, A.; Salamon, S.; Walecki, D.; Smekhova, A.; Wende, H.; Potzger, K.
Magnetic phase transition in the Fe60Al40 transition metal aluminide from the ferromagnetic disordered A2-phase to the paramagnetic ordered B2-phase as a function of annealing up to 1000 ░C has been investigated by means of magneto-optical and spectroscopy techniques, i.e., Kerr effect, positron annihilation, and M÷ssbauer spectroscopy. The positron annihilation spectroscopy has been performed in-situ sequentially after each annealing step at the Apparatus for In-situ Defect Analysis that is a unique tool combining positron annihilation spectroscopy with temperature treatment, material evaporation, ion irradiation, and sheet resistance measurement techniques. The overall goal was to investigate the importance of the open volume defects onto the magnetic phase transition. No evidence of variation in the vacancy concentration in matching the magnetic phase transition temperature range (400-600 ░C) has been found, whereas higher temperatures showed an increase in the vacancy concentration.
Age and metallicity gradients in early-type galaxies: a dwarf-to-giant sequence
NASA Astrophysics Data System (ADS)
Koleva, Mina; Prugniel, Philippe; de Rijcke, Sven; Zeilinger, Werner W.
We studied the stellar populations of 40 early-type galaxies using medium-resolution long-slit spectroscopy along their major axes (and along the minor axis for two of them). The sample, including elliptical and lenticular galaxies as well as dwarf galaxies, is combined with other previously published data in order to discuss the systematics of the radial gradients of age and metallicity over a large mass range, from 107 M? to 1012 M? (-9.2 > MB > -22.4 mag). The well-known mass-metallicity relation is continuous throughout the whole mass range, in the sense that more massive galaxies are more metal-rich. The age-mass relation is consistent with the idea of downsizing: smaller galaxies have more extended star formation histories than more massive ones. The transition-type dwarfs (intermediate between dwarf irregular and dwarf elliptical galaxies) deviate from this relation having younger mean age, and the low-mass dwarf spheroidals have older ages, marking a discontinuity in the relation, possibly due to selection effects. In all mass regimes, the mean metallicity gradients are approximately -0.2 and the mean age gradients +0.1 dex per decade of radius. The individual gradients are widely spread: -0.1 < ?Age < 0.4 and -0.54 < ?[Fe/H] < +0.2. We do not find evidence for a correlation between the metallicity gradient and luminosity, velocity dispersion, central age or age gradient. Likewise, we do not find a correlation between the age gradient and any other parameter in bright early-type galaxies. In faint early-types with MB?-17 mag, on the other hand, we find a strong correlation between the age gradient and luminosity: the age gradient becomes more positive for fainter galaxies. Together with the observed downsizing phenomenon this indicates that, as time passes, star formation persists in dwarf galaxies and becomes more centrally concentrated. However, this prolonged central star formation is not reflected in the metallicity profiles of the dwarfs in our sample. We conclude that various physical mechanisms can lead to similar gradients and that these gradients are robust against the environmental effects. In particular, the gradients observed in dwarf galaxies certainly survived the transformation of the progenitors through tidal harassment or/and ram-pressure stripping. The diversity of metallicity gradients amongst dwarf elliptical galaxies may reflect a plurality of progenitors' morphologies. The dwarfs with steep metallicity gradients could have originated from blue compact dwarfs and those with flat profiles from dwarf irregulars and late-type spirals.
Alternative Kondo breakdown mechanism: Orbital-selective orthogonal metal transition
Zhong, Yin; Wang, Yong-Qiang; Luo, Hong-Gang
In a recent paper of Nandkishore and Senthil [arxiv:1201.5998 (2012)], a concept of orthogonal metal has been introduced to reinterpret the disordered state of slave-spin representation in the Hubbard model as an exotic gapped metallic state. We extend this concept to study the corresponding quantum phase transition in the extended Anderson lattice model. It is found that the disordered state of slave spins in this model is an orbital-selective orthogonal metal, a generalization of the concept of the orthogonal metal in the Hubbard model. Near the quantum critical point the essential behaviors are dominated by a z = 3 critical mode, which is in contrast to the naive expectation in the Hubbard model. The result provides alternative Kondo breakdown mechanism for heavy fermion compounds underlying the physics of the orbital-selective orthogonal metal in the disordered state, which is different from the conventional Kondo breakdown mechanism with the fractionalized Fermi liquid picture. This work is expected to b...
Alternative Kondo breakdown mechanism: Orbital-selective orthogonal metal transition
Yin Zhong; Ke Liu; Yong-Qiang Wang; Hong-Gang Luo
In a recent paper of Nandkishore and Senthil [arxiv:1201.5998 (2012)], a concept of orthogonal metal has been introduced to reinterpret the disordered state of slave-spin representation in the Hubbard model as an exotic gapped metallic state. We extend this concept to study the corresponding quantum phase transition in the extended Anderson lattice model. It is found that the disordered state of slave spins in this model is an orbital-selective orthogonal metal, a generalization of the concept of the orthogonal metal in the Hubbard model. Near the quantum critical point the essential behaviors are dominated by a z = 3 critical mode, which is in contrast to the naive expectation in the Hubbard model. The result provides alternative Kondo breakdown mechanism for heavy fermion compounds underlying the physics of the orbital-selective orthogonal metal in the disordered state, which is different from the conventional Kondo breakdown mechanism with the fractionalized Fermi liquid picture. This work is expected to be useful in understanding the quantum criticality happening in some heavy fermion materials and other related strongly correlated systems.
Catalytic Fusion and the Interface Between Insulators and Transition Metals
NASA Astrophysics Data System (ADS)
Chubb, Talbot A.
Cold fusion uses a catalyzed configuration change to replace plasma fusion's need for high-energy particle collisions.1 In radiationless cold fusion, the configuration change is a coherent partitioning of deuterons into fractional pieces within a set of potential wells provided by a hosting lattice.2 The coherently partitioned matter distribution is a Bloch wave function. Alpha addition transmutations3 require active deuterium in the form of Bloch function deuterons with 2-dimensional periodic symmetry.4,5 The configuration change to Bloch form has been modeled as occurring in the interface volume between a salt and Pd metal. In Arata and Zhang radiationless cold fusion6-8 reactive deuterons are modeled by Bloch ions with 3-dimensional periodic symmetry hosted in metallic nano crystals.5 The nano crystals are isolated by salt-metal interfaces. In both cases, the fusion process is modeled as a Li-Feshbach resonance transition to an excited nucleus state, with subsequent energy transfer to a metal lattice by phonon cascade.5 The lattice structure of the deuterons is preserved in the product nucleus until the energy transfer is completed. For the 2-dimensional symmetry case, the intermediate nucleus or many-body nuclear system can sometimes be observed in "flake" lattice form, providing insight about the process.5 Research on salt-metal interfaces could facilitate cold fusion technology.
Criteria for identifying homogeneous transition metal cluster-catalyzed reactions
Laine, R.M.
Four criteria were developed for distinguishing homogeneous cluster from mononuclear catalysis. A criterion based on reaction kinetics, which distinguishes catalysis by clusters from catalysis by mononuclear complexes in equilibrium with the clusters, is illustrated with the hydroformylation of 1-pentene by Ru/sub 3/(CO)/sub 12/ clusters in equilibrium with 3Ru(CO)/sub 5/ complexes in the presence of CO. A criterion based on selectivity is illustrated by the water gas shift and deuterium-exchange reactions catalyzed by ruthenium and rhodium carbonyl clusters in methanol and triethylamine; the scope of this criterion is limited to deuterium exchange reactions. A criterion based on mixed-metal catalysis, in which a mixed-metal system has higher catalytic activity than each transition metal cluster alone, is illustrated with hydroformylation of 1-pentene and the water gas shift reaction catalyzed by Ru/sub 3/(CO)/sub 12//Fe/sub 3/(CO)/sub 2/ and Rh/sub 6/(CO)/sub 16//Fe/sub 3/(CO)/sub 12/. A criterion based on changes in asymmetric induction is a special case of the selectivity criterion, in which specific starting metal complexes may produce either of two enantiomers. Catalysis by metal clusters is an analytical tool for modeling heterogeneous catalytic mechanisms.
A study of three transition metal compounds and their applications
NASA Astrophysics Data System (ADS)
Qin, Zhen
The La5/8-yPryCa3/8MnO3 with colossal magnetoresistance (CMR) effect has a rich phase diagram and is well studied for its electronic phase separation phenomenon, where the ferromagnetic (FM) metal order co-exists and competes with the charge-ordered (CO) insulator phase. High Pr doping will favor the CO order, leading a sharp FM to CO dominated phase transition around Pr concentration of 0.3 and above. Hydrostatic pressure favors FM metallic order without damaging the sample and can be tuned continuously. In this study, pressurized-magnetic and resistivity measurements was done on a La0.25Pr0.375Ca0.375MnO3 single crystal. The sample, at first sitting in CO dominated phase, changed into FM upon a small amount of pressure. This transition was verified both by magnetic and resistivity measurement results. FeSe1-x is one of the newly discovered iron-based superconductors. As a binary transition metal compound, it is of great research interest due to the simple stacking 2d-layered structure. The itinerant or localized nature of the electrons in Fe2+ ion has been debated but not concluded. In this research, Raman scattering measurements on FeSe0.97 were applied within a temperature range from 5 K to 300 K. The excitation near 185 cm-1 was assigned to B1g phonon excitation. Broad and intensive excitation peaks were found in a wide region between 200 cm-1 and 700 cm-1, and they are classified as the Fe2+ crystal field excitations. These excitations suggest a low Hund coupling constant and thus support the itinerant nature of 3d electrons in Fe2+ ion indirectly. Evanescent wave was discovered to be able to tunnel through a negative reflectance index material and gets strengthened inside an alternating metal-high K material 1d photonic crystal structure. Where the regular light eventually fails in sub-micron photolithography due to diffraction limit, evanescent wave can carry the information of small structure below diffraction limit. In our study, HfO2, a transitional metal oxide widely used in IC fabrication, was used as the high-K material to construct a sub-wavelength length imaging device for nano scale photolithography applications.
Supramolecular control of transition metal complexes in water by a hydrophobic cavity: a bio-inspired strategy.
Bistri, Olivia; Reinaud, Olivia
Supramolecular chemistry in water is a very challenging research area. In biology, water is the universal solvent where transition metal ions play major roles in molecular recognition and catalysis. In enzymes, it participates in substrate binding and/or activation in the heart of a pocket defined by the folded protein. The association of a hydrophobic cavity with a transition metal ion is thus a very appealing strategy for controlling the metal ion properties in the very competitive water solvent. Various systems based on intrinsically water-soluble macrocyclic structures such as cyclodextrins, cucurbituryls, and metallo-cages have been reported. Others use calixarenes and resorcinarenes functionalized with hydrophilic substituents. One approach for connecting a metal complex to these cavities is to graft a ligand for metal ion binding at their edge. Early work with cyclodextrins has shown Michaelis-Menten like catalysis displaying enhanced kinetics and substrate-selectivity. Remarkable examples of regio- and stereo-selective transformation of substrates have been reported as well. Dynamic two-phase systems for transition metal catalysis have also been developed. They rely on either water-transfer of the metal complex through ligand embedment or synergistic coordination of a metal ion and substrate hosting. Another strategy consists in using metallo-cages, which provide a well-defined hydrophobic space, to stabilize metal complexes in water. When the cages can host simultaneously a substrate and a reactive metal complex, size- and regio-selective catalysis was obtained. Finally, construction of a polydentate coordination site closely interlocked with a calixarene or resorcinarene macrocycle has been shown to be a very fruitful strategy for obtaining metal complexes with remarkable hosting properties. For each of these systems, the synergism resulting from the biomimetic association of a hydrophobic cavity and a metal ion is discussed within the objective of developing new tools for either selective molecular recognition (with analytical perspectives) or performant catalysis, in water. PMID:25608497
Pressure-induced electronic phase transitions in transition metal oxides and rare earth metals
Brian Ross Maddox
Electron correlation can affect profound changes in a material's bulk properties. When the degree of correlation is changed, phase transitions can sometimes result. Applying pressure can inducing changes in the degree of electron correlation by altering the interatomic distances of crystalline materials. This dissertation presents a study of a number of correlated systems at ultrahigh pressures generated by diamond-anvil cells.
Critical Transitions in Early Embryonic Aortic Arch Patterning and Hemodynamics
Kowalski, William J.; Dur, Onur; Wang, Yajuan; Patrick, Michael J.; Tinney, Joseph P.; Keller, Bradley B.; Pekkan, Kerem
Transformation from the bilaterally symmetric embryonic aortic arches to the mature great vessels is a complex morphogenetic process, requiring both vasculogenic and angiogenic mechanisms. Early aortic arch development occurs simultaneously with rapid changes in pulsatile blood flow, ventricular function, and downstream impedance in both invertebrate and vertebrate species. These dynamic biomechanical environmental landscapes provide critical epigenetic cues for vascular growth and remodeling. In our previous work, we examined hemodynamic loading and aortic arch growth in the chick embryo at Hamburger-Hamilton stages 18 and 24. We provided the first quantitative correlation between wall shear stress (WSS) and aortic arch diameter in the developing embryo, and observed that these two stages contained different aortic arch patterns with no inter-embryo variation. In the present study, we investigate these biomechanical events in the intermediate stage 21 to determine insights into this critical transition. We performed fluorescent dye microinjections to identify aortic arch patterns and measured diameters using both injection recordings and high-resolution optical coherence tomography. Flow and WSS were quantified with 3D computational fluid dynamics (CFD). Dye injections revealed that the transition in aortic arch pattern is not a uniform process and multiple configurations were documented at stage 21. CFD analysis showed that WSS is substantially elevated compared to both the previous (stage 18) and subsequent (stage 24) developmental time-points. These results demonstrate that acute increases in WSS are followed by a period of vascular remodeling to restore normative hemodynamic loading. Fluctuations in blood flow are one possible mechanism that impacts the timing of events such as aortic arch regression and generation, leading to the variable configurations at stage 21. Aortic arch variations noted during normal rapid vascular remodeling at stage 21 identify a temporal window of increased vulnerability to aberrant aortic arch morphogenesis with the potential for profound effects on subsequent cardiovascular morphogenesis. PMID:23555940
An Analysis of Grain Production Decline During the Early Transition in Ukraine: Bayesian Inference
An Analysis of Grain Production Decline During the Early Transition in Ukraine: Bayesian Inference in production. Identifying the reasons for the decline in agricultural output in countries like Ukraine of the decline in grain production in early transition Ukraine. Much of the literature devoted to the analyses
Transition metal complexes of an isatinic quinolyl hydrazone
Background The importance of the isatinic quinolyl hydrazones arises from incorporating the quinoline ring with the indole ring in the same compound. Quinoline ring has therapeutic and biological activities. On the other hand, isatin (1H-indole-2,3-dione) and its derivatives exhibit a wide range of biological activities. Also, the indole ring occurs in Jasmine flowers and Orange blossoms. Recently, the physiological and biological activities of quinolyl hydrazones arise from their tendency to form metal chelates with transition metal ions. In this context, we have reported to isolate, characterize and study the biological activity of some transition metal complexes of an isatinic quinolyl hydrazone; 3-[2-(4-methyl quinolin-2-yl)hydrazono] indolin-2-one. Results Mono- and binuclear as well as dimeric chelates were obtained from the reaction of a new isatinic quinolyl hydrazone with Fe(III), Co(II), Ni(II), Cu(II), VO(II) and Pd(II) ions. The ligand showed a variety of modes of bonding viz. (NNO)2-, (NO)- and (NO) per each metal ion supporting its ambidentate and flexidentate characters. The mode of bonding and basicity of the ligand depend mainly on the type of the metal cation and its counter anion. All the obtained Pd(II)- complexes have the preferable square planar geometry (D4h- symmetry) and depend mainly on the mole ratio (M:L). Conclusion The effect of the type of the metal ion for the same anion (Cl-) is obvious from either structural diversity of the isolated complexes (Oh, Td and D4h) or the various modes of bonding. The isatinic hydrazone uses its lactim form in all complexes (Cl-) except complex 5 (SO42-) in which it uses its lactam form. The obtained Pd(II)- complexes (dimeric, mono- and binuclear) are affected by the mole ratio (M:L) and have the square planar (D4h) geometry. Also, the antimicrobial activity is highly influenced by the nature of the metal ion and the order for S. aureus bacteria is as follows: Nickel(II) > Vanadyl(II) > Cobalt(II) > Copper(II) ? Palladium(II) >> Iron(III). PMID:21708023
Low temperature synthesis of transition metal oxides containing surfactant ions
NASA Astrophysics Data System (ADS)
Janauer, Gerald Gilbert
Recently there has been much interest in reacting vanadium oxides hydrothermally with cationic surfactants to form novel layered compounds. A series of new transition metal oxides, however, has also been formed at or near room temperature in open containers. Synthesis, characterization, and proposed mechanisms of formation are the focus of this work. Low temperature reactions of vanadium pentoxide and ammonium transition metallates with long chain amine surfactants, such as dodecyltrimethylammonium bromide yielded interesting new products many of which are layered phases. DTAsb4\\ Hsb2Vsb{10}Osb{28}. 8Hsb2O, a layered highly crystalline phase, is the first such phase for which a single crystal X-ray structure has been determined. The unit cell for this material was found to be triclinic with space group P1-, cell parameters a=9.8945(3)A, b=11.5962(1)A, c=21.9238(2)A, alpha=95.153(2)sp░,\\ beta=93.778(1)sp░, and gamma=101.360(1)sp░. Additionally, a novel tungsten, a molybdenum and a dichromate phase will be discussed. Both the tungsten and the dichromate materials were indexed from their powder diffraction patterns yielding monoclinic unit cells. The tungsten material was found to have a=50.56(4)A, b=54.41(4)A, c=13.12(1)A, and beta=99.21sp░. The dichromate compound was determined to have a=26.757(5)A, b=10.458(2)A, c=14.829(3)A and beta=98.01(1)sp░. Interlayer spacings for the lamellar dichromate and molybdenum phases were d001 = 28.7 A, and d001 = 22.9 A. The synthesis, characterization, composition, and structure of these transition metal oxide-surfactant materials will be discussed.
Kondo Effects in Single Layer Transition Metal Dichalcogenides
NASA Astrophysics Data System (ADS)
Phillips, Michael; Aji, Vivek
Inversion symmetry breaking and strong spin orbit coupling in two dimensional transition metal dichalcogenides leads to interesting new phenomena such as the valley hall and spin hall effects. They display optical circular dichroism and the ability to generate excitation with valley specificity. In this talk we report on the consequences of these properties on correlated states in hole doped systems focussing on the physics of the screening of magnetic impurities. Unlike typical metals, the breaking of inversion symmetry leads to the mixing of a triplet component to the Kondo cloud. Using a variational wave function approach we determine the nature of the many body state. With the ground state in hand we analyze the excitations generated by valley discriminating perturbations. Graduate Student.
Energy level statistics in disordered metals with an Anderson transition
Evangelou, S.N. [Univ. of Ioannina (Greece)]|[Institute for Electronic Structure & Lasers, Crete (Greece); Katsanos, D.E. [Univ. of Ioannina (Greece)
We present numerical scaling results for the energy level statistics in orthogonal and symplectic tight-binding Hamiltonian random matrix ensembles defined on disordered two and three-dimensional electronic systems with and without spin-orbit coupling (SOC), respectively. In the metallic phase for weak disorder the nearest level spacing distribution function P(S), the number variance <({delta}N){sup 2}>, and the two-point correlation function K{sub 2}({epsilon}) are shown to be described by the Gaussian random matrix theories. In the insulating phase, for strong disorder, the correlations vanish for large scales and the ordinary Poisson statistics is asymptotically recovered, which is consistent with localization of the corresponding eigenstates. At the Anderson metal-insulator transition we obtain new universal scale-invariant distribution functions describing the critical spectral density fluctuations.
Surface entropy of liquid transition and noble metals
NASA Astrophysics Data System (ADS)
Gosh, R. C.; Das, Ramprosad; Sen, Sumon C.; Bhuiyan, G. M.
Surface entropy of liquid transition and noble metals has been investigated using an expression obtained from the hard-sphere (HS) theory of liquid. The expression is developed from the Mayer's extended surface tension formula [Journal of Non-Crystalline Solids 380 (2013) 42-47]. For interionic interaction in metals, Brettonet-Silbert (BS) pseudopotentials and embedded atom method (EAM) potentials have been used. The liquid structure is described by the variational modified hypernetted chain (VMHNC) theory. The essential ingredient of the expression is the temperature dependent effective HS diameter (or packing fraction), which is calculated from the aforementioned potentials together with the VMHNC theory. The obtained results for the surface entropy using the effective HS diameter are found to be good in agreement with the available experimental as well as other theoretical values.
Structural and Electronic Transitions in Ytterbium Metal to 202 GPa
Chesnut, G.N.; Vohra, Y.K. [Department of Physics, University of Alabama at Birmingham (UAB), Birmingham, Alabama 35294-1170 (United States)] [Department of Physics, University of Alabama at Birmingham (UAB), Birmingham, Alabama 35294-1170 (United States)
Ytterbium (Yb), a member of the rare earth group metals with a filled f shell and a divalent electronic character, has been studied under static high pressures in a diamond anvil cell to 202thinspthinspGPa. We report a new transformation from a face-centered-cubic structure to a hexagonal phase with 3thinspthinspatoms/cell (space group P3{sub 1}21 referred to as hP3 ) at 98{plus_minus}5 GPa . This hP3 phase is similar to that observed in trivalent neodymium and samarium at pressures near 40thinspthinspGPa. Our high pressure study thus clearly demonstrates that Yb is no longer an {open_quotes}irregular{close_quotes} member of the rare earth metals, and divalent to trivalent electronic transition is completed at megabar pressures. {copyright} {ital 1999} {ital The American Physical Society}
Augmenting Molecular Junctions with Different Transition Metal Contacts
NASA Astrophysics Data System (ADS)
Kaur, Rupan Preet; Sawhney, Ravinder Singh; Engles, Derick
In this research paper, the effect of the material of electrodes at the nanometer scale was elucidated towards measuring the electron transport properties of a single molecular junction comprising of anthracenedithiol molecule (ADT) stringed to two semi-infinite metallic electrodes using Extended Huckle Theory (EHT)-based semi-empirical modelling approach. The electron transport parameters i.e., I-V curves, Conductance-Voltage curves and transmission spectrum were investigated through ADT molecule by buffering it between different electrodes composed of rhodium, palladium, nickel and copper, all from transition metals series, under finite bias voltages within Keldysh's non equilibrium green function formulism (NEGF). The simulated results revealed that the copper electrodes showed maximum conduction whereas palladium showed least. The maximum conductance of 0.82 G0 and 43 ?A current was exhibited by copper and thus affirmed to be the most effective electrode at nanometre scale when compared with other electrodes viz. nickel, rhodium and palladium.
Investigations into Transition Metal Catalyzed Arene Trifluoromethylation Reactions
Ye, Yingda; Sanford, Melanie S.
Trifluoromethyl-substituted arenes and heteroarenes are widely prevalent in pharmaceuticals and agrochemicals. As a result, the development of practical methods for the formation of aryl¢CF3 bonds has become an active field of research. Over the past five years, transition metal catalyzed cross-coupling between aryl¢X (X = halide, organometallic, or H) and various ōCF3ö reagents has emerged as a particularly exciting approach for generating aryl¢CF3 bonds. Despite many recent advances in this area, current methods generally suffer from limitations such as poor generality, harsh reaction conditions, the requirement for stoichiometric quantities of metals, and/or the use of costly CF3 sources. This Account describes our recent efforts to address some of these challenges by: (1) developing aryl trifluoromethylation reactions involving high oxidation state Pd intermediates, (2) exploiting AgCF3 for C¢H trifluoromethylation, and (3) achieving Cu-catalyzed trifluoromethylation with photogenerated CF3Ģ. PMID:25838638
The Holographic Disorder-Driven Supeconductor-Metal Transition
Arean, Daniel; Landea, Ignacio Salazar; Scardicchio, Antonello
We implement the effects of disorder on a holographic superconductor by introducing a random chemical potential on the boundary. We demonstrate explicitly that increasing disorder leads to the formation of islands where the superconducting order is enhanced and subsequently to the transition to a metal. We study the behavior of the superfluid density and of the conductivity as a function of the strength of disorder. We find explanations for various marked features in the conductivities in terms of hydrodynamic quasi-normal modes of the holographic superconductors. These identifications plus a particular disorder-dependent spectral weight shift in the conductivity point to a signature of the Higgs mode in the context of disordered holographic superconductors. We observe that the behavior of the order parameter close to the transition is not mean-field type as in the clean case, rather we find robust agreement with $\\exp(- A\\, |T-T_c|^{-\
VO2 films with strong semiconductor to metal phase transition prepared by the precursor oxidation process
Luryi, Serge
VO2 films with strong semiconductor to metal phase transition prepared by the precursor oxidation, the precursor oxidation process, for making VO2 films with strong semiconductor-to-metal phase transition. Sputter-deposited metal precursor V films were oxidized in situ in the deposition chamber for 2.5Ł7 h
Hybrid density functional calculations of redox potentials and formation energies of transition metal compounds
Ceder, Gerbrand
, and formation energies of transition metal compounds. Conventional DFT functionals significantly underesti- mate-Fock ex- change. For formation energies, HSE06 performs well for transition metal compounds, which the results of nontransi- tion metal oxides. Hence, we find that hybrid functionals provide a good alternative
Strain effects in low-dimensional transition metal oxides Jinbo Cao a,b,
Wu, Junqiao
Strain effects in low-dimensional transition metal oxides Jinbo Cao a,b, *, Junqiao Wu a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 1.1. SpinŁlatticeŁcharge coupling in transition metal oxides/nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3.2. Phase transitions and domain physics under strain
Boyer, Edmond
L-279 ON THE PRESSURE DEPENDENCE OF SUPERCONDUCTIVITY IN TRANSITION METAL DICHALCOGENIDE LAYER measurements of the pressure dependence of superconductivity in transition metal dichalco- genide layer to 10 kbar (Fig.1 a, b). FIG. 1. - Pressure dependence of the critical temperature 7. in transition
Metal-insulator transition by holographic charge density waves.
Ling, Yi; Niu, Chao; Wu, Jian-Pin; Xian, Zhuo-Yu; Zhang, Hongbao
We construct a gravity dual for charge density waves (CDWs) in which the translational symmetry along one spatial direction is spontaneously broken. Our linear perturbation calculation on the gravity side produces the frequency dependence of the optical conductivity, which exhibits the two familiar features of CDWs, namely, the pinned collective mode and gapped single-particle excitation. These two features indicate that our gravity dual also provides a new mechanism to implement the metal to insulator phase transition by CDWs, which is further confirmed by the fact that dc conductivity decreases with the decreased temperature below the critical temperature. PMID:25215974
Resonant electron-exchange excitations in transition-metal oxides
B. Fromme; M. M÷ller; C. Bethke; U. Brunokowski; E. Kisker
Electron-exchange processes in dipole-forbidden d-d excitations of the 3d transition-metal oxides NiO, CoO, and MnO have been investigated by spin-polarized electron-energy-loss spectroscopy over a wide primary energy range and for different scattering geometries. At primary energies corresponding to certain excitation thresholds, the d-d excitations in CoO and MnO show resonant enhancement, similar to the resonances previously found for NiO. The
IVb Transition Metal Oxides and Silicates: An Ab Initio Study
Gian-Marco Rignanese
Using density-functional theory, we investigate the structural, vibrational and dielectric properties of group IVb transition\\u000a metals (M=Hf, Zr, Ti) oxides and silicates which have drawn considerable attention as alternative high-k materials. For the\\u000a oxides, three crystalline phases of dioxide are considered. The first two are the cubic and tetragonal structures which exist\\u000a for hafnia HfO2 or zirconia ZrO2, while it
Ionically-mediated electromechanical hysteresis in transition metal oxides
Kim, Yunseok [ORNL] [ORNL; Kumar, Amit [ORNL] [ORNL; Jesse, Stephen [ORNL] [ORNL; Kalinin, Sergei V [ORNL] [ORNL
Electromechanical activity, remanent polarization states, and hysteresis loops in paraelectric TiO2 and SrTiO3 are observed. The coupling between the ionic dynamics and incipient ferroelectricity in these materials is analyzed using extended Ginsburg Landau Devonshire (GLD) theory. The possible origins of electromechanical coupling including ionic dynamics, surface-charge induced electrostriction, and ionically-induced ferroelectricity are identified. For the latter, the ionic contribution can change the sign of first order GLD expansion coefficient, rendering material effectively ferroelectric. These studies provide possible explanation for ferroelectric-like behavior in centrosymmetric transition metal oxides.
Transition metal complexes of bidentate Schiff base ligands
Mehmet T³mer; Cumali Ūelik; H³seyin K÷ksal; Selahattin Serin
New Schiff base ligands derived from vanillin (HL1), 4-dimethylaminobenzaldehyde (HL2) and 3,5-di-t-butyl-4-hydroxybenzaldehyde (HL3) with N-(pyridyl)-3-methoxy-4-hydroxy-5-aminobenzylamine (2) and their copper(II), cobalt(II), nickel(II), oxovanadium(IV) and zinc(II) transition metal complexes have been synthesized and characterized by elemental analyses, electronic and i.r. spectra, molar conductance data and by 1H and 13C n.m.r. spectra. The results indicate that the ligands coordinate through azomethine nitrogen and
Rare earth-transition metal scrap treatment method
Schmidt, F.A.; Peterson, D.T.; Wheelock, J.T.; Jones, L.L.; Lincoln, L.P.
Rare earth-transition metal (e.g. iron) scrap (e.g. Nd-Fe-B scrap) is melted to reduce the levels of tramp oxygen and nitrogen impurities therein. The tramp impurities are reduced in the melt by virtue of the reaction of the tramp impurities and the rare earth to form dross on the melt. The purified melt is separated from the dross for reuse. The oxygen and nitrogen of the melt are reduced to levels acceptable for reuse of the treated alloy in the manufacture of end-use articles, such as permanent magnets. 3 figs.
Metal-insulator transition in graphene on boron nitride.
Titov, M; Katsnelson, M I
Electrons in graphene aligned with hexagonal boron nitride are modeled by Dirac fermions in a correlated random-mass landscape subject to a scalar- and vector-potential disorder. We find that the system is insulating in the commensurate phase since the average mass deviates from zero. At the transition the mean mass is vanishing and electronic conduction in a finite sample can be described by a critical percolation along zero-mass lines. In this case graphene at the Dirac point is in a critical state with the conductivity sqrt[3]e(2)/h. In the incommensurate phase the system behaves as a symplectic metal. PMID:25216000
Correlated Electron Pseudopotentials for 3d-Transition Metals
Trail, John
A recently published correlated electron pseudopotentials (CEPPs) method has been adapted for application to the 3d-transition metals, and to include relativistic effects. New CEPPs are reported for the atoms Sc$-$Fe, constructed from atomic quantum chemical calculations that include an accurate description of correlated electrons. Dissociation energies, molecular geometries, and zero-point vibrational energies of small molecules are compared with all electron results, with all quantities evaluated using coupled cluster singles doubles and triples (CCSD(T)) calculations. The CEPPs give better results in the correlated-electron calculations than Hartree-Fock-based pseudopotentials available in the literature.
Thiosulfate oxidation: Catalysis of synthetic sphalerite doped with transition metals
NASA Astrophysics Data System (ADS)
Xu, Yong; Schoonen, Martin A. A.; Strongin, Daniel R.
The rate of thiosulfate oxidation to tetrathionate by dissolved molecular oxygen was measured in aqueous suspensions of synthetic sphalerites doped with first row transition metals (Mn, Fe, Co, Ni, Cu). Pure sphalerite and Mn-doped sphalerite show no catalytic activity for this reaction. Catalytic activity increases from Fe to Ni, which has maximum activity, with a slight decrease moving to Cu. The catalytic activity of the doped sphalerites is proportional to the concentration of dopant. This trend in catalytic activity from pure sphalerite to Cu-doped sphalerite can be explained using the band structures of these solids.
Negative ion photoelectron spectroscopy of bare transition metal dimers
NASA Astrophysics Data System (ADS)
Barker, Beau J.
This thesis contains gas phase negative ion photoelectron spectra of Mo2, MoV, CrCu, MoCu and Cu2. Spectra were taken with 488 nm and 514 nm light at a resolution of 4-5 meV. Information such as electron affinities, vibrational frequencies, anharmonicities and bond dissociation energies are reported for the ground and excited electronic states of both the anion and neutral species. Theoretical calculations at the density functional level are also reported for these species. Experiment and theory are used to analyze the bonding in these bare transition metal dimers.
Cross-plane thermal properties of transition metal dichalcogenides
Muratore, C. [Department of Chemical and Materials Engineering, University of Dayton, Dayton, Ohio 45469 (United States) [Department of Chemical and Materials Engineering, University of Dayton, Dayton, Ohio 45469 (United States); Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States); Varshney, V. [Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States) [Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States); Universal Technology Corporation, Dayton, Ohio 45432 (United States); Gengler, J. J. [Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States) [Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States); Spectral Energies LLC, Dayton, Ohio 45431 (United States); Hu, J. J.; Bultman, J. E. [Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States) [Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States); University of Dayton Research Institute, Dayton, Ohio 45469 (United States); Smith, T. M. [Department of Materials Science and Engineering, Ohio State University, Columbus, Ohio 43210 (United States)] [Department of Materials Science and Engineering, Ohio State University, Columbus, Ohio 43210 (United States); Shamberger, P. J.; Roy, A. K.; Voevodin, A. A. [Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States)] [Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433 (United States); Qiu, B.; Ruan, X. [Department of Mechanical Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907 (United States)] [Department of Mechanical Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907 (United States)
In this work, we explore the thermal properties of hexagonal transition metal dichalcogenide compounds with different average atomic masses but equivalent microstructures. Thermal conductivity values of sputtered thin films were compared to bulk crystals. The comparison revealed a >10 fold reduction in thin film thermal conductivity. Structural analysis of the films revealed a turbostratic structure with domain sizes on the order of 5-10 nm. Estimates of phonon scattering lengths at domain boundaries based on computationally derived group velocities were consistent with the observed film microstructure, and accounted for the reduction in thermal conductivity compared to values for bulk crystals.
Stability and thermal evolution of transition metal and silicon clusters
NASA Astrophysics Data System (ADS)
Polukhin, V. A.; Vatolin, N. A.
The results of computer simulation studies of transition metal and silicon clusters published in the last decade are summarized. Comparative analysis of the stability and thermal evolution of nanoclusters is performed depending on the preparation method, type of bonds, atom packing, coherence of the constituent nanofragments, surface morphology and change in the relationship between the short- and long-range ordering with increasing size. Taking account of the substrate nature and dimensionality of the cluster disperse systems being simulated, most important structure-dependent kinetic and mechanical characteristics are discussed, including specific temperature ranges of disordering corresponding to isomerization and quasi-melting. The bibliography includes 263 references.
Rare earth-transition metal scrap treatment method
Schmidt, Frederick A. (Ames, IA); Peterson, David T. (Ames, IA); Wheelock, John T. (Nevada, IA); Jones, Lawrence L. (Des Moines, IA); Lincoln, Lanny P. (Woodward, IA)
Rare earth-transition metal (e.g. iron) scrap (e.g. Nd-Fe-B scrap) is melted to reduce the levels of tramp oxygen and nitrogen impurities therein. The tramp impurities are reduced in the melt by virtue of the reaction of the tramp impurities and the rare earth to form dross on the melt. The purified melt is separated from the dross for reuse. The oxygen and nitrogen of the melt are reduced to levels acceptable for reuse of the treated alloy in the manufacture of end-use articles, such as permanent magnets.
Electrochromic displays with transition-metal oxide as counterelectrode
Katsumi Kuwabara; Shoichi Ichikawa; Kohzo Sugiyama
Amorphous WO3 electrochromic displays (ECD) with transition-metal oxides were investigated to determine fundamental factors for developing all-solid-state ECDs. The oxides examined were coO, NiO, SnO, Fe3O4, Co3O4, Ir2O3, VO2 and MnO2. Layers of solid electrolyte (tin phosphate) and each oxide were prepared by a spray method. The ECDs with VO2, Ir2O3 and MnO2 were found to give excellent characteristics such
Transition-metal-catalyzed oxidation of metallic Sn in NiO/SnO2 nanocomposite.
Hua, Chunxiu; Fang, Xiangpeng; Wang, Zhaoxiang; Chen, Liquan
It is well accepted that metallic tin as a discharge (reduction) product of SnO(x) cannot be electrochemically oxidized below 3.00?V versus Li(+)/Li(0) due to the high stability of Li2O, though a similar oxidation can usually occur for a transition metal formed from the corresponding oxide. In this work, nanosized Ni2 SnO4 and NiO/SnO2 nanocomposite were synthesized by coprecipitation reactions and subsequent heat treatment. Owing to the catalytic effect of nanosized metallic nickel, metallic tin can be electrochemically oxidized to SnO2 below 3.00?V. As a result, the reversible lithium-storage capacities of the nanocomposite reach 970?mAh?g(-1) or above, much higher than the theoretical capacity (ca. 750?mAh?g(-1)) of SnO2, NiO, or their composites. These findings extend the well-known electrochemical conversion reaction to non-transition-metal compounds and may have important applications, for example, in constructing high-capacity electrode materials and efficient catalysts. PMID:24648283
Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers.
Lin, Junhao; Cretu, Ovidiu; Zhou, Wu; Suenaga, Kazu; Prasai, Dhiraj; Bolotin, Kirill I; Cuong, Nguyen Thanh; Otani, Minoru; Okada, Susumu; Lupini, Andrew R; Idrobo, Juan-Carlos; Caudel, Dave; Burger, Arnold; Ghimire, Nirmal J; Yan, Jiaqiang; Mandrus, David G; Pennycook, Stephen J; Pantelides, Sokrates T
In the pursuit of ultrasmall electronic components, monolayer electronic devices have recently been fabricated using transition-metal dichalcogenides. Monolayers of these materials are semiconducting, but nanowires with stoichiometry MX (M = Mo or W, X = S or Se) have been predicted to be metallic. Such nanowires have been chemically synthesized. However, the controlled connection of individual nanowires to monolayers, an important step in creating a two-dimensional integrated circuit, has so far remained elusive. In this work, by steering a focused electron beam, we directly fabricate MX nanowires that are less than a nanometre in width and Y junctions that connect designated points within a transition-metal dichalcogenide monolayer. In situ electrical measurements demonstrate that these nanowires are metallic, so they may serve as interconnects in future flexible nanocircuits fabricated entirely from the same monolayer. Sequential atom-resolved Z-contrast images reveal that the nanowires rotate and flex continuously under momentum transfer from the electron beam, while maintaining their structural integrity. They therefore exhibit self-adaptive connections to the monolayer from which they are sculpted. We find that the nanowires remain conductive while undergoing severe mechanical deformations, thus showing promise for mechanically robust flexible electronics. Density functional theory calculations further confirm the metallicity of the nanowires and account for their beam-induced mechanical behaviour. These results show that direct patterning of one-dimensional conducting nanowires in two-dimensional semiconducting materials with nanometre precision is possible using electron-beam-based techniques. PMID:24776648
Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers
NASA Astrophysics Data System (ADS)
Lin, Junhao; Cretu, Ovidiu; Zhou, Wu; Suenaga, Kazu; Prasai, Dhiraj; Bolotin, Kirill I.; Cuong, Nguyen Thanh; Otani, Minoru; Okada, Susumu; Lupini, Andrew R.; Idrobo, Juan-Carlos; Caudel, Dave; Burger, Arnold; Ghimire, Nirmal J.; Yan, Jiaqiang; Mandrus, David G.; Pennycook, Stephen J.; Pantelides, Sokrates T.
In the pursuit of ultrasmall electronic components, monolayer electronic devices have recently been fabricated using transition-metal dichalcogenides. Monolayers of these materials are semiconducting, but nanowires with stoichiometry MX (M = Mo or W, X = S or Se) have been predicted to be metallic. Such nanowires have been chemically synthesized. However, the controlled connection of individual nanowires to monolayers, an important step in creating a two-dimensional integrated circuit, has so far remained elusive. In this work, by steering a focused electron beam, we directly fabricate MX nanowires that are less than a nanometre in width and Y junctions that connect designated points within a transition-metal dichalcogenide monolayer. In situ electrical measurements demonstrate that these nanowires are metallic, so they may serve as interconnects in future flexible nanocircuits fabricated entirely from the same monolayer. Sequential atom-resolved Z-contrast images reveal that the nanowires rotate and flex continuously under momentum transfer from the electron beam, while maintaining their structural integrity. They therefore exhibit self-adaptive connections to the monolayer from which they are sculpted. We find that the nanowires remain conductive while undergoing severe mechanical deformations, thus showing promise for mechanically robust flexible electronics. Density functional theory calculations further confirm the metallicity of the nanowires and account for their beam-induced mechanical behaviour. These results show that direct patterning of one-dimensional conducting nanowires in two-dimensional semiconducting materials with nanometre precision is possible using electron-beam-based techniques.
Metal-insulator transition in pyrochlore Eu2Ir2O7 studied by infrared spectroscopy
NASA Astrophysics Data System (ADS)
Sushkov, Andrei; Drew, Dennis; Ishikawa, Jun; Nakatsuji, Satoru; Luo, Xuan; Cheong, Sang-Wook
The large family of pyrochlores with formula A2B2C7 attracted a lot of early attention due to strong geometric magnetic frustration. Recent band structure calculations predict that the iridate pyrochlores A2Ir2O7 may have nontrivial topological states. We will report the results of an infrared spectroscopic study of the metal-insulator transition in Eu2Ir2O7 single crystal and Y2Ir2O7 polycrystal. We will report the broad band IR reflection as a function of temperature for an overview of the M-I transition and the low frequency transmission which is more sensitive for detection of a 10 meV gap and other possible excitations. We will discuss possible implications of the semimetal Weyl states. The large family of pyrochlores with formula A2B2C7 attracted a lot of early attention due to strong geometric magnetic frustration. Recent band structure calculations predict that the iridate pyrochlores A2Ir2O7 may have nontrivial topological states. We will report the results of an infrared spectroscopic study of the metal-insulator transition in Eu2Ir2O7 single crystal and Y2Ir2O7 polycrystal. We will report the broad band IR reflection as a function of temperature for an overview of the M-I transition and the low frequency transmission which is more sensitive for detection of a 10 meV gap and other possible excitations. We will discuss possible implications of the semimetal Weyl states. Work supported in part by NSF-MRSEC grant DMR-0520471
Characteristics of thermal decomposition products of rare earth, alkali earth metal and transition metal p -toluenesulfonates
Z.-L. Sun
Summary A series of alkali earth (Ca, Ba), lanthanide (La, Ce, Pr, Nd,) and transition metal (Fe, Mn, Co, Ni, Cu, Zn, Cd) p-toluenesulfonates were synthesized. Thermal analysis, infrared spectrophotometry, and XRD powder diffraction patterns were employed to characterize the final thermal decomposition products. By heating in dynamic air atmosphere at 30-850░C, it has been found that the alkali earth
Wannier function analysis of charge states in transition metal oxides
NASA Astrophysics Data System (ADS)
Quan, Yundi; Pickett, Warren
The charge (or oxidation) state of a cation has been a crucial concept in analyzing the electronic and magnetic properties of oxides as well as interpreting ``charge ordering'' metal-insulator transitions. In recent years a few methods have been proposed for the objective identification of charge states, beyond the conventional (and occasionally subjective) use of projected densities of states, weighted band structures (fatbands), and Born effective charges. In the past two decades Wannier functions (WFs) and particularly maximally localized WFs (MLWFs), have become an indispensable tool for several different purposes in electronic structure studies. These developments have motivated us to explore the charge state picture from the perspective of MLWFs. We will illustrate with a few transition metal oxide examples such as AgO and YNiO3 that the shape, extent, and location of the charge centers of the MLWFs provide insights into how cation-oxygen hybridization determines chemical bonding, charge distribution, and ``charge ordering.'' DOE DE-FG02-04ER46111.
Anderson metal-insulator transitions with classical magnetic impurities
NASA Astrophysics Data System (ADS)
Jung, Daniel; Kettemann, Stefan
We study the effects of classical magnetic impurities on the Anderson metal-insulator transition (AMIT) numerically. In particular we find that while a finite concentration of Ising impurities lowers the critical value of the site-diagonal disorder amplitude Wc, in the presence of Heisenberg impurities, Wc is first increased with increasing exchange coupling strength J due to time-reversal symmetry breaking. The resulting scaling with J is compared to analytical predictions by Wegner [1]. The results are obtained numerically, based on a finite-size scaling procedure for the typical density of states [2], which is the geometric average of the local density of states. The latter can efficiently be calculated using the kernel polynomial method [3]. Although still suffering from methodical shortcomings, our method proves to deliver results close to established results for the orthogonal symmetry class [4]. We extend previous approaches [5] by combining the KPM with a finite-size scaling analysis. We also discuss the relevance of our findings for systems like phosphor-doped silicon (Si:P), which are known to exhibit a quantum phase transition from metal to insulator driven by the interplay of both interaction and disorder, accompanied by the presence of a finite concentration of magnetic moments [6].
Stabilization of Small Boron Cage by Transition Metal Encapsulation
NASA Astrophysics Data System (ADS)
Zhang, Lijun; Lv, Jian; Wang, Yanchao; Ma, Yanming
The discovery of chemically stable fullerene-like structures formed by elements other than carbon has been long-standing desired. On this aspect significant efforts have centered around boron, only one electron deficient compared with carbon. However, during the past decade a large number of experimental and theoretical studies have established that small boron clusters are either planar/quasi-planar or forming double-ring tubular structures. Until recently, two all-boron fullerenes have been independently discovered: B38 proposed by our structure searching calculations and B40 observed in a joint experimental and theoretical study. Here we extend our work to the even smaller boron clusters and propose an effective routine to stabilize them by transition metal encapsulation. By combining swarm-intelligence structure searching and first-principles calculations, we have systematically investigated the energy landscapes of transition-metal-doped MB24 clusters (M = Ti, Zr, Hf, Cr, Mo, W, Fe, Ru and Os). Two stable symmetric endohedral boron cages, MoB24 and WB24 are identified. The stability of them can be rationalized in terms of their unique 18-electron closed-shell electronic structures. Funded by Recruitment Program of Global Experts of China and China Postdoctoral Science Foundation.
Metal-Insulator Transitions in Crystalline Phase Change Materials
NASA Astrophysics Data System (ADS)
Zhang, Wei; Thiess, Alexander; Zalden, Peter; Zeller, Rudolf; Dederichs, Peter; Raty, Jean-Yves; Wuttig, Matthias; Bl³gel, Stefan; Mazzarello, Riccardo
Phase-change materials are capable of undergoing fast and reversible transitions between amorphous and crystalline phase upon heating and have been exploited in data storage applications based on the strong optical/electrical contrast between the two phases. Recently, compelling evidence for a metal-insulator transition (MIT) solely due to disorder has been observed in the crystalline PCM Ge1Sb2Te4 (GST) and similar compounds: upon annealing at temperatures T below 548K, the system exhibits insulating behavior due to Anderson localization; at higher T, it shows metallic behavior. In contrast to the MITs observed in other systems such as P-doped Si, in GST correlation effects do not play a role and the MIT occurs at fixed stoichiometry. In this work, we present a Density Functional Theory study of this effect. We consider a set of very large models of GST containing one to several thousand atoms and different degree of disorder. We identify the microscopic mechanism that localizes the electron wavefunctions near the Fermi energy in the insulating phase: these states are localized inside regions having large vacancy consequent dissolution of these vacancy clusters. These results could help to develop new device based on multiple resistance states.
Superconductivity Series of Ion-gated Transition Metal Dichalcogenides
NASA Astrophysics Data System (ADS)
Shi, Wu; Ye, Jianting; Zhang, Yijing; Suzuki, Ryuji; Yoshida, Masaro; Inoue, Naoko; Saito, Yu; Iwasa, Yoshihiro
Semiconducting transition metal dichalcogenides (TMDs) have attracted considerable interest as typical two-dimensional (2D) materials. By mechanical cleavage, atomically flat and chemically stable thin flakes of TMDs can be readily obtained from bulk crystals. Recently, coupling with high efficient ionic media, TMD thin flakes have exhibited extraordinary electronic and opto-valleytronic properties in the form of electrical double layer transistors (EDLTs). The introduction of high-density carriers have also induced metal-insulator transition and superconductivity in MoS2, revealing an enhanced Tc and a dome-like phase diagram that are inaccessible through conventional chemically doping. In this work, we report the discovery of a superconductivity series based on a further exploration of other available semiconducting TMDs (MoSe2, MoTe2, WS2, WSe2) by using different ionic media. The present results not only complement important superconducting properties in TMDs, but also suggest a close correlation between transistor operation and the possibility of inducing superconductivity, providing general guidelines for the optimization of charge accumulation and the induction of superconductivity in other material series via ionic gating. Present address: University of Groningen.
Potential Energy Functions for Transition Metals and Their Aluminides
NASA Astrophysics Data System (ADS)
Zou, Jun
Transition metal aluminides have a wide range of potential uses in applications requiring high strength and light weight. Obtaining accurate potential energy functions is the first step in trying to understand the structural and defect properties of these materials. Using the simple Anderson model and ab-initio results as inputs, we are able to derive strong medium- and long-ranged interactions among the transition metal atoms in aluminides. Treating the pseudopotentials as a perturbation, we obtain a complete set of pair potentials for any pair of atoms in the aluminides. These potentials agree well with experiments on the structure of liquid aluminides, predict the correct chemical trends in structural stability, and in most cases reproduce the large structural energy differences that have been demonstrated by ab-initio calculations. The calculated results on the structural energies agree fairly well with the experimental phase diagram and the existing ab-initio results. On the basis of these potentials, we propose an explanation for quasicrystal formation in the aluminides. Four-body angular forces recently developed by Carlsson are used in the molecular dynamics simulation of liquid W to examine the effect of angular forces on the structure and thermodynamic properties of the liquid. We do not find any significant effect of the angular forces as compared with radial interatomic forces. However, the angular forces are much more accurate overall in treating the thermodynamic properties.
Altered transition metal homeostasis in the cuprizone model of demyelination.
Moldovan, Nataliya; Al-Ebraheem, Alia; Lobo, Lianne; Park, Raina; Farquharson, Michael J; Bock, Nicholas A
In the cuprizone model of demyelination, the neurotoxin cuprizone is fed to mice to induce a reproducible pattern of demyelination in the brain. Cuprizone is a copper chelator and it has been hypothesized that it induces a copper deficiency in the brain, which leads to demyelination. To test this hypothesis and investigate the possible role of other transition metals in the model, we fed C57Bl/6 mice a standard dose of cuprizone (0.2% dry chemical to dry food weight) for 6 weeks then measured levels of copper, manganese, iron, and zinc in regions of the brain and visceral organs. As expected, this treatment induced demyelination in the mice. We found, however, that while the treatment significantly reduced copper concentrations in the blood and liver in treated animals, there was no significant difference in concentrations in brain regions relative to control. Interestingly, cuprizone disrupted concentrations of the other transition metals in the visceral organs, with the most notable changes being decreased manganese and increased iron in the liver. In the brain, manganese concentrations were also significantly reduced in the cerebellum and striatum. These data suggest a possible role of manganese deficiency in the brain in the cuprizone model. PMID:25749275
Anderson metal-insulator transitions with classical magnetic impurities
Jung, Daniel [School of Engineering and Science, Jacobs University Bremen gGmbH, Campus Ring 1, 28759 Bremen, Germany and Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-dong, Nam-gu, Pohang 790-784 (Korea, Republic of); Kettemann, Stefan [School of Engineering and Science, Jacobs University Bremen gGmbH,Campus Ring 1, 28759 Bremen, Germany and Division of Advanced Materials Science, Pohang University of Science and Technology (POSTECH), San 31, Hyoja-dong, Nam-gu, Pohang 790-784 (Korea, Republic of)
We study the effects of classical magnetic impurities on the Anderson metal-insulator transition (AMIT) numerically. In particular we find that while a finite concentration of Ising impurities lowers the critical value of the site-diagonal disorder amplitude W{sub c}, in the presence of Heisenberg impurities, W{sub c} is first increased with increasing exchange coupling strength J due to time-reversal symmetry breaking. The resulting scaling with J is compared to analytical predictions by Wegner [1]. The results are obtained numerically, based on a finite-size scaling procedure for the typical density of states [2], which is the geometric average of the local density of states. The latter can efficiently be calculated using the kernel polynomial method [3]. Although still suffering from methodical shortcomings, our method proves to deliver results close to established results for the orthogonal symmetry class [4]. We extend previous approaches [5] by combining the KPM with a finite-size scaling analysis. We also discuss the relevance of our findings for systems like phosphor-doped silicon (Si:P), which are known to exhibit a quantum phase transition from metal to insulator driven by the interplay of both interaction and disorder, accompanied by the presence of a finite concentration of magnetic moments [6].
A theory of the pressure-induced high-spin-low-spin transition of transition-metal oxides
Shuhei Ohnishi
The pressure-induced high-spin-low-spin transition of the transition-metal ions in octahedral coordination is studied theoretically. The relation between the crossover point and the transition point is discussed and the formula to determine the transition point is given in terms of the crystal-field splitting Delta and the spin-pairing energy Pi asDelta = alphaPi. The numerical coefficient alpha is determined by the ratio
The 1s x-ray absorption pre-edge structures in transition metal oxides
We develop a general procedure to analyse the pre-edges in 1s x-ray absorption near edge structure (XANES) of transition metal oxides and coordination complexes. Transition metal coordination complexes can be described from a local model with one metal ion. The 1s 3d quadrupole transitions are calculated with the charge-transfer multiplet program. Tetrahedral coordination complexes have more intense pre-edge structures due
The 1s x-ray absorption pre-edge structures in transition metal oxides
Frank de Groot; Gy÷rgy Vank¾; Pieter Glatzel
We develop a general procedure to analyse the pre-edges in 1s x-ray absorption near edge structure (XANES) of transition metal oxides and coordination complexes. Transition metal coordination complexes can be described from a local model with one metal ion. The 1s 3d quadrupole transitions are calculated with the charge-transfer multiplet program. Tetrahedral coordination complexes have more intense pre-edge structures due
Transition metal-catalyzed oxidation of sulfur(IV) oxides. Atmospheric-relevant processes and mechanisms
Christian. Brandt; Rudi. van Eldik
The transition metal-catalyzed oxidation of sulfur(IV) oxides has been known for more than 100 years. There is a significant lack of information on the actual role of the transition metal-catalyzed reactions, and much of the earlier work was performed without a detailed knowledge of the chemical system. For this reason attention is focused on the role of transition metal ions
Charge-transfer gap closure in transition-metal halides under pressure
Chen, A.L.; Yu, P.Y.
Insulator-to-metal transition induced by pressure has been studied in three transition metal iodides: NiI{sub 2}, CoI{sub 2} and FeI{sub 2} using optical absorption and resistivity measurements at room temperature. Comparisons between the results obtained by these two techniques suggested that the closure of the charge-transfer gap is the principal mechanism responsible for the insulator-to-metal transition in these materials.
XRD and XPS characterisation of transition metal silicide thin films
NASA Astrophysics Data System (ADS)
Tam, P. L.; Cao, Y.; Nyborg, L.
Binary transition metal silicides based on the systems Ti-Si, Fe-Si, Ni-Si and Cr-Si were fabricated on Si wafers by means of ion-beam co-sputter deposition and subsequent annealing. The crystalline structures of the phases formed were identified from the characteristic patterns acquired by means of X-ray diffraction (XRD) measurements. The phase formation sequences were described by means of the Pretorius' effective heat of formation (EHF) model. For the Ti-Si, Fe-Si and Ni-Si systems, single phase thin films of TiSi2, ?-FeSi2 and NiSi2 were generated as the model predicts, while a mixture of CrSi + CrSi2 phases was obtained for the Cr-Si system. The surface chemical condition of individual specimens was analysed by using X-ray photoelectron spectroscopy (XPS). The chemical shifts of transition metal 2p3/2 peaks from their metallic to silicide states were depicted by means of the Auger parameters and the Wagner plots. The positive chemical shift of 2.0 eV for Ni 2p3/2 peak of NiSi2 is mainly governed by the initial-state effects. For the other silicide specimens, the initial-state and final-state effects may oppose one another with similar impact. Consequently, smaller binding energy shifts of both negative and positive character are noted; a positive binding energy shift of 0.3 eV for the Fe 2p3/2 level was shown for ?-FeSi2 and negative binding energy shifts of 0.1 and 0.3 eV were determined for CrSi + CrSi2 and TiSi2, respectively.
Electrochemical lithiation and delithiation for control of magnetic properties of nanoscale transition metal oxides
Sivakumar, Vikram
Transition metal oxides comprise a fascinating class of materials displaying a variety of magnetic and electronic properties, ranging from half-metallic ferromagnets like CrO2, ferrimagnetic semiconductors like Fey's, and ...
Quantum Spin Liquids and the Metal-Insulator Transition in Doped Semiconductors
Potter, Andrew Cole
We describe a new possible route to the metal-insulator transition in doped semiconductors such as Si:P or Si:B. We explore the possibility that the loss of metallic transport occurs through Mott localization of electrons ...
K-CO on transition metals: A local ionic interaction
NASA Astrophysics Data System (ADS)
Patterson, Charles H.; Schultz, Peter A.; Messmer, Richard P.
The nature of the K-CO interaction on a transition metal surface is addressed when the K:CO stoichiometry is approx. 1. The interaction proposed is transfer of the K 4s electron to CO. A BORN-Haber cycle for this process on a surface has been calculated. The reference state is K plus CO, bound separately to the surface (a). A key point in the cycle is that removal of the electron from K bound to a metal surface (b) is less costly in energy (approx. metal/K surface, i.e., approx. +2.0 eV) than from K in the gas phase (IP = +4.3 eV). This means that the complex is significantly stabilized on the surface. The removed K electron is then transported to the isolated adsorbed CO (c) at a cost of the electron affinity of CO (approx. = 1.5 eV). When the isolated ions are brought together (d) the net stabilization at the equilibrium geometry is approx. - 1.8 eV.
Metal-insulator transition in a switchable mirror
NASA Astrophysics Data System (ADS)
Roy, Arunabha Shasanka
Rare earth hydride films can be converted reversibly from metallic mirrors to insulating windows simply by changing the surrounding hydrogen gas pressure at room temperature. At low temperatures, in situ doping is not possible in this way as hydrogen cannot diffuse. However, our finding of persistent photoconductivity under ultraviolet illumination enables us to tune yttrium hydride through the T = 0 metal-insulator transition. Conductivity and Hall measurements are used to determine the critical exponents in this system: mu = 1.0 +/- 0.1 and zv = 6.0 +/- 0.5. The simultaneous validity of finite-size scaling in the metal and its failure in the insulator motivates a study of the insulating phase of YHx in greater detail. A new transport regime is discovered which is consistent with our earlier determined values for the critical exponents but entails a new scaling dependence of the conductivity on the carrier density n and temperature T. The unusually large value for the product of the static and dynamic critical exponents appears to signify the important role played by electron-electron interactions in this system. Finally, possible schemes for the mechanism underlying the unusual photoconductivity are discussed.
The Timing of School Transitions and Early Adolescent Problem Behavior
Lippold, Melissa A.; Powers, Christopher J.; Syvertsen, Amy K.; Feinberg, Mark E.; Greenberg, Mark T.
This longitudinal study investigates whether rural adolescents who transition to a new school in sixth grade have higher levels of risky behavior than adolescents who transition in seventh grade. Our findings indicate that later school transitions had little effect on problem behavior between sixth and ninth grades. Cross-sectional analyses found a small number of temporary effects of transition timing on problem behavior: Spending an additional year in elementary school was associated with higher levels of deviant behavior in the Fall of Grade 6 and higher levels of antisocial peer associations in Grade 8. However, transition effects were not consistent across waves and latent growth curve models found no effects of transition timing on the trajectory of problem behavior. We discuss policy implications and compare our findings with other research on transition timing. PMID:24089584
Synthesis, characterization and formation process of transition metal oxide nanotubes using carbon nanofibers as templates
Ogihara, Hitoshi, E-mail: firstname.lastname@example.com [Catalysis Research Center, Hokkaido University, N21-W10 Kita-ku, Sapporo 001-002 (Japan); Japan Society for the Promotion of Science, Chiyoda-ku, Tokyo 102-8472 (Japan); Masahiro, Sadakane [Catalysis Research Center, Hokkaido University, N21-W10 Kita-ku, Sapporo 001-002 (Japan); Nodasaka, Yoshinobu [Oral Functional Science, Graduate School of Dental Medicine, Hokkaido University, N13-W7 Kita-ku, Sapporo 060-8586 (Japan); Ueda, Wataru, E-mail: email@example.com [Catalysis Research Center, Hokkaido University, N21-W10 Kita-ku, Sapporo 001-002 (Japan)
Mono and binary transition metal oxide nanotubes could be synthesized by the immersion of carbon nanofiber templates into metal nitrate solutions and removal of the templates by heat treatment in air. The transition metal oxide nanotubes were composed of nano-crystallites of metal oxides. The functional groups on the carbon nanofiber templates were essential for the coating of these templates: they acted as adsorption sites for the metal nitrates, ensuring a uniform metal oxide coating. During the removal of the carbon nanofiber templates by calcination in air, the metal oxide coatings promoted the combustion reaction between the carbon nanofibers and oxygen. - Graphical abstract: Mono and binary transition metal-oxide nanotubes could be synthesized by the immersion of carbon nanofiber templates into metal nitrate solutions and removal of the templates by heat treatment in air.
Transition metal chalcogenides: ultrathin inorganic materials with tunable electronic properties.
Heine, Thomas
CONSPECTUS: After the discovery of graphene and the development of powerful exfoliation techniques, experimental preparation of two-dimensional (2D) crystals can be expected for any layered material that is known to chemistry. Besides graphene and hexagonal boron nitride (h-BN), transition metal chalcogenides (TMC) are among the most studied ultrathin materials. In particular, single-layer MoS2, a direct band gap semiconductor with ?1.9 eV energy gap, is popular in physics and nanoelectronics, because it nicely complements semimetallic graphene and insulating h-BN monolayer as a construction component for flexible 2D electronics and because it was already successfully applied in the laboratory as basis material for transistors and other electronic and optoelectronic devices. Two-dimensional crystals are subject to significant quantum confinement: compared with their parent layered 3D material, they show different structural, electronic, and optical properties, such as spontaneous rippling as free-standing monolayer, significant changes of the electronic band structure, giant spin-orbit splitting, and enhanced photoluminescence. Most of those properties are intrinsic for the monolayer and already absent for two-layer stacks of the same 2D crystal. For example, single-layer MoS2 is a direct band gap semiconductor with spin-orbit splitting of 150 meV in the valence band, while the bilayer of the same material is an indirect band gap semiconductor without observable spin-orbit splitting. All these properties have been observed experimentally and are in excellent agreement with calculations based on density-functional theory. This Account reports theoretical studies of a subgroup of transition metal dichalcogenides with the composition MX2, with M = Mo, or W and X = Se or S, also referred to as "MoWSeS materials". Results on the electronic structure, quantum confinement, spin-orbit coupling, spontaneous monolayer rippling, and change of electronic properties in the presence of an external electric field are reported. While all materials of the MoWSeS family share the same qualitative properties, their individual values can differ strongly, for example, the spin-orbit splitting in WSe2 reaches the value of 428 meV, nearly three times that of MoS2. Further, we discuss the effect of strain on the electronic properties (straintronics). While MoWSeS single layers are very robust against external electric fields, bilayers show a linear reduction of the band gap, even reaching a semiconductor-metal phase transition, and an increase of the spin-orbit splitting from zero to the monolayer value at rather small fields. Strain is yet another possibility to control the band gap in a linear way, and MoWSeS monolayers become metallic at strain values of ?10%. The density-functional based tight-binding model is a useful tool to investigate the electronic and structural properties, including electron conductance, of large MoS2 structures, which show spontaneous rippling in finite-temperature molecular dynamics simulations. Structural defects in MoS2 result in anisotropy of the electric conductivity. Finally, DFT predictions on the properties of noble metal dichalcogenides are presented. Most strikingly, 1T PdS2 is an indirect band gap semiconductor in its monolayer form but becomes metallic as a bilayer. PMID:25489917
Comparative reactivities of two isoelectronic transition-metal hydrides with transition-metal carbonyls and alkyls
Jones, W.D.; Huggins, J.M.; Bergman, R.G.
The two isoelectronic hydrides PPN/sup +/CpV(CO)/sub 3/H/sup -/ (1) and -CpMo(CO)/sub 3/H (2) react with a variety of metal carbonyls and alkyls. Treatment of Fe(CO)/sub 5/, Cr(CO)/sub 6/, (CH/sub 3/C/sub 5/H/sub 4/)V(CO)/sub 4/, CH/sub 3/Re(CO)/sub 5/, and (CH/sub 3/CO)Re(CO)/sub 5/ with 1 produces HFe(CO)/sub 4//sup -/, HCr(CO)/sub 5//sup -/, (CH/sub 3/C/sub 5/H/sub 4/)V(CO)/sub 3/H/sup -/, (H)(CH/sub 3/)Re(CO)/sub 4//sup -/, and (H)(CH/sub 3/CO)Re(CO)/sub 4//sup -/, respectively, and CpV(CO)/sub 4/ (3). l also catalyzes ligand substitution reactions in 3 and CpFe(CO)(PPh/sub 3/)(COCH/sub 3/). In comparison 2 reacts only with CH/sub 3/Mn(CO)/sub 5/ and CpMo(CO)/sub 3/R(R = CH/sub 3/, C/sub 2/H/sub 5/, CH/sub 2/C/sub 6/H/sub 5/) producing aldehydes and the dimers (CpMo(CO)/sub 3/)/sub 2/ (4a) and (CpMo(CO)/sub 2/)/sub 2/ (5a). Reaction of 2 with ethylene produces ethane and diethyl ketone. l is proposed to react by an electron-transfer mechanism, whereas 2 is proposed to react by hydrogen transfer to a vacant coordination site. The relationship of the molybdenum hydride/alkyl reaction to the final step in hydroformylation (oxo process) is discussed.
Multiplet structures of metal L(2,3) absorption spectra of ionic 3d transition-metal compounds
NASA Astrophysics Data System (ADS)
Ohno, Y.; Nakai, S.-I.
The results obtained by a study of the L(2,3) absorption spectra of metal ions in layered 3d transition metal thiophosphates of general composition MPS3, where M = Mn, Fe, Ni, are compared both with other transition metal compound spectra and with the theoretical calculation results of Gupta and Sen (1974, 1975). The spectra are interpreted in terms of the multiplet structure generated by the interaction of the 2p hole and 3d electrons of a localized metal ion. It is concluded that the near-edge structure of the metal L(2,3) absorption spectrum for an ionic 3d transition metal compound exhibits the multiplet structure, while that of a covalent compound represents the projected density of states in the conduction band.
Dense organic-inorganic framework materials containing transition metal ions
NASA Astrophysics Data System (ADS)
Feller, Russell Kenneth
Hybrid inorganic-organic framework materials built upon metal cations and polyfunctional anionic ligands have arrived at the forefront of chemical research, boasting unique properties that derive from their diverse structures. The majority of these compounds are prepared through mild hydrothermal or solvothermal synthesis, at temperatures below 225░C. There has been a great deal of emphasis on porous coordination polymers, or "metal-organic frameworks" (MOFs), which exhibit a wide range of useful sorption and catalytic properties, but there is growing interest in a second class of hybrid frameworks which are denser and often have extended inorganic connectivity, which may be thought of as hybrid metal oxides. These denser compounds provide access to more oxide-like properties such as magnetism, along with improved thermal stability. The flourishing diversity of structures and dimensionalities seen in these materials has led to their growth into a major field of research. Here, the hydrothermal syntheses and crystal structures of fourteen new hybrid framework materials are reported, and additional properties are measured for several of them. The terminology used to describe this class of materials is laid out, and a classification scheme is introduced based on the dimensionalities of their structures. While a combinatorial approach for new compound discovery is often used, strategies for rational synthesis are also discussed. A family of isostructural 3,4,5-trihydroxybenzoate (gallate) hybrid frameworks of the first-row transition metals is produced, with chiral structures imparted by helical packing arrangements. Complex magnetic behavior is characterized in a pillared layered cobalt ethanedisulfonate, the structure of which features intricate metal-oxygen-metal connectivity. Frameworks of new, sometimes unreported, ligand molecules are created through the reactivities of aspartic acid and 5-hydroxyisophthalic acid under hydrothermal conditions. A family of two-ligand frameworks are created using 5-hydroxyisophthalic acid and 4,4'-bipyridyl, which form very different structures when reacted with Mn, Ni, Cu and Zn. Finally, reactions of Cu and Zn together with these two ligands lead to a family of four- and five-component hybrid frameworks showing even greater degrees of structural complexity.
Early transitional metal alkyl, alkylidene, and alkylidyne chemistry
Tonzetich, Zachary John
CHAPTER 1. Zirconium and hafnium complexes of several new unsymmetric diamide ligands have been prepared and their proficiency in olefin polymerization reactions evaluated. The first set of supporting ligands examined are ...
Electronic structure studies of materials properties and stability in transition metal-metalloid compounds
NASA Astrophysics Data System (ADS)
Hart, Gus Lewis Warren
Using first-principles electronic-structure calculations (i.e., the linearized-augmented-planewave method), we have studied the materials properties and stability of a number of transition metal-metalloid compounds. The classes of systems examined include: (1) B1-structure (NaCl-type) compounds of early transition-metal carbides and nitrides; (2) several platinum and iridium silicides; and (3) some a spinel-structure cuprous sulfide, CuIr2S4. 1. The nitrides and carbides we studied have remarkable physical and chemical properties, including extraordinary hardness and strength, high melting temperatures, superconductivity, and superior chemical stability. Attempts at systematic improvements of the remarkable properties of these materials are often frustrated by structural instabilities driven by elastic or phonon instabilities, as well as intrinsic non-stoichiometry and the formation of vacancies and defect structures. The instability of MoN at high pressures is demonstrated, in contrast to conjectures in the literature concerning its metastability. We also find a dynamic instability (zone boundary phonon) in MoC despite its elastic stability. 2. Platinum and iridium silicide compounds have been used in infrared detectors for a number of years. The optical properties of these compounds are explored, and suggestions for increasing their effectiveness are made based on theoretical predictions of anisotropic absorption and structurally dependent absorption in the infrared region. The Fermi surfaces and their relationship to the observed optical properties are discussed. An extensive study of the equilibrium properties of these Pt/Ir-Si compounds is also presented. 3. Spinel-structure chalcogenide compounds have a wide array of interesting and unusual properties. We have studied the as-yet-not-understood metal-insulator transition (MIT) in CuIr2S 4. First-principles calculations have failed to reproduce the insulating state. Possible explanations are explored including relativistic effects (spin-orbit interactions), relaxations of internal coordinates, and LDA errors.
Radiation damage of transition metal carbides. Final technical report
Dixon, G.
In this grant period we have investigated electrical properties of transition metal carbides and radiation-induced defects produced by low-temperature electron irradiation in them. Special attention has been given to the composition VC{sub 0.88} in which the vacancies on the carbon sublattice of this fcc crystal order to produce a V{sub 8}C{sub 7} superlattice. The existence of this superlattice structure was found to make the crystal somewhat resistant to radiation damage at low doses and/or at ambient temperature. At larger doses significant changes in the resistivity are produced. Annealing effects were observed which we believe to be connected with the reconstitution of the superlattice structure.
Tailoring Nanoscale Friction in MX2 Transition Metal Dichalcogenides.
Cammarata, Antonio; Polcar, TomßÜ
Lattice dynamics of MX2 transition metal dichalcogenides (M = Mo, W; X = S, Se, Te) have been studied with density functional theory techniques to control the macroscopic tribological behavior. Long-range van der Waals forces have been modeled with Grimme correction to capture the interlayer interactions. A new lattice dynamic metric, named cophonicity, is proposed and used in combination with electronic and geometric descriptors to relate the stability of the lattice distortions with the electro-structural features of the system. The cophonicity analysis shows that the distortion modes relevant to the microscopic friction can be controlled by tuning the relative M/X atomic contributions to the phonon density of states. Guidelines on how to engineer macroscopic friction at nanoscale are formulated, and finally applied to design a new Ti-doped MoS2 phase with enhanced tribologic properties. PMID:26000720
Semiconductor-metal transition of titanium sesquioxide nanopowder
NASA Astrophysics Data System (ADS)
Yan, Weiwei; Fang, Ming; Liu, Mao; Kang, Shenghong; Wang, Ruining; Zhang, Lide; Liu, Ling
Titanium sesquioxide (Ti2O3) nanopowders have been successfully synthesized using TiCl4 assisted hydrogen reduction method from P25 (TiO2) powders at 970 ░C. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy reveal its high purity. The temperature dependent XRD, resistance, and infrared absorption investigations exhibit Z-shaped curves (cell parameter a, resistance, and infrared transmittance) indicating the semiconductor-metal transition (SMT) in the range of 135-220 ░C. With prolonging the annealing time, the starting temperature of SMT is found to move toward the low-temperature side, and this was also discussed. The c/a ratio alteration is considered to result in the modulation of property. Besides, the results are verified comparing with the theoretical calculation of band structure.
Intrinsic Disorder in Graphene on Transition Metal Dichalcogenide Heterostructures
NASA Astrophysics Data System (ADS)
Yankowitz, Matthew; Larentis, Stefano; Kim, Kyounghwan; Xue, Jiamin; McKenzie, Devin; Huang, Shengqiang; Paggen, Marina; Ali, Mazhar N.; Cava, Robert J.; Tutuc, Emanuel; LeRoy, Brian J.
The electronic properties of two-dimensional materials such as graphene are extremely sensitive to their environment, especially the underlying substrate. Planar van der Waals bonded substrates such as hexagonal boron nitride (hBN) have been shown to greatly improve the electrical performance of graphene devices by reducing topographic variations and charge fluctuations compared to amorphous insulating substrates}. Semiconducting transition metal dichalchogenides (TMDs) are another family of van der Waals bonded materials that have recently received interest as alternative substrates to hBN for graphene as well as for components in novel graphene-based device heterostructures. Additionally, their semiconducting nature permits dynamic gate voltage control over the interaction strength with graphene. Through local scanning probe measurements we find that crystalline defects intrinsic to TMDs induce scattering in graphene which results in significant degradation of the heterostructure quality, particularly compared to similar graphene on hBN devices.
Transition-metal profiles in a multicrystalline silicon ingot
NASA Astrophysics Data System (ADS)
Macdonald, Daniel; Cuevas, Andrķs; Kinomura, A.; Nakano, Y.; Geerligs, L. J.
The concentrations of transition-metal impurities in a photovoltaic-grade multicrystalline silicon ingot have been measured by neutron activation analysis. The results show that the concentrations of Fe, Co, and Cu are determined by segregation from the liquid-to-solid phase in the central regions of the ingot. This produces high concentrations near the top of the ingot, which subsequently diffuse back into the ingot during cooling. The extent of this back diffusion is shown to correlate to the diffusivity of the impurities. Near the bottom, the concentrations are higher again due to solid-state diffusion from the crucible after crystallization has occurred. Measurement of the interstitial Fe concentration along the ingot shows that the vast majority of the Fe is precipitated during ingot growth. Further analysis suggests that this precipitation occurs mostly through segregation to extrinsic defects at high temperature rather than through solubility-limit-driven precipitation during ingot cooling.
Bifunctional transition metal-based molecular catalysts for asymmetric syntheses.
Ikariya, Takao; Murata, Kunihiko; Noyori, Ryoji
The discovery and development of conceptually new chiral bifunctional transition metal-based catalysts for asymmetric reactions is described. The chiral bifunctional Ru catalyst was originally developed for asymmetric transfer hydrogenation of ketones and imines and is now successfully applicable to enantioselective C-C bond formation reaction with a wide scope and high practicability. The deprotonation of 1,3-dicarbonyl compounds with the chiral amido Ru complexes leading to the amine Ru complexes bearing C- or O-bonded enolates, followed by further reactions with electrophlies gives C-C bond formation products. The present bifunctional Ru catalyst offers a great opportunity to open up new fundamentals for stereoselective molecular transformation including enantioselective C-H and C-C as well as C-O, C-N bond formation. PMID:16446796
Transition Metal Dichalcogenide Growth via Close Proximity Precursor Supply
O'Brien, Maria; McEvoy, Niall; Hallam, Toby; Kim, Hye-Young; Berner, Nina C.; Hanlon, Damien; Lee, Kangho; Coleman, Jonathan N.; Duesberg, Georg S.
Reliable chemical vapour deposition (CVD) of transition metal dichalcogenides (TMDs) is currently a highly pressing research field, as numerous potential applications rely on the production of high quality films on a macroscopic scale. Here, we show the use of liquid phase exfoliated nanosheets and patterned sputter deposited layers as solid precursors for chemical vapour deposition. TMD monolayers were realized using a close proximity precursor supply in a CVD microreactor setup. A model describing the growth mechanism, which is capable of producing TMD monolayers on arbitrary substrates, is presented. Raman spectroscopy, photoluminescence, X-ray photoelectron spectroscopy, atomic force microscopy, transmission electron microscopy, scanning electron microscopy and electrical transport measurements reveal the high quality of the TMD samples produced. Furthermore, through patterning of the precursor supply, we achieve patterned growth of monolayer TMDs in defined locations, which could be adapted for the facile production of electronic device components. PMID:25487822
Intrinsic disorder in graphene on transition metal dichalcogenide heterostructures
NASA Astrophysics Data System (ADS)
Yankowitz, Matthew; Larentis, Stefano; Kim, Kyounghwam; Xue, Jiamin; McKenzie, Devin; Huang, Shengqiang; Paggen, Marina; Ali, Mazhar; Cava, Robert; Tutuc, Emanuel; Leroy, Brian J.
Recently, semiconducting materials in the transition metal dichalcogenide (TMD) family have gained great popularity for use in novel graphene-based heterostructure devices such as tunneling transistors, highly efficient flexible photovoltaic devices, and nonvolatile memory cells. TMDs have also been explored as alternatives to hexagonal boron nitride (hBN) as substrates for pristine graphene devices. However, their quality has thus far been significantly worse than comparable hBN devices. We examine graphene on numerous TMD substrates (MoS2, WS2, WSe2, MoTe2) with scanning tunneling microscopy and spectroscopy and find that point and line defects intrinsic to all TMD crystals (both of natural and synthetic origin) result in scattering of electrons in graphene. Our findings suggest that the quality of graphene on TMD heterostructures is limited by the intrinsic crystalline quality of the TMDs.
Phase stability and electronic structure of transition-metal aluminides
Carlsson, A.E.
This paper will describe the interplay between die electronic structure and structural energetics in simple, complex, and quasicrystalline Al-transition metal (T) intermetallics. The first example is the Ll{sub 2}-DO{sub 22} competition in Al{sub 3}T compounds. Ab-initio electronic total-energy calculations reveal surprisingly large structural-energy differences, and show that the phase stability of both stoichiometric and ternary-substituted compounds correlates closely with a quasigap in the electronic density of states (DOS). Secondly, ab-initio calculations for the structural stability of the icosahedrally based Al{sub 12}W structure reveal similar quasigap effects, and provide a simple physical explanation for the stability of the complex aluminide structures. Finally, parametrized tight-binding model calculations for the Al-Mn quasicrystal reveal a large spread in the local Mn DOS behavior, and support a two-site model for the quasicrystal`s magnetic behavior.
Exchange coupling in transition metal monoxides: Electronic structure calculations
Fischer, Guntram [University Halle-Wittenberg, Halle Germany; Daene, Markus W [ORNL; Ernst, Arthur [Max Planck Institute of Microstructure Physics; Bruno, Patrick [European Synchrotron Radiation Facility (ESRF); Lueders, Martin [Daresbury Laboratory, UK; Szotek, Zdzislawa [Daresbury Laboratory, UK; Temmerman, Walter M [Daresbury Laboratory, UK; Wolfam, Hergert [Martin-Luther University Halle-Wittenberg
An ab initio study of magnetic-exchange interactions in antiferromagnetic and strongly correlated 3d transition metal monoxides is presented. Their electronic structure is calculated using the local self-interaction correction approach, implemented within the Korringa-Kohn-Rostoker band-structure method, which is based on multiple scattering theory. The Heisenberg exchange constants are evaluated with the magnetic force theorem. Based on these the corresponding Neel temperatures TN and spin-wave dispersions are calculated. The Neel temperatures are obtained using mean-field approximation, random-phase approximation and Monte Carlo simulations. The pressure dependence of TN is investigated using exchange constants calculated for different lattice constants. All the calculated results are compared to experimental data.
First-principles prediction of superplastic transition-metal alloys
NASA Astrophysics Data System (ADS)
Souvatzis, P.; Katsnelson, M. I.; Simak, S.; Ahuja, R.; Eriksson, O.; Mohn, P.
Superplastic transition metal alloys and compounds are predicted from first principles calculations. Provided a suitable tuning of the alloying is done, materials with vanishingly low shear modulus C' have recently been identified among the 3d , 4d , and 5d elements if the valence electron average number is close to 4.24 (i.e., Ti-Ta-Nb-V-Zr-O and Ti-Nb-Ta-Zr-O alloys). The vanishingly low C' elastic constant of these bcc alloys is, according to the joint experimental and theoretical studies [T. Saito , Science 300, 464 (2003)], the crucial material parameter that is responsible for the superplasticity. We predict here, using first principles calculations, that superplastic alloys should also be found for alloys with drastically different valence electron concentrations, i.e., for W-Re-, W-Tc-, Mo-Re-, Mo-Tc-, and Fe-Co-based alloys.
TOPICAL REVIEW: Transition metal oxides using quantum Monte Carlo
NASA Astrophysics Data System (ADS)
Wagner, Lucas K.
The transition metal-oxygen (TM-O) bond appears prominently throughout chemistry and solid-state physics. Many materials, from biomolecules to ferroelectrics to the components of supernova remnants contain this bond in some form. Many of these materials' properties strongly depend on fine details of the TM-O bond and intricate correlation effects, which make accurate calculations of their properties very challenging. We present quantum Monte Carlo, an explicitly correlated class of methods, to improve the accuracy of electronic structure calculations over that of more traditional methods like density functional theory. We find that unlike s-p type bonding, the amount of hybridization of the d-p bond in TM-O materials is strongly dependent on electronic correlation.
Tightly Bound Trions in Transition Metal Dichalcogenide Heterostructures.
Bellus, Matthew Z; Ceballos, Frank; Chiu, Hsin-Ying; Zhao, Hui
We report the observation of trions at room temperature in a van der Waals heterostructure composed of MoSe2 and WS2 monolayers. These trions are formed by excitons excited in the WS2 layer and electrons transferred from the MoSe2 layer. Recombination of trions results in a peak in the photoluminescence spectra, which is absent in monolayer WS2 that is not in contact with MoSe2. The trion origin of this peak is further confirmed by the linear dependence of the peak position on excitation intensity. We deduced a zero-density trion binding energy of 62 meV. The trion formation facilitates electrical control of exciton transport in transition metal dichalcogenide heterostructures, which can be utilized in various optoelectronic applications. PMID:26046238
Defect-induced photoluminescence in monolayer semiconducting transition metal dichalcogenides.
Chow, Philippe K; Jacobs-Gedrim, Robin B; Gao, Jian; Lu, Toh-Ming; Yu, Bin; Terrones, Humberto; Koratkar, Nikhil
It is well established that defects strongly influence properties in two-dimensional materials. For graphene, atomic defects activate the Raman-active centrosymmetric A1g ring-breathing mode known as the D-peak. The relative intensity of this D-peak compared to the G-band peak is the most widely accepted measure of the quality of graphene films. However, no such metric exists for monolayer semiconducting transition metal dichalcogenides such as WS2 or MoS2. Here we intentionally create atomic-scale defects in the hexagonal lattice of pristine WS2 and MoS2 monolayers using plasma treatments and study the evolution of their Raman and photoluminescence spectra. High-resolution transmission electron microscopy confirms plasma-induced creation of atomic-scale point defects in the monolayer sheets. We find that while the Raman spectra of semiconducting transition metal dichalcogenides (at 532 nm excitation) are insensitive to defects, their photoluminescence reveals a distinct defect-related spectral feature located ?0.1 eV below the neutral free A-exciton peak. This peak originates from defect-bound neutral excitons and intensifies as the two-dimensional (2D) sheet is made more defective. This spectral feature is observable in air under ambient conditions (room temperature and atmospheric pressure), which allows for a relatively simple way to determine the defectiveness of 2D semiconducting nanosheets. Controlled defect creation could also enable tailoring of the optical properties of these materials in optoelectronic device applications. PMID:25603228
Theoretical Study of Surface Related Phenomena of Bcc Transition Metals
NASA Astrophysics Data System (ADS)
Xu, Wei
We have carried out a theoretical study of surface related phenomena for three bcc transition metals: tungsten (W), molybdenum (Mo) and vanadium (V), using a newly developed atomic potential, the fourth moment method. The fourth moment method is based on the second, third and fourth moment approximation of tight-binding theory in which three and four body terms are included to describe the partially filled d-bands. This method was fit to twelve bulk properties and the fitting results of these bulk properties are in good agreement with experimental results, except for some phonon modes. This potential was used to study surface related phenomena. (1) Structure of W(100), Mo(100), V(100), W(110), W(111), W(210), W(211), W(310) and W(321) surfaces. (2) Self-diffusion of W single adatoms on the W(110), (211) and (321) surfaces. The binding site, the activation energy and pre-exponential factor of single W adatom diffusion on these surfaces have been calculated. (3) Small cluster interaction on W(110). The present calculations indicate a strong angular anisotropy and oscillatory behavior of W adatom-adatom interactions on W(110). For long ranges, the interaction is slightly repulsive. (4) Small cluster diffusion on W(110) and (211). We studied the diffusion mechanism of W dimer on those surfaces. The activation energy barrier and pre-exponential factor for W dimer migration have been determined. In general, most of the calculations are in good agreement with experimental results and ab initio calculations. Some of our results are for systems not previously studied. These calculations have resulted in a more detailed understanding of surface structure and diffusion mechanisms on those surfaces. We found that the inclusion of angular terms does significantly improve the description of the bcc transition metals. This potential is a significant improvement over previous empirical models.
Electron-energy-loss spectroscopy of transition-metal carbides
Allison, C.
Electron-energy-loss spectroscopy (EELS) was applied to several problems dealing with the transition-metal carbides. First, narrow (20/sup 0/A) precipitates of TiC were identified in a single crystal of titanium diboride. Second, the carbon concentration in individual stacking faults of TaC was determined to be less than in the surrounding matrix, and the implications of this finding for crystal structure and mechanical properties were discussed. Next, thin films of amorphous carbon and TiO were studied with EELS. Experimental cross sections for C and Ti were obtained from these spectra and used for quantitative analysis of a material of known stoichiometry, V/sub 6/C/sub 5/ (by assuming V and Ti cross sections equal), and one of unknown stoichiometry, a cemented carbide composed of 1 to 5 ..mu..m grains of TiC/sub x/ embedded in a Ni-Mo binder. These experimental cross sections gave a C/V ratio of 0.84, very close to the expected value of 0.833, and a C/Ti ratio of 0.81 for a grain of TiC/sub x/, a reasonable value of x in these materials. Two other methods of determining cross sections were applied: calculated cross sections based on a hydrogenic model gave inaccurate results, perhaps due to the inadequacy of the model for these transition metals; and efficiency factors applied to the Bethe cross section gave reasonable results, but inferior to those from the experimental cross sections.
Goodbyes and Hellos: Effective Strategies for Bridging Early Services. Project APPLES. Bridging Early Services Transition Project.
ERIC Educational Resources Information Center
Rosenkoetter, Sharon; Shotts, Cynthia
This packet of information items presents a variety of resources for the effective transition of young children with disabilities from one program or service to another. Typical items include: challenges and truths of transition; six key points of transition; regulations from relevant federal laws; a flowchart of interagency transition planning; aģ
Solubility Behavior and Phase Stability of Transition Metal Oxides in Alkaline Hydrothermal Environments
S.E. Ziemniak
The solubility behavior of transition metal oxides in high temperature water is interpreted by recognizing three types of chemical reaction equilibria: metal oxide hydration/dehydration, metal oxide dissolution and metal ion hydroxocomplex formation. The equilibria are quantified using thermodynamic concepts and the thermochemical properties of the metal oxides/ions representative of the most common constituents of construction metal alloys, i.e., element shaving atomic numbers between Z = 22 (Ti) and Z = 30 (Zn), are summarized on the basis of metal oxide solubility studies conducted in the laboratory. Particular attention is devoted to the uncharged metal ion hydrocomplex, M{sup Z}(OH){sub Z}(aq), since its thermochemical properties define minimum solubilities of the metal oxide at a given temperature. Experimentally-extracted values of standard partial molal entropy (S{sup 0}) for the transition metal ion neutral hydroxocomplex are shown to be influenced by ligand field stabilization energies and complex symmetry.
Deviance as an antecedent and consequence of early transitions to adulthood: mediating effects and moderating conditionsĀ
Halim, Shaheen
, were used to estimate Structural Equation Models, testing the unmediated and mediated relationships between adolescent deviance, early timing of transitions to adult roles, and adult deviance. First, a simplified three latent variable model...
Reactivity of small transition-metal clusters with CO
NASA Astrophysics Data System (ADS)
Andersson, Mats T.; Gronbeck, H.; Holmgren, L.; Rosen, Arne
The size-dependent reactivity of several transition-metal clusters: Con, Nbn, Rhn, and Wn with CO has been investigated in a cluster beam experiment. The reactions occur at single-collision-like conditions and the results are evaluated in terms of the reaction probability (S) in a collision. For all the four metals, clusters with more than 10 - 15 atoms show a high reaction probability, S >= 0.4, rather independent of size. For smaller Nbn and Wn, the reaction probability is lower, and for Nbn, large variations in the CO reactivity are observed in the n equals 8 - 13 range with a distinct minimum at Nb10. Using an LCAO approach within the local spin density approximation (LSDA) the adsorption of molecular CO on Nbn has also been investigated theoretically. The geometries of the bare clusters were optimized and two different sites for CO were investigated. The discussion is based on a detailed analysis of Nb4. The calculations show that compact structures with high coordination numbers are the most stable ones for the bare Nb clusters and hollow sites, also maximizing the coordination, are preferred for CO adsorption. The calculations indicate that a high CO-Nbn bond strength is obtained for clusters with a high density of states close to the Fermi level and for which the HOMO level has a symmetry that allows for an efficient back-donation of electrons to the 2(pi) *-orbital of CO. A particularly low chemisorption energy was calculated for the Nb10 cluster.
Photoinduced energy transfer in transition metal complex oligomers
The work done over the past three years has been directed toward the preparation, characterization and photophysical examination of mono- and bimetallic diimine complexes. The work is part of a broader project directed toward the development of stable, efficient, light harvesting arrays of transition metal complex chromophores. One focus has been the synthesis of rigid bis-bidentate and bis-tridentate bridging ligands. The authors have managed to make the ligand bphb in multigram quantities from inexpensive starting materials. The synthetic approach used has allowed them to prepare a variety of other ligands which may have unique applications (vide infra). They have prepared, characterized and examined the photophysical behavior of Ru(II) and Re(I) complexes of the ligands. Energy donor/acceptor complexes of bphb have been prepared which exhibit nearly activationless energy transfer. Complexes of Ru(II) and Re(I) have also been prepared with other polyunsaturated ligands in which two different long lived (> 50 ns) excited states exist; results of luminescence and transient absorbance measurements suggest the two states are metal-to-ligand charge transfer and ligand localized {pi}{r_arrow}{pi}* triplets. Finally, the authors have developed methods to prepare polymetallic complexes which are covalently bound to various surfaces. The long term objective of this work is to make light harvesting arrays for the sensitization of large band gap semiconductors. Details of this work are provided in the body of the report.
Photoinduced energy transfer in transition metal complex oligomers
The work we have done over the past three years has been directed toward the preparation, characterization and photophysical examination of mono- and bimetallic diimine complexes. The work is part of a broader project directed toward the development of stable, efficient, light harvesting arrays of transition metal complex chromophores. One focus has been the synthesis of rigid bis-bidentate and bis-tridentate bridging ligands. We have managed to make the ligand bphb in multigram quantities from inexpensive starting materials. The synthetic approach used has allowed us prepare a variety of other ligands which may have unique applications (vide infra). We have prepared, characterized and examined the photophysical behavior of Ru(II) and Re(I) complexes of the ligands. Energy donor/acceptor complexes of bphb have been prepared which exhibit nearly activationless energy transfer. Complexes of Ru(II) and Re(I) have also been prepared with other polyunsaturated ligands in which two different long lived ( > 50 ns) excited states exist; results of luminescence and transient absorbance measurements suggest the two states are metal-to-ligand charge transfer and ligand localized {pi}{r_arrow}{pi}* triplets. Finally, we have developed methods to prepare polymetallic complexes which are covalently bound to various surfaces. The long term objective of this work is to make light harvesting arrays for the sensitization of large band gap semiconductors. Details of this work are provided in the body of the report.
Metal-insulator transition in dirty Kondo insulators
Schlottmann, P.; Hellberg, C.S. [Department of Physics, Florida State University, Tallahassee, Florida 32306 (United States)] [Department of Physics, Florida State University, Tallahassee, Florida 32306 (United States)
We consider arbitrary clusters of Kondo holes in a Kondo insulator described by the nondegenerate symmetric Anderson lattice with a nearest-neighbor tight-binding conduction band on a simple cubic lattice. The {ital f}-electron self energy is considered within the local approximation. Each Kondo hole introduces a boundstate in the gap. The quantum interference in the scattering off the impurities gives rise to interactions among the Kondo holes. The spectral weight of the bound states is predominantly localized on the sites neighboring the Kondo holes. Clusters of impurities separated by more than one lattice site are disconnected for boundstates at the Fermi level. On a simple cubic lattice the metal-insulator transition in the impurity band then reduces to the site percolation of Kondo holes with first, second and fourth nearest neighbors. We use the low density mean cluster size expansion and a small cell renormalization to estimate the critical concentration. Hopping in the conduction band beyond nearest neighbors reduces the percolation threshold. Hence, 9.9{percent} of Kondo holes is an upper bound for the insulator to become a metal. {copyright} {ital 1996 American Institute of Physics.}
Orbital reconstruction in nonpolar tetravalent transition-metal oxide layers
Bogdanov, Nikolay A.; Katukuri, Vamshi M.; Romhßnyi, Judit; Yushankhai, Viktor; Kataev, Vladislav; B³chner, Bernd; van den Brink, Jeroen; Hozoi, Liviu
A promising route to tailoring the electronic properties of quantum materials and devices rests on the idea of orbital engineering in multilayered oxide heterostructures. Here we show that the interplay of interlayer charge imbalance and ligand distortions provides a knob for tuning the sequence of electronic levels even in intrinsically stacked oxides. We resolve in this regard the d-level structure of layered Sr2IrO4 by electron spin resonance. While canonical ligand-field theory predicts g||-factors less than 2 for positive tetragonal distortions as present in Sr2IrO4, the experiment indicates g|| is greater than 2. This implies that the iridium d levels are inverted with respect to their normal ordering. State-of-the-art electronic-structure calculations confirm the level switching in Sr2IrO4, whereas we find them in Ba2IrO4 to be instead normally ordered. Given the nonpolar character of the metal-oxygen layers, our findings highlight the tetravalent transition-metal 214 oxides as ideal platforms to explore d-orbital reconstruction in the context of oxide electronics. PMID:26105992
Orbital reconstruction in nonpolar tetravalent transition-metal oxide layers.
Bogdanov, Nikolay A; Katukuri, Vamshi M; Romhßnyi, Judit; Yushankhai, Viktor; Kataev, Vladislav; B³chner, Bernd; van den Brink, Jeroen; Hozoi, Liviu
A promising route to tailoring the electronic properties of quantum materials and devices rests on the idea of orbital engineering in multilayered oxide heterostructures. Here we show that the interplay of interlayer charge imbalance and ligand distortions provides a knob for tuning the sequence of electronic levels even in intrinsically stacked oxides. We resolve in this regard the d-level structure of layered Sr2IrO4 by electron spin resonance. While canonical ligand-field theory predicts g||-factors less than 2 for positive tetragonal distortions as present in Sr2IrO4, the experiment indicates g|| is greater than 2. This implies that the iridium d levels are inverted with respect to their normal ordering. State-of-the-art electronic-structure calculations confirm the level switching in Sr2IrO4, whereas we find them in Ba2IrO4 to be instead normally ordered. Given the nonpolar character of the metal-oxygen layers, our findings highlight the tetravalent transition-metal 214 oxides as ideal platforms to explore d-orbital reconstruction in the context of oxide electronics. PMID:26105992
Photoinduced Magnetism of Ternary Transition Metal Prussian Blue Analogs
NASA Astrophysics Data System (ADS)
Pajerowski, D. M.; Meisel, M. W.; Gardner, J. E.; Talham, D. R.
The magnetism of Prussian blue analog materials (PBAs) can be tuned with external stimuli such as temperature, pressure, and light. Recently, novel effects have been seen in PBAs with substitutionally mixed ternary and quaternary transition metals, rather than the usual binary analogs. One noteworthy material we have studied is a NaaNi1-xCox[Fe(CN)6]b.nH2O powder, which can show either a photoinduced increase or decrease in magnetization depending upon the Ni substitution, the applied magnetic field, and the temperature. This result is the first example of a photoinduced decrease in magnetization while generating new spins via a charge transfer induced spin transition (CTIST) in a bulk material. Constrastingly, the photodecrease observed in PBA binary thin films has different microscopic origins [1-2]. Insight into the underlying mechanisms can be obtained by using mean field models, which qualitatively reproduce the experimental data. SQUID magnetometer, FT-IR, TEM, and EDS data will be presented.[1] J.-H. Park, et al., Appl. Phys. Lett. 85, 3797 (2004). [2] F. A. Frye et al., Chem. Mater. 20, 5706 (2008).
Magneto-photoconductivitity of atomically thin transition metal dichalcogenides
NASA Astrophysics Data System (ADS)
Eginligil, M.; Zou, C.; Peimyou, N.; Cao, B.; Shen, X.; Shang, J.; Cong, C.; Yu, T.
Photoinduced effects of two-dimensional (2D) transition metal dichalcogenides (TMDs) are of great interest since the bandgap of these materials corresponds to visible range of spectrum. For instance, in molybdenum disulphide (MoS2) - a 2D semiconductor TMD and a non-centrosymmertic crystal, inherent broken inversion symmetry in monolayers leads to a large spin-orbit interaction which splits the valence band (VB) by 160 meV and gives rise to strong excitonic transitions due to the direct band gap at low energy K and -K valleys. The same broken inversion symmetry together with time reversal symmetry is responsible for spin-valley coupling in monolayer MoS2 and similar TMDs (such as tungsten disulphide, WS2) . Spin-valley coupled band edges in TMDs result in different localization behaviors for different scattering mechanisms. In this work, we present our magneto-photoconductivity studies of mono- and bilayer field-effect transistor devices of MoS2 and WS2, and discuss our results in terms of localization effects. Supported in part by the Singapore Ministry of Education under MOE2013-T1-2-235, MOE2012-T2-2-049, and MOE2013-T2-1-044.
M÷ssbauer-Active Transition Metals Other than Iron
NASA Astrophysics Data System (ADS)
G³tlich, Philipp; Bill, Eckhard; Trautwein, Alfred X.
The previous chapters are exclusively devoted to the measurements and interpretation of 57Fe spectra of various iron-containing systems. Iron is, by far, the most extensively explored element in the field of chemistry compared with all other M÷ssbauer-active elements because the M÷ssbauer effect of 57Fe is very easy to observe and the spectra are, in general, well resolved and they reflect important information about bonding and structural properties. Besides iron, there are a good number of other transition metals suitable for M÷ssbauer spectroscopy which is, however, less extensively studied because of technical and/or spectral resolution problems. In recent years, many of these difficulties have been overcome, and we shall see in the following sections a good deal of successful M÷ssbauer spectroscopy that has been performed on compounds of nickel (61Ni), zinc (67Zn), ruthenium (mainly 99Ru), tantalum (181Ta), tungsten (mainly 182W, 183W), osmium (mainly 189Os), iridium (191Ir, 193Ir), platinum (195Pt), and gold (197Au). The nuclear ?-resonance effect in the case of technetium (99Tc), silver (107Ag), hafnium (176Hf, 177Hf, 178Hf, 180Hf), rhenium (187Re), and mercury (199Hg, 201Hg) has been of relatively little use to the chemists, so far. There are various reasons responsible for this, viz., (1) extraordinary difficulties in measuring the resonance effect because of the long lifetime of the excited M÷ssbauer level and hence the extremely small transition line width (e.g., in 67Zn), (2) poor resolution of the resonance lines due to either very small nuclear moments or the very short lifetime of the excited M÷ssbauer level resulting in very broad resonance lines, (3) insufficient resonance effects due to unusually high transition energies between the excited and the ground nuclear levels, which in turn increase the recoil energy and thus reduces the recoilless fraction of emitted and observed ?-rays.
Methods are developed for evaluating the electronic properties of non-simple liquid and amorphous metals, their alloys and liquid semi-conductors. By non simple metals we mean those for which the nearly free electron model does not hold, such as transition, noble and rare earth metals. Explicit expressions for the one electron Green's function of such systems are evaluated within the self
XAFS Studies of Transition Metal and Halogen Biomaterials in Invertebrate Tools
Scott, Robert A.
XAFS Studies of Transition Metal and Halogen Biomaterials in Invertebrate Tools Y. Taoad , J, invertebrate tools, biomaterials PACS: 87.64.Fb INTRODUCTION There exists a large amount of transition metals and halide elements in many invertebrate tools [1]. What is the biological role of these elements
First Principles Hartree-Fock Cluster Study Of Very Dilute Transition Metal And Rare-Earth
Chow, Lee
First Principles Hartree-Fock Cluster Study Of Very Dilute Transition Metal And Rare-Earth Ion INTRODUCTION There is currently great interest in the doping of silicon with transition metal and rare-earth temperature with potential applications for spintronics2 . As far as the rare-earth ion Er3+ is concerned
[Photoreduction of Se (VI) by marine algae-transitional metals-light system].
Li, Shun-Xing; Zheng, Feng-Ying; Deng, Nan-Sheng; Hong, Hua-Sheng; Zhu, Guo-Hui
Seven marine phytoplankton, including five green algae (Tetraselmis levis, Chlorella autotrophica, Dunaliella salina, Nannochloropsis sp. and Tetraselmis subcordiformis), one diatom (Phaeodactylum tricornutum), one red alga (Porphyridium purpureum), and three usual transitional metals (Fe(III), Cu(II), Mn(II)) were used to make up marine phytoplankton-light or transitional metals-light or marine phytoplankton-transitional metals-light system. In such system, Se(VI) could be transformed into Se(IV) by photoreduction. The species transformation of selenium could be photo-induced by redox reaction of transitional metals. The photochemical activity of marine phytoplankton was confirmed for the first time, because marine phytoplankton could adsorb and concentrated of selenium, transitional metals and organic substances (including the exudation of algae, as reducing agent) which redox potentials were changed. The ratios of Se(VI) to Se(IV) were dominated by the species, the concentration of marine phytoplankton and transitional metals, and it could be enhanced through increasing the concentration of marine algae or the combined effect from marine algae and transitional metals. After photoreduction by ternary system, the ratio of Se(VI) to Se(IV) ranges from 1.17 to 2.85, which is close to the actual value in euphotic layer of seawater. The photochemical process that is induced by marine algae and transitional metals dominative the leading effects on the distribution of oxidation states of selenium. PMID:16212166
Paris-Sud XI, Universitķ de
596. NEUTRON DIFFRACTION STUDIES OF MAGNETIC MOMENTS IN DILUTE TRANSITION METAL ALLOYS By M. F 1964, Some rather direct information concerning the electronic structure of magnetic transition metals of the magnetic incoherent scattering from a crystal containing magnetic defects. In this se- cond case
Mott metal-insulator transition in a metallic liquid - Gutzwiller molecular dynamics simulations
NASA Astrophysics Data System (ADS)
Barros, Kipton; Chern, Gia-Wei; Batista, Cristian D.; Kress, Joel D.; Kotliar, Gabriel
Molecular dynamics (MD) simulations are crucial to modern computational physics, chemistry, and materials science, especially when combined with potentials derived from density-functional theory. However, even in state of the art MD codes, the on-site Coulomb repulsion is only treated at the self-consistent Hartree-Fock level. This standard approximation may miss important effects due to electron correlations. The Gutzwiller variational method captures essential correlated-electron physics yet is much faster than, e.g., the dynamical-mean field theory approach. We present our efficient Gutzwiller-MD implementation. With it, we investigate the Mott metal-insulator transition in a metallic fluid and uncover several surprising static and dynamic properties of this system.
Dissolution energetics and its strain dependence of transition metal alloying elements in tungsten
NASA Astrophysics Data System (ADS)
Yan, Wen-Li; Zhou, Hong-Bo; Jin, Shuo; Zhang, Ying; Lu, Guang-Hong
We have systematically investigated the dissolution energetics of all 3d-5d transition metal alloying elements (AEs) in tungsten (W) using a first-principles method. It is found that Ti is the easiest one to dissolve with the solution energy of -0.81 eV in the intrinsic bulk W, while La is the most difficult one with the solution energy as large as 3.91 eV. The d-electrons play a determining role in the dissolution of AEs in W. We have demonstrated that the solution energies of AEs is a linear monotonic function of strain. The binding energy results indicate that the interaction between AEs is repulsive for the early elements and then becomes attractive for the late elements in each series. This study provides a good reference for developing W materials as a plasma facing material.
Metal on metal hip resurfacing versus uncemented custom total hip replacement - early results
Introduction There is no current consensus on the most appropriate prosthesis for treating symptomatic osteoarthritis (OA) of the hip in young, active patients. Modern metal on metal hip resurfacing arthroplasty (HR) has gained popularity as it is theoretically more stable, bone conserving and easier to revise than total hip arthroplasty. Early results of metal on metal resurfacing have been encouraging. We have compared two well matched cohorts of patients with regard to function, pain relief and patient satisfaction. Methods This prospective study compares 2 cohorts of young, active patients treated with hip resurfacing (137 patients, 141 hips) and custom uncemented (CADCAM) stems (134 patients, 141 hips). All procedures were performed by a single surgeon. Outcome measures included Oxford, WOMAC and Harris hip scores as well as an activity score. Statistical analysis was performed using the unpaired student's t-test. Results One hundred and thirty four and 137 patients were included in the hip replacement and resurfacing groups respectively. The mean age of these patients was 54.6 years. The mean duration of follow up for the hip resurfacing group was 19.2 months compared to 13.4 months for the total hip replacement group. Pre operative oxford, Harris and WOMAC scores in the THA group were 41.1, 46.4 and 50.9 respectively while the post operative scores were 14.8, 95.8 and 5.0. In the HR group, pre- operative scores were 37.0, 54.1 and 45.9 respectively compared to 15.0, 96.8 and 6.1 post operatively. The degree of improvement was similar in both groups. Conclusion There was no significant clinical difference between the patients treated with hip resurfacing and total hip arthroplasty in the short term. PMID:20167056
Kindergarten Transition: Does Family Involvement Make a Difference in Children's Early School Adjustment?
ERIC Educational Resources Information Center
Kang, Jean
Kindergarten transition is a critical experience for children because of its potential long-term impact on school performance. Recognizing this impact, the field of early childhood education has been making great efforts to facilitate children's smooth transition to kindergarten through various approaches. One of the most widely acceptedģ
Bridging Early Services Transition Project--Outreach July, 1993 - June, 1997. Final Report.
ERIC Educational Resources Information Center
Rosenkoetter, Sharon; Shotts, Cynthia
This report discusses the outcomes of a project designed to help young children with special needs and their families prepare for and adjust to new service settings. The Bridging Early Service Transition (BEST) Project has helped administrators, service providers, and families plan and coordinate transitions for young children with disabilities orģ
Sociocultural Perspectives on Transition to School from Pacific Islands Early Childhood Centres.
ERIC Educational Resources Information Center
Podmore, Valerie N.; Sauvao, Le'Autuli'ilagi M.; Mapa, Lia
Summarizes research investigating children's transition to primary school from Pacific early childhood centers in New Zealand. Key issues emerging from the review include continuity of Pacific Islands languages and culture between home, early childhood center, and school; home-school partnership; teachers' and parents' expectations regardingģ
Virtual vs. Physical Materials in Early Science Instruction: Transitioning to an Autonomous
Klahr, David
Virtual vs. Physical Materials in Early Science Instruction: Transitioning to an Autonomous Tutor and effects of virtual materials vs. physical materials in early science instruction. In this paper we first- senting instructional materials in laboratory science. The advantages of computer- based science
A Qualitative Study of Early Family Histories and Transitions of Homeless Youth
ERIC Educational Resources Information Center
Tyler, Kimberly A.
Using intensive qualitative interviews with 40 homeless youth, this study examined their early family histories for abuse, neglect, and other family problems and the number and types of transitions that youth experienced. Multiple forms of child maltreatment, family alcoholism, drug use, and criminal activity characterized early family historiesģ
Assaying the catalytic potential of transition metal sulfides for abiotic carbon fixation
NASA Astrophysics Data System (ADS)
Cody, G. D.; Boctor, N. Z.; Brandes, J. A.; Filley, T. R.; Hazen, R. M.; Yoder, H. S.
A suite of nickel, cobalt, iron, copper, and zinc containing sulfides are assayed for the promotion of a model carbon fixation reaction with relevance to local reducing environments of the early Earth. The assay tests the promotion of hydrocarboxylation (the Koch reaction) wherein a carboxylic acid is synthesized via carbonyl insertion at a metal-sulfide-bound alkyl group. The experimental conditions are chosen for optimal assay, i.e., high reactant concentrations and pressures (200 MPa) to enhance chemisorption, and high temperature (250░C) to enhance reaction kinetics. All of the metal sulfides studied, with the exception CuS, promote hydrocarboxylation. Two other significant reactions involve the catalytic reduction of CO to form a surface-bound methyl group, detected after nucleophilic attack by nonane thiol to form methyl nonyl sulfide, and the formation of dinonyl sulfide via a similar reaction. Estimation of the catalytic turnover frequencies for each of the metal sulfides with respect to each of the primary reactions reveals that NiS, Ni 3S 2, and CoS perform comparably to commonly employed industrial catalysts. A positive correlation between the yield of primary product to NiS and Ni 3S 2 surface areas provides strong evidence that the reactions are surface catalytic in these cases. The sulfides FeS and Fe (1-x)S are unique in that they exhibit evidence of extensive dissolution, thus, complicating interpretation regarding heterogeneous vs. homogeneous catalysis. With the exception of CuS, each of the metal sulfides promotes reactions that mimic key intermediate steps manifest in the mechanistic details of an important autotrophic enzyme, acetyl-CoA synthase. The relatively high temperatures chosen for assaying purposes, however, are incompatible with the accumulation of thioesters. The results of this study support the hypothesis that transition metal sulfides may have provided useful catalytic functionality for geochemical carbon fixation in a prebiotic world (at least intially) devoid of peptide-based enzymes.
Watching Transitions Unfold: A Mixed-Method Study of Transitions within Early Childhood Care and Education Settings
ERIC Educational Resources Information Center
O'Farrelly, Christine; Hennessy, Eilis
Unlike the transitions children make between settings, those they undertake between age groups within early childhood care and education (ECCE) settings are seldom studied. Accordingly, this exploratory study followed seven preschool children (three boys and four girls) as they moved to new rooms in five ECCE settings. Structured observations ofģ
Aneuploidy as an Early Mechanistic Event in Metal Carcinogenesis
Wise, Sandra S.; Wise, John Pierce
Aneuploidy has recently been proposed as an initiating event for carcinogenesis. There is significant evidence that carcinogenic metals induce aneuploidy. Here we review the mechanisms for how carcinogenic metals may induce aneuploidy and the evidence that carcinogenic metals induce an aneugenic effect which can destabilize the genome leading to genomic instability and cancer. PMID:21118142
Transition-Metal Oxides in Warm Circumstellar Environments
NASA Astrophysics Data System (ADS)
Schmidt, Miros?aw R.; Kaminski, Tomasz; Tylenda, Romuald
We report on detections and simulations of electronic bands of transition-metal oxides, i.e. ScO, TiO, VO, CrO, YO, and of AlO, in spectra of two red novae V838 Mon and V4332 Sgr. These objects experienced a stellar merger event in 2002 and 1994, respectively, and have very rich circumstellar environments abundant in dust and molecules. We analyzed optical spectra of V838 Mon which show a presence of outflowing material. In this object, electronic systems of oxides are observed in absorption against a photospheric spectrum which resembles that of a late-type supergiant. We present simulations of the absorption bands which allowed us to derive the excitation temperatures of 300-500 K and constrain column densities, which turned out to be very high. Among many interesting features discovered, we identified forbidden transitions of TiO in the b^1?-X^3? and c^{1}?-X^{3}? systems, which are seen owing to the high column densities and the relatively low temperatures. In the case of the older red nova V4332 Sgr, the main object is surrounded by a circumstellar disc which is seen almost edge-on and obscures the central star. The molecular spectra are seen in emission in this object, what is very unusual in astrophysical sources observed at optical wavelengths. We show that these emission bands arise owing to the special geometry of the star-disk system and that radiative pumping is responsible for excitation of the molecules. From the shapes of the rotational contours, we derive temperatures of about 120 K in this object. Remarkably, the spectra of V4332 Sgr contain features of CrO, which is the first identified signature of this molecule in an astrophysical object. In addition to the excitation and radiative-transfer analysis of the molecular spectra, we discuss chemical pathways that could lead to the observed variety of metal oxides seen in these enigmatic sources. T. Kaminski, M. Schmidt, R. Tylenda, M. Konacki, and M. Gromadzki ApJSuppl., {182} (33), 2009. T. Kaminski, M. Schmidt and R. Tylenda Astronomy and Astrophysics, {522} (A75), 2010.
Voltage controlled magnetism in 3d transitional metals
NASA Astrophysics Data System (ADS)
Wang, Weigang
Despite having attracted much attention in multiferroic materials and diluted magnetic semiconductors, the impact of an electric field on the magnetic properties remains largely unknown in 3d transitional ferromagnets (FMs) until recent years. A great deal of effort has been focused on the voltage-controlled magnetic anisotropy (VCMA) effect where the modulation of anisotropy field is understood by the change of electron density among different d orbitals of FMs in the presence of an electric field. Here we demonstrate another approach to alter the magnetism by electrically controlling the oxidation state of the 3d FM at the FM/oxide interface. The thin FM film sandwiched between a heavy metal layer and a gate oxide can be reversibly changed from an optimally-oxidized state with a strong perpendicular magnetic anisotropy to a metallic state with an in-plane magnetic anisotropy, or to a fully-oxidized state with nearly zero magnetization, depending on the polarity and time duration of the applied electric fields. This is a voltage controlled magnetism (VCM) effect, where both the saturation magnetization and anisotropy field of the 3d FM layer can be simultaneously controlled by voltage in a non-volatile fashion. We will also discuss the impact of this VCM effect on magnetic tunnel junctions and spin Hall switching experiments. This work, in collaboration with C. Bi, Y.H. Liu, T. Newhouse-Illige, M. Xu, M. Rosales, J.W. Freeland, O. Mryasov, S. Zhang, and S.G.E. te Velthuis, was supported in part by NSF (ECCS-1310338) and by C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA.
Metal-insulator transitions in effective J=1/2 insulating iridates
NASA Astrophysics Data System (ADS)
Zhang, Hongbin
The competition between spin-orbit coupling, crystal field splitting and electron correlations with comparable magnitude gives rise to many interesting phenomena. For instance, the so-called effective J = 1 / 2 state has been observed in many iridates compounds, e.g., Ruddlesden-Popper (RP) Srn+1IrnO3n+1 and pyrochlore RE2Ir2O7 (RE=Bi, Pr, Nd, Sm, Eu, Y) iridates, where metal-insulator transitions occur driven by the interplay of electron correlations with magnetic ordering. Using first-principles methods, for correlated solids based on density functional theory and dynamical mean field theory (DFT+DMFT), we have investigated the metal-insulator transitions in both classes of iridates. We explore the robustness of the effective J = 1 / 2 state against band effects due to itineracy, structural distortion, and strain. We show how single-particle spectra, optical conductivities, and orbital and spin moments change with strain, and we demonstrate that the ground state can be well characterized in terms of an effective energy-dependent J = 1 / 2 state. For RP compounds, we demonstrate that the crystal field splittings induced by local structural distortions and hybridization are critical to understand previous experimental results. For pyrochlore compounds, the total energies obtained using charge self-consistent DFT+DMFT method reveal that the all-in-all-out magnetic ordering is stable at low temperature in late rare earth pyrochlores, while a bad metallic state is found in early rare earth pyrochlores, in agreement with experiments. This work was supported by NSF Grant No. DMREF-12-33349.
STM Image Contrast Interpretation and its Role in Determining the Structure of Transition Metal Oxide Surfaces
NASA Astrophysics Data System (ADS)
Lu, Weier
The surfaces of transition metal oxides play a critical role in many applications such as heterogeneous catalysis, gas detection, thermionic emission, electrolysis, and photolysis. Understanding the mechanisms of such surface processes requires a detailed knowledge of the surface microscopic structure. Since its invention in the early 1980's, scanning tunneling microscopy (STM) has come to be a popular tool for oxide surface studies. However, despite some experimental successes, interpretation of the contrast in STM images of metal oxides has remained challenging due to the numerous contributing factors such as nonstoichiometry, structural complexity, surface disorder, and uncertainties regarding the bonding and termination layers in such multicomponent systems. In this thesis work, a computer simulation scheme that explores these effects separately has been developed to assist the interpretation of atomic-scale contrast in STM images. A semiquantitative technique, based on the one -dimensional square well tunneling model, is used to simulate constant current STM images. This model provides an efficient mechanism to test and explore effects of various ill-defined experimental parameters. The method was applied to the atomic-scale resolution STM study of three transition metal oxides: {rm M_{x}WO _3} (M=Rb, Na), {rm Mo_{18}O_{52}}, and V_2{rm O}_5. Our observations include surface termination layer variations, surface ordering, surface relaxations, surface steps caused by crystallographic shear (CS) planes, surface oxygen vacancies and other defects. In each case, the competition between geometric and electronic contributions to the image contrast is evident. Tunneling spectroscopy experiments and calculations were also performed on the sodium tungsten bronzes and the implication to their electronic structures is discussed.
Fingerprints of spin-orbital entanglement in transition metal oxides
Andrzej M. Ole?
The concept of spin-orbital entanglement on superexchange bonds in transition metal oxides is introduced and explained on several examples. It is shown that spin-orbital entanglement in superexchange models destabilizes the long-range (spin and orbital) order and may lead either to a disordered spin-liquid state or to novel phases at low temperature which arise from strongly frustrated interactions. Such novel ground states cannot be described within the conventionally used mean field theory which separates spin and orbital degrees of freedom. Even in cases where the ground states are disentangled, spin-orbital entanglement occurs in excited states and may become crucial for a correct description of physical properties at finite temperature. As an important example of this behaviour we present spin-orbital entanglement in the $R$VO$_3$ perovskites, with $R$=La,Pr,...,Yb,Lu, where such finite temperature properties of these compounds can be understood only using entangled states: ($i$) thermal evolution of the optical spectral weights, ($ii$) the dependence of transition temperatures for the onset of orbital and magnetic order on the ionic radius in the phase diagram of the $R$VO$_3$ perovskites, and ($iii$) dimerization observed in the magnon spectra for the $C$-type antiferromagnetic phase of YVO$_3$. Finally, it is shown that joint spin-orbital excitations in an ordered phase with coexisting antiferromagnetic and alternating orbital order introduces topological constraints for the hole propagation and will thus radically modify transport properties in doped Mott insulators where hole motion implies simultaneous spin and orbital excitations.
Fingerprints of spin-orbital entanglement in transition metal oxides.
Ole?, Andrzej M
The concept of spin-orbital entanglement on superexchange bonds in transition metal oxides is introduced and explained on several examples. It is shown that spin-orbital entanglement in superexchange models destabilizes the long-range (spin and orbital) order and may lead either to a disordered spin-liquid state or to novel phases at low temperature which arise from strongly frustrated interactions. Such novel ground states cannot be described within the conventionally used mean field theory which separates spin and orbital degrees of freedom. Even in cases where the ground states are disentangled, spin-orbital entanglement occurs in excited states and may become crucial for a correct description of physical properties at finite temperature. As an important example of this behaviour we present spin-orbital entanglement in the RV O(3) perovskites, with R = La,Pr,ģ,Y b,Lu, where the finite temperature properties of these compounds can be understood only using entangled states: (i) the thermal evolution of the optical spectral weights, (ii) the dependence of the transition temperatures for the onset of orbital and magnetic order on the ionic radius in the phase diagram of the RV O(3) perovskites, and (iii) the dimerization observed in the magnon spectra for the C-type antiferromagnetic phase of Y V O(3). Finally, it is shown that joint spin-orbital excitations in an ordered phase with coexisting antiferromagnetic and alternating orbital order introduce topological constraints for the hole propagation and will thus radically modify the transport properties in doped Mott insulators where hole motion implies simultaneous spin and orbital excitations. PMID:22776856
The Electronic Properties of Semiconductors, Semiconductor - Metal Interfaces, and Transition Metal Alloys.
NASA Astrophysics Data System (ADS)
Nelson, Jeffrey Steven
The first principles self-consistent pseudopotential method and the supercell concept are used to investigate the electronic properties of three vastly different condensed matter systems. First, we will look at the electronic states associated with a heavily doped GaAs/AlAs superlattice. The main questions we will address in this study are, (1) what is the origin of the band edge states; (2) what is the charge distribution of the Si donor state; (3) how will Si doping of the GaAs/AlAs interface affect the band edge states. Secondly, we will study the bonding properties and Schottky barrier formation at the Si-Al interface. The change in various physical quantities such as the Si-Al interlayer-distance, work functions, total valence charge densities, Schottky barrier heights, and metallization of the Al overlayer are studied as a function of Al coverage. An important metal-surface distance relaxation is also projected. The third system we will investigate is a Co-Cr transition metal alloy. The main focus here will be to study the changes in the charge distribution and band structure of hexagonal close packed Co due to alloying with Cr. We will also examine the effects of stress on the electronic properties of these alloys. The results of the first principles pseudopotential calculations provide detailed information and physical insight not generally accessible with more empirical techniques.
Oligomeric rare-earth metal cluster complexes with endohedral transition metal atoms
NASA Astrophysics Data System (ADS)
Steinberg, Simon; Zimmermann, Sina; Br³hmann, Matthias; Meyer, Eva; Rustige, Christian; Wolberg, Marike; Daub, Kathrin; Bell, Thomas; Meyer, Gerd
Comproportionation reactions of rare-earth metal trihalides (RX3) with the respective rare-earth metals (R) and transition metals (T) led to the formation of 22 oligomeric R cluster halides encapsulating T, in 19 cases for the first time. The structures of these compounds were determined by single-crystal X-ray diffraction and are composed of trimers ({T3R11}X15-type, P63/m), tetramers ({T4R16}X28{R4} (P-43m), {T4R16}X20 (P42/nnm), {T4R16}X24(RX3)4 (I41/a) and {T4R16}X23 (C2/m) types of structure) and pentamers ({Ru5La14}2Br39, Cc) of {TRr}n (n=2-5) clusters. These oligomers are further enveloped by inner (Xi) as well as outer (Xa) halido ligands, which possess diverse functionalities and interconnect like oligomers through i-i, i-a and/or a-i bridges. The general features of the crystal structures for these new compounds are discussed and compared to literature entries as well as different structure types with oligomeric T centered R clusters. Dimers and tetramers originating from the aggregation of {TR6} octahedra via common edges are more frequent than trimers and pentamers, in which the {TRr} clusters share common faces.
Transition metal activation and functionalization of carbon-hydrogen bonds
Jones, W.D.
We are investigating the fundamental thermodynamic and kinetic factors that influence carbon-hydrogen bond activation at homogeneous transition metal centers and the conversion of hydrocarbons into functionalized products of potential use to the chemical industry. Advances have been made in both understanding the interactions of hydrocarbons with metals and in the functionalization of hydrocarbons. We have found that RhCl(PR{sub 3}){sub 2}(CNR) complexes can catalyze the insertion of isonitriles into the C-H bonds or arenes upon photolysis. The mechanism of these reactions was found to proceed by way of initial phosphine dissociation, followed by C-H activation and isonitrile insertion. We have also examined reactions of a series of arenes with (C{sub 5}Me{sub 5})Rh(PMe{sub 3})PhH and begun to map out the kinetic and thermodynamic preferences for arene coordination. The effects of resonance, specifically the differences in the Hueckel energies of the bound vs free ligand, are now believed to fully control the C-H activation/{eta}{sup 2}-coordination equilibria. We have begun to examine the reactions of rhodium isonitrile pyrazolylborates for alkane and arene C-H bond activation. A new, labile, carbodiimide precursor has been developed for these studies. We have completed studies of the reactions of (C{sub 5}Me{sub 5})Rh(PMe{sub 3})H{sub 2} with D{sub 2} and PMe{sub 3} that indicate that both {eta}{sup 5} {yields} {eta}{sup 3} ring slippage and metal to ring hydride migration occur more facilely than thermal reductive elimination of H{sub 2}. We have examined the reactions of heterocycles with (C{sub 5}Me{sub 5})Rh(PMe{sub 3})PhH and found that pyrrole and furan undergo C-H or N-H activation. Thiophene, however, undergoes C-S bond oxidative addition, and the mechanism of activation has been shown to proceed through sulfur coordination prior to C-S insertion.
Recent advances in transition metal-catalyzed Csp2-monofluoro-, difluoro-, perfluoromethylation and trifluoromethylthiolation
Landelle, Grķgory; Panossian, Armen; Pazenok, Sergiy; Vors, Jean-Pierre
Summary In the last few years, transition metal-mediated reactions have joined the toolbox of chemists working in the field of fluorination for Life-Science oriented research. The successful execution of transition metal-catalyzed carbon¢fluorine bond formation has become a landmark achievement in fluorine chemistry. This rapidly growing research field has been the subject of some excellent reviews. Our approach focuses exclusively on transition metal-catalyzed reactions that allow the introduction of ¢CFH2, ¢CF2H, ¢CnF2 n +1 and ¢SCF3 groups onto sp▓ carbon atoms. Transformations are discussed according to the reaction-type and the metal employed. The review will not extend to conventional non-transition metal methods to these fluorinated groups. PMID:24367416
Metal-insulator transitions in IZO, IGZO, and ITZO films
Makise, Kazumasa, E-mail: email@example.com [National Institute of Information and Communications Technology, Kobe 651-2492 (Japan); Hidaka, Kazuya; Ezaki, Syohei; Asano, Takayuki; Shinozaki, Bunju [Department of Physics, Kyushu University, Fukuoka 810-8560 (Japan); Tomai, Shigekazu; Yano, Koki; Nakamura, Hiroaki [Central Research Laboratories, Idemitsu Kosan Co. Ltd, Chiba 299-0293 (Japan)
In this study, we measured the low-temperature resistivity of amorphous two- and three-dimensional (2D and 3D) indium-zinc oxide, indium-gallium-zinc oxide, and indium-tin-zinc oxide films with a wide range of carrier densities. To determine their critical characteristics at the metal-insulator transition (MIT), we used the Ioffe¢Regel criterion. We found that the MIT occurs in a narrow range between k{sub F}??=0.13 and k{sub F}??=0.25, where k{sub F} and ? are the Fermi wave number and electron mean free path, respectively. For films in the insulating region, we analyzed ?(T) using a procedure proposed by Zabrodskii and Zinov'eva. This analysis confirmed the occurrence of Mott and Efros¢Shklovskii (ES) variable-range hopping. The materials studied show crossover behavior from exp(T{sub Mott}/T){sup 1/4} or exp(T{sub Mott}/T){sup 1/3} for Mott hopping conduction to exp(T{sub ES}/T){sup 1/2} for ES hopping conduction with decreasing temperature. For both 2D and 3D materials, we found that the relationship between T{sub Mott} and T{sub ES} satisfies T{sub ES}?T{sub Mott}{sup 2/3}.
Correlations in rare-earth transition-metal permanent magnets
NASA Astrophysics Data System (ADS)
Skomski, R.; Manchanda, P.; Kashyap, A.
It is investigated how electron-electron correlations affect the intrinsic properties of rare-earth transition-metal magnets. Focusing on orbital moment and anisotropy, we perform model calculations for 3d-4f alloys and density-functional theory (DFT) calculations for NdCo5. On an independent-electron level, the use of a single Slater determinant with broken spin symmetry introduces Hund's rule correlations, which govern the behavior of rare-earth ions and of alloys described by the local spin density approximation (LSDA) and LSDA + U approximations to DFT. By contrast, rare-earth ions in intermetallics involve configuration interactions between two or more Slater determinants and lead to phenomena such as spin-charge distribution. Analyzing DFT as a Legendre transformation and using Bethe's crystal-field theory, we show that the corresponding density functionals are very different from familiar LSDA-type expressions and outline the effect of spin-charge separation on the magnetocrystalline anisotropy.
Metal-insulator transition driven by short range ferromagnetic correlations
NASA Astrophysics Data System (ADS)
RodrĒguez-N·±ez, J. J.; Figueira, M. S.; Anda, E. V.; Ventura, C. I.; Calegari, E. J.
One of the methods used to study strongly correlated electron systems is the moment approach of Nolting, which is based on a two-pole Ansatz for the one electron Green function, G( k?,?) . The two energy bands and the two spectral weights are calculated via four sum rules or spectral moments. In a new hybrid approach, we use the equation of motion for G( k?,?) , generating G 2(i,j,?)?<
Induce magnetism into silicene by embedding transition-metal atoms
NASA Astrophysics Data System (ADS)
Sun, Xiaotian; Wang, Lu; Lin, Haiping; Hou, Tingjun; Li, Youyong
Embedding transition-metal (TM) atoms into nonmagnetic nanomaterials is an efficient way to induce magnetism. Using first-principles calculations, we systematically investigated the structural stability and magnetic properties of TM atoms from Sc to Zn embedded into silicene with single vacancy (SV) and double vacancies (DV). The binding energies for different TM atoms correlate with the TM d-shell electrons. Sc, Ti, and Co show the largest binding energies of as high as 6 eV, while Zn has the lowest binding energy of about 2 eV. The magnetic moment of silicene can be modulated by embedding TM atoms from V to Co, which mainly comes from the 3d orbitals of TM along with partly contributions from the neighboring Si atoms. Fe atom on SV and Mn atom on DV have the largest magnetic moment of more than 3 ?B. In addition, we find that doping of N or C atoms on the vacancy site could greatly enhance the magnetism of the systems. Our results provide a promising approach to design silicene-based nanoelectronics and spintronics device.
Magnetic ground state of semiconducting transition metal trichalcogenide monolayers
Sivadas, Mr. Nikhil [Carnegie Mellon University (CMU); Daniels, Matthew W. [Carnegie Mellon University (CMU); Swendsen, Robert H. [Carnegie Mellon University (CMU); Okamoto, Satoshi [ORNL; Xiao, Di [Carnegie Mellon University (CMU)
Layered transition-metal trichalcogenides with the chemical formula ABX3 have attracted recent interest as potential candidates for two-dimensional magnets. Using first-principles calculations within density functional theory, we investigate the magnetic ground states of monolayers of Mn- and Cr-based semiconducting trichalcogenides.We show that the second and third nearest-neighbor exchange interactions (J2 and J3) between magnetic ions, which have been largely overlooked in previous theoretical studies, are crucial in determining the magnetic ground state. Specifically, we find that monolayer CrSiTe3 is an antiferromagnet with a zigzag spin texture due to significant contribution from J3, whereas CrGeTe3 is a ferromagnet with a Curie temperature of 106 K. Monolayers of Mn compounds (MnPS3 and MnPSe3) always show antiferromagnetic N eel order. We identify the physical origin of various exchange interactions, and demonstrate that strain can be an effective knob for tuning the magnetic properties. Possible magnetic ordering in the bulk is also discussed. Our study suggests that ABX3 can be a promising platform to explore two-dimensional magnetic phenomena.
APCVD Transition Metal Oxides ¢ Functional Layers in "Smart windows"
NASA Astrophysics Data System (ADS)
Gesheva, K. A.; Ivanova, T. M.; Bodurov, G. K.
Transition metal oxides (TMO) exhibit electrochromic effect. Under a small voltage they change their optical transmittance from transparent to collored (absorbing) state. The individual material can manifest its electrochromic properties only when it is part of electrochromic (EC) multilayer system. Smart window is controlling the energy of solar flux entering the building or car and makes the interiors comfortable and energy utilization more effective. Recently the efforts of material researchers in this field are directed to price decreasing. APCVD technology is considered as promissing as this process permits flowthrough large-scale production process. The paper presents results on device optimization based on WO3-MoO3 working electrode. Extensive research reveals that WO3-MoO3 structure combines positive features of single oxides: excellent electrochromic performance of WO3 and better kinetic properties of MoO3 deposition. The achieved color efficiency of APCVD WO3-MoO3 films is 200cm2/C and optical modulation of 65-70% are practically favorable electrochromic characteristics. To respond to low cost requirement, the expensive hexacarbonyl can be replaced with acetylacetonate. We have started with this precursor to fabricate mixed WxV1-xO3 films. The films possess excellent surface coverage and high growth-rate. CVD deposition of VO2, a promissing thermochromic thin film material is also presented.
Exciton Radiative Lifetimes in Layered Transition Metal Dichalcogenides
NASA Astrophysics Data System (ADS)
Palummo, Maurizia; Benardi, Marco; Grossman, Jeffrey C.
Light emission in two-dimensional (2D) transition metal dichalcogenides (TMDs) changes significantly with number of layers and stacking sequence. While the electronic structure and optical absorption are well understood in 2D-TMDs, much less is known about exciton dynamics and radiative recombination. In this talk, we show first-principles calculations of intrinsic exciton radiative lifetimes at low temperature (4 K) and room temperature (300 K) in TMD monolayers with chemical formula MX2 (M=Mo,W and X=S,Se), in bilayer and bulk MoS2, and in two MX2 hetero-bilayers. Our results elucidate the time scale and microscopic origin of light emission in TMDs, which have been the subjects of recent intense investigation. We find radiative lifetimes of a few ps at low temperature and a few ns at room temperature in the monolayers, and slower radiative recombination in bulk and bilayer than in monolayer MoS2. The MoS2/WS2 and MoSe2/WSe2 hetero-bilayers exhibit long-lived (~30 ns at room temperature) inter-layer excitons constituted by electrons localized on the Mo-based and holes on the W-based monolayer; this finding agrees with recent ultrafast spectroscopy experiments. We discuss how the radiative lifetime tunability can be employed to manipulate excitons in 2D-TMDs.
Optoelectronics of Transition Metal Dichalcogenide Monolayers and Heterostructures
NASA Astrophysics Data System (ADS)
Schaibley, John
Monolayer transition metal dichalcogenides (TMDs) contain 2D valley excitons which reside in two degenerate momentum space valleys at the edges of the Brillouin zone. It is crucially important to understand fundamental 2D exciton properties in TMD monolayers and van der Waals heterostructures. By performing coherent nonlinear optical spectroscopy with high spectral resolution, we observe nanosecond decay dynamics in single monolayers of MoSe2, implying the presence of a previously unreported long-lived state that appears to trap the exciton population. In MoSe2-WSe2 vertical heterostructures, we observe intralayer excitons, where the electron and hole are confined to different monolayers, and show evidence of strong exciton-exciton interaction effects and long lifetimes. Based on TMD monolayer excitons, we have also investigated a variety of fundamental quantum devices, including a nano-cavity laser and a second-harmonic generation transistor. Finally, we report a new type of single quantum emitter, based on single localized excitons spatially confined to defects in monolayers of WSe2. The photoluminescence from these localized excitons is spectrally narrow and shows strong anti-bunching, demonstrating the single photon nature of the emission.
Dislocations and Plasticity in bcc Transition Metals at High Pressure
Yang, L H; Tang, M; Moriarty, J A
Using first-principles electronic structure calculations, quantum-based atomistic simulations and atomistically informed dislocation dynamics (DD) simulations, we have studied individual dislocation behavior and the multiscale modeling of single-crystal plasticity in the prototype bcc transition metals Ta, Mo and V under both ambient and high pressure conditions. The primary focus in this work is on the pressure-dependent structure, mobility and interaction of a/2<111> screw dislocations, which dominate the plastic deformation properties of these materials. At the electronic scale, first-principles calculations of elasticity, ideal strength and generalized stacking fault energy surfaces have been used to validate quantum-based multi-ion interatomic potentials. At the atomistic scale, these potentials have been used in flexible Green's function boundary condition simulations to study the core structure, Peierls stress {tau}{sub P}, thermally activated kink-pair formation and mobility below {tau}{sub P}, and phonon-drag mobility above {tau}{sub P}. These results have then been distilled into analytic velocity laws and used directly in predictive microscale DD simulations of flow stress and resolved yield stress over wide ranges of pressure, temperature and strain rate.
Two-dimensional transition metal dichalcogenide nanosheet-based composites.
Tan, Chaoliang; Zhang, Hua
Ultrathin two-dimensional (2D) nanosheets of layered transition metal dichalcogenides (TMDs), such as MoS2, TiS2, TaS2, WS2, MoSe2, WSe2, etc., are emerging as a class of key materials in chemistry and electronics due to their intriguing chemical and electronic properties. The ability to prepare these TMD nanosheets in high yield and large scale via various methods has led to increasing studies on their hybridization with other materials to create novel functional composites, aiming to engineer their chemical, physical and electronic properties and thus achieve good performance for some specific applications. In this critical review, we will introduce the recent progress in hybrid nanoarchitectures based on 2D TMD nanosheets. Their synthetic strategies, properties and applications are systematically summarized and discussed, with emphasis on those new appealing structures, properties and functions. In addition, we will also give some perspectives on the challenges and opportunities in this promising research area. PMID:25292209
RKKY interaction in transition-metal dichalcogenide nanoflakes
NASA Astrophysics Data System (ADS)
Avalos-Ovando, Oscar; Mastrogiuseppe, Diego; Ulloa, Sergio
Transition metal dichalcogenides (TMDs) are layered crystals with unique electronic and optical properties, and are promising candidates for a new generation of semiconductor-based devices, mainly when exfoliated to one or a few layers. The process of exfoliation often produces nanoscale samples -flakes- with different shapes and boundaries. These flakes might have applications as quantum dots with novel characteristics. One interesting topic relates to the presence of magnetic impurities and their interaction. In combination with strong spin-orbit coupling and valley degrees of freedom, TMDs might have a great impact in the field of spintronics. Using an effective low-energy two-orbital tight-binding model, we study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic impurities in 2D TMD nanoflakes. We consider different geometries and terminations, analyzing the effect of the sample size. Our results show the behavior of the interaction for impurities sitting at different positions in the flake, and its possible tunability with the electron/hole concentration. The magnetic impurities can be intrinsic to the sample production process or can be introduced extrinsically. Our results can be tested with local probes, such as spin-polarized STM. Supported by NSF DMR MWN/CIAM.
Ionic bonding of transition-metal halides: A spectroscopic approach
NASA Astrophysics Data System (ADS)
Thomas, J.; Pollini, I.
The dielectric theory of the chemical bond has been applied to crystals with either Cd(OH)2 or CdCl2 structure, namely to layered Mn, Fe, Co, and Ni dihalides (MX2) with octahedral coordination, in order to evaluate the fractional ionic character fi for this class of insulators. The crystalline spectroscopic energy gap Eg has been measured via the optical data, related to the dominant exciton peaks ?, and then evaluated either through the Phillips model (EPhillipsg) or the measured dielectric constant ?1(0) in the framework of the Penn model (EPenng). The obtained scales of ionicities, fi or fDTi, ranging from fi~=0.72 of NiI2 to fi~=0.80 of MnCl2 are then compared to the ionicity scale fXPSi based on x-ray photoelectron spectroscopy. For transition-metal chlorides, for which photoemission spectra are available, the different ionicity scales are in good agreement. Furthermore, the ionicity parameters scale rather well with the ionicity trend given by the fitted values of the net charge Z, the electrostatic parameter for dealing with crystals not completely ionic. The overall agreement between the spectroscopically determined ionicity, the structural, thermochemical, and electronic properties of these compounds seems to indicate that the dielectric theory of Phillips and Van Vechten can be successfully applied to layered materials with reduced ionicity and open d-shell configuration.
Proton extraction by laser ablation of transition metals
NASA Astrophysics Data System (ADS)
Velardi, L.; Delle Side, D.; Krßsa, J.; Nassisi, V.
A study on the proton beams extraction from a plasma generated by pulsed laser ablation by targets containing transition metals is presented. The targets used were pure disks of titanium and tantalum and disks of TiH2, obtained by compression of TiH2 powder. The plasma was produced by means of a nanosecond excimer KrF laser operating at low irradiance (109-1010 W/cm2). The proton and ions emission was analyzed by the time-of-flight technique using a Faraday cup as ion collector. Studies on the produced protons and ions at different laser irradiances from 2 to 15 GW/cm2 were performed. The characterization showed that it is possible to obtain good proton fluxes from these targets, up to 1011 proton/pulse. The results obtained are very interesting if compared with those available in literature where proton fluxes per pulse ranging from 108 to 109 by hydride targets were obtained, at the same laser irradiances.
Magnetic ground state of semiconducting transition-metal trichalcogenide monolayers
NASA Astrophysics Data System (ADS)
Sivadas, Nikhil; Daniels, Matthew W.; Swendsen, Robert H.; Okamoto, Satoshi; Xiao, Di
Layered transition-metal trichalcogenides with the chemical formula A B X3 have attracted recent interest as potential candidates for two-dimensional magnets. Using first-principles calculations within density functional theory, we investigate the magnetic ground states of monolayers of Mn- and Cr-based semiconducting trichalcogenides. We show that the second and third nearest-neighbor exchange interactions (J2 and J3) between magnetic ions, which have been largely overlooked in previous theoretical studies, are crucial in determining the magnetic ground state. Specifically, we find that monolayer CrSiTe3 is an antiferromagnet with a zigzag spin texture due to significant contribution from J3, whereas CrGeTe3 is a ferromagnet with a Curie temperature of 106 K. Monolayers of Mn compounds (MnPS3 and MnPSe3) always show antiferromagnetic Nķel order. We identify the physical origin of various exchange interactions, and demonstrate that strain can be an effective knob for tuning the magnetic properties. Possible magnetic ordering in the bulk is also discussed. Our study suggests that A B X3 can be a promising platform to explore two-dimensional magnetic phenomena.
Density functional localized orbital corrections for transition metals
Rinaldo, David; Tian, Li; Harvey, Jeremy N.; Friesner, Richard A.
This paper describes the development of the B3LYP localized orbital correction model which improves the accuracy of the B3LYP thermochemical predictions for compounds containing transition metals. The development of this model employs a large data set containing 36 experimental atomic energies and 71 bond dissociation energies. B3LYP calculations were carried out on these systems with different basis sets. Based on an electronic structure analysis and physical arguments, we built a set of 10 parameters to correct atomic data and a set of 21 parameters to correct bond dissociation energies. Using the results from our biggest basis set, the model was shown to reduce the mean absolute deviation from 7.7 to 0.4 kcal?mol for the atomic data and from 5.3 to 1.7 kcal?mol for the bond dissociation energies. The model was also tested using a second basis set and was shown to give relatively accurate results too. The model was also able to predict an outlier in the experimental data that was further investigated with high level coupled-cluster calculations. PMID:19045248
Exciton radiative lifetimes in two-dimensional transition metal dichalcogenides.
Palummo, Maurizia; Bernardi, Marco; Grossman, Jeffrey C
Light emission in two-dimensional (2D) transition metal dichalcogenides (TMDs) changes significantly with the number of layers and stacking sequence. While the electronic structure and optical absorption are well understood in 2D-TMDs, much less is known about exciton dynamics and radiative recombination. Here, we show first-principles calculations of intrinsic exciton radiative lifetimes at low temperature (4 K) and room temperature (300 K) in TMD monolayers with the chemical formula MX2 (X = Mo, W, and X = S, Se), as well as in bilayer and bulk MoS2 and in two MX2 heterobilayers. Our results elucidate the time scale and microscopic origin of light emission in TMDs. We find radiative lifetimes of a few picoseconds at low temperature and a few nanoseconds at room temperature in the monolayers and slower radiative recombination in bulk and bilayer than in monolayer MoS2. The MoS2/WS2 and MoSe2/WSe2 heterobilayers exhibit very long-lived (?20-30 ns at room temperature) interlayer excitons constituted by electrons localized on the Mo-based and holes on the W-based monolayer. The wide radiative lifetime tunability, together with the ability shown here to predict radiative lifetimes from computations, hold unique potential to manipulate excitons in TMDs and their heterostructures for application in optoelectronics and solar energy conversion. PMID:25798735
Valency configuration of transition metal impurities in ZnO
Petit, Leon [ORNL; Schulthess, Thomas C [ORNL; Svane, Axel [University of Aarhus, Denmark; Temmerman, Walter M [Daresbury Laboratory, UK; Szotek, Zdzislawa [Daresbury Laboratory, UK; Janotti, Anderson [University of California, Santa Barbara
We use the self-interaction corrected local spin-density approximation to investigate the ground state valency configuration of transition metal (TM=Mn, Co) impurities in n- and p-type ZnO. We find that in pure Zn{sub 1-x}TM{sub x}O, the localized TM{sup 2+} configuration is energetically favored over the itinerant d-electron configuration of the local spin density (LSD) picture. Our calculations indicate furthermore that the (+/0) donor level is situated in the ZnO gap. Consequently, for n-type conditions, with the Fermi energy {epsilon}F close to the conduction band minimum, TM remains in the 2+ charge state, while for p-type conditions, with {epsilon}F close to the valence band maximum, the 3+ charge state is energetically preferred. In the latter scenario, modeled here by co-doping with N, the additional delocalized d-electron charge transfers into the entire states at the top of the valence band, and hole carriers will only exist, if the N concentration exceeds the TM impurity concentration.
Surface Metal-Insulator Transition on a Vanadium Pentoxide (001) Single Crystal R.-P Blum,1,2
Peters, Achim
phases. In addition, several vanadium oxides undergo metal-to-insulator transitions (MIT), e.g., V2O3Surface Metal-Insulator Transition on a Vanadium Pentoxide (001) Single Crystal R.-P Blum,1,2 H and a concomitant oxygen loss driven metal-to-insulator transition at the surface. At elevated temperatures
Calculated Momentum Dependence of Zhang-Rice States in Transition Metal Oxides Alexey Gordienko,1,2
Savrasov, Sergej Y.
Calculated Momentum Dependence of Zhang-Rice States in Transition Metal Oxides Quan Yin,1 Alexey in insulating transition metal oxides (TMOs) such as classical MottŁHubbard systems or undoped high temperature to obtain accurate spectra of transition metal oxides and, in particular, describe full momentum dependent
Comprehensive study of the metal-insulator transition in pulsed laser deposited epitaxial VO2 thin films
Wu, Junqiao
://dx.doi.org/10.1063/1.4788804] I. INTRODUCTION Vanadium oxides exhibiting metal-insulator transition (MITComprehensive study of the metal-insulator transition in pulsed laser deposited epitaxial VO2 thin properties of high-quality VO2 thin films across its metal-insulator phase transition. Detailed x
Electronic structure of mixed valence transition metal oxides Institute of Physics, Academy of Sciences of the Czech Republic,
Tebbens, Jurjen Duintjer
Electronic structure of mixed valence transition metal oxides P. Nov┤ak1 1 Institute of Physics: 1 #12;I. INTRODUCTION The character of the charge carriers in oxides of transition metals belongs repeatedly tried to understand the physics of the mixed valence transition metal oxides. Number of results
Oxidation energies of transition metal oxides within the GGA+U framework Lei Wang, Thomas Maxisch, and Gerbrand Ceder*
Ceder, Gerbrand
Oxidation energies of transition metal oxides within the GGA+U framework Lei Wang, Thomas Maxisch; published 4 May 2006 The energy of a large number of oxidation reactions of 3d transition metal oxides error makes it possible to address the correlation effects in 3d transition metal oxides with the GGA
Electronic Structure of Transition Metals. I. Quantum Defects and Model Potential
A. O. Animalu
Recent applications of the concept of quantum defects in setting up model pseudopotentials for simple or transition-metal ions presuppose that the atomic-spectroscopy data of such metals can be expressed in terms of certain quantum defects. Experience indicates, however, that the old quantum-defect idea applies only to group I and II metals at best, but not to metals of higher chemical
Late transition metal-oxo compounds and open-framework materials that catalyze aerobic oxidations
Rui Cao; Jong Woo Han; Travis M. Anderson; Daniel A. Hillesheim; Kenneth I. Hardcastle; Elena Slonkina; Britt Hedman; Keith O. Hodgson; Martin L. Kirk; Djamaladdin G. Musaev; Keiji Morokuma; Yurii V. Geletii; Craig L. Hill
After decades of speculation and attempted synthesis, terminal metal-oxo compounds of the late transition metal elements have been prepared. Terminal Pt-, Pd-, and Au-oxo complexes have been made using polytungstate ligands that share geometrical and electronic structural features and chemical properties (acid-base and others) in common with the metal oxides used as supports for these metals in myriad industrial catalytic
The Early College Challenge: Navigating Disadvantaged Students' Transition to College
ERIC Educational Resources Information Center
Rosenbaum, James E.; Becker, Kelly Iwanaga
Successful early college high schools (ECHSs) are formed through partnerships between high schools and colleges (usually community colleges). Think of it as preparation through acceleration. ECHSs enroll disadvantaged students who have not excelled with ordinary grade-level academic content and have them take college courses while still in highģ
Theory of elastic phase transitions in metals at high pressures. Application to vanadium
Krasil'nikov, O. M., E-mail: firstname.lastname@example.com; Vekilov, Yu. Kh.; Isaev, E. I.; Bondarenko, N. G. [National Research Technological University MISiS (Russian Federation)
Structural transformations in elementary metals under high pressures are considered using the Landau theory of phase transitions, in which the finite strain tensor components play the role of the order parameter. As an example, the phase transition in vanadium observed at a pressure of 69 GPa is analyzed. It is shown that it is a first-order elastic phase transition, which is close to a second-order transition.
Transition metal ions: charge carriers that mediate the electron capture dissociation pathways of peptides.
Chen, Xiangfeng; Fung, Yi Man Eva; Chan, Wai Yi Kelly; Wong, Pui Shuen; Yeung, Hoi Sze; Chan, T-W Dominic
Electron capture dissociation (ECD) of model peptides adducted with first row divalent transition metal ions, including Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+), were investigated. Model peptides with general sequence of ZGGGXGGGZ were used as probes to unveil the ECD mechanism of metalated peptides, where X is either V or W; and Z is either R or N. Peptides metalated with different divalent transition metal ions were found to generate different ECD tandem mass spectra. ECD spectra of peptides metalated by Mn(2+) and Zn(2+) were similar to those generated by ECD of peptides adducted with alkaline earth metal ions. Series of c-/z-type fragment ions with and without metal ions were observed. ECD of Fe(2+), Co(2+), and Ni(2+) adducted peptides yielded abundant metalated a-/y-type fragment ions; whereas ECD of Cu(2+) adducted peptides generated predominantly metalated b-/y-type fragment ions. From the present experimental results, it was postulated that electronic configuration of metal ions is an important factor in determining the ECD behavior of the metalated peptides. Due presumably to the stability of the electronic configuration, metal ions with fully-filled (i.e., Zn(2+)) and half filled (i.e., Mn(2+)) d-orbitals might not capture the incoming electron. Dissociation of the metal ions adducted peptides would proceed through the usual ECD channel(s) via "hot-hydrogen" or "superbase" intermediates, to form series of c-/z(Ģ)- fragments. For other transition metal ions studied, reduction of the metal ions might occur preferentially. The energy liberated by the metal ion reduction would provide enough internal energy to generate the "slow-heating" type of fragment ions, i.e., metalated a-/y- fragments and metalated b-/y- fragments. PMID:21952786
X-ray absorption spectra for transition metal high-dielectrics: Final state differences for intra-and inter-atomic transitions
in transition metal TM and lanthanide series rare earth RE atom oxides, and their silicate and aluminate alloysX-ray absorption spectra for transition metal high- dielectrics: Final state differences for intra- and inter-atomic transitions G. Lucovskya) Departments of Physics, Materials Science and Engineering
Quantum phase transition between orbital-selective Mott states in Hund's metals
NASA Astrophysics Data System (ADS)
Rinc¾n, Julißn; Moreo, Adriana; Alvarez, Gonzalo; Dagotto, Elbio
We report a quantum phase transition between orbital-selective Mott states, with different localized orbitals, in a Hund's metals model. Using the density matrix renormalization group, the phase diagram is constructed varying the electronic density and Hubbard U , at robust Hund's coupling. We demonstrate that this transition is preempted by charge fluctuations and the emergence of free spinless fermions, as opposed to the magnetically driven Mott transition. The Luttinger correlation exponent is shown to have a universal value in the strong-coupling phase, whereas it is interaction dependent at intermediate couplings. At weak coupling we find a second transition from a normal metal to the intermediate-coupling phase.
Scanning Tunneling Microscopy of the Transition Metal Dichalcogenide Tantalum Disulfide.
NASA Astrophysics Data System (ADS)
Han, Wenhai
The transition metal dichalcogenide tantalum disulfide (TaS_2) in the 2H, 1T, and 4Hb polytypes has been systematically studied with a scanning tunneling microscope (STM), typically operating in air at room temperature. A variety of atomic and electronic structures on the surfaces of those materials have been observed in real space at the atomic scale. The 2H polytype was intercalated with silver and found to have two different structures on the surface of the stage-2 compound. One clearly shows a 2 x 1 superlattice, while the other demonstrates the same hexagonal pattern as appearing on pure 2H-TaS_2. The 2 x 1 superlattice is interpreted as a possible surface reconstruction caused by Ag ions intercalated in the first van der Waals gap underneath the surface. On 1T-TaS_2, charge density waves (CDWs) have been studied with many different Pt/Ir, W, Ni, and Co tips. Images of CDWs with strong atomic modulation, as well as those with weakly superimposed atomic modulation, have been observed with all different kinds of tips. Changes from one kind of CDW image to the other under the same conditions are also observed. The different CDW images are believed to be caused by the different surface electronic states of the tip. These observations are consistent with the atomic-orbital STM theory. The H and T-layer mixed structure of 4Hb-TaS _2 exhibits mixed atomic and electronic structures. The T-type surface shows a commensurate sqrt {13} x sqrt{13} CDW superlattice, independent of sample bias, while the H-type surface appears bias-dependent. Increase of the positive sample bias from ~50 mV to ~170 mV results in a continuous change of images from a dominant atomic lattice with a very weak CDW modulation superimposed to a fully developed commensurate sqrt{13} x sqrt{13} CDW superlattice. With a negative bias (down to -300 mV), however, there is hardly any sign of a CDW modulation. The bias -dependent behavior is thought to be associated with the strong interaction between the electronic structure of the sample and that of the tip. STM observations of the CDWs on 4Hb-TaS _2 over 315 K indicate that the phase transition at 315 K involves CDW correlations among the T-type layers.
Properties of molecular beam epitaxy grown Eu{sub x}(transition metal){sub y} films (transition metals: Mn, Cr)
Balin, K. [A. Chelkowski Institute of Physics, University of Silesia, Katowice, 40-007 (Poland); Center for Magnetism and Magnetic Nanostructures, University of Colorado at Colorado Springs, Colorado Springs, Colorado 80918 (United States); Nowak, A. [A. Chelkowski Institute of Physics, University of Silesia, Katowice, 40-007 (Poland); Laboratoire de Physique de l'Etat Condense, University du Maine, Le Mans Cedex, 72085 (France); Gibaud, A. [Laboratoire de Physique de l'Etat Condense, University du Maine, Le Mans Cedex, 72085 (France); Szade, J. [A. Chelkowski Institute of Physics, University of Silesia, Katowice, 40-007 (Poland); Celinski, Z. [Center for Magnetism and Magnetic Nanostructures, University of Colorado at Colorado Springs, Colorado Springs, Colorado 80918 (United States)
The electronic and crystallographic structures, as well as the magnetic properties, of Eu{sub x}(transition metal){sub y} (transition metals: Mn, Cr) thin films grown by molecular beam epitaxy were studied. Relative changes of the Eu/Mn and Eu/Cr ratios derived from the XPS lines, as well as x-ray reflectivity, indicate mixing of the Eu/Mn and Eu/Cr layers. Valency transitions from Eu{sup 2+} to Eu{sup 3+} were observed in both systems for most studied stoichiometries. A transition to a magnetically ordered phase was observed at 15 K, 40 K, and 62 K for selected films in the Eu-Mn system, and at 50 K for the film with a Eu/Cr ratio of 0.5.
Functionalized Silicone Nanospheres: Synthesis, Transition Metal Immobilization, and Catalytic Applications
Bradley, Christopher A.; Yuhas, Benjamin D.; McMurdo, Meredith J.; Tilley, T. D.
Silicone nanospheres containing a variety of functional groups (pyridines, phosphines, thiols, amines, etc.) have been prepared by emulsion copolymerization of methyltrimethoxysilane, MeSi(OMe)3, and the functionalized monomer of interest, RSi(OMe)3. This procedure provides a reproducible synthesis of spherical particles in the 12-28 nm size regime as determined by transmission electronSilicone nanospheres containing a variety of functional groups (pyridines, phosphines, thiols, amines, etc.) have been prepared by emulsion copolymerization of methyltrimethoxysilane, MeSi(OMe)?, and the functionalized monomer of interest, RSi(OMe)?. This procedure provides a reproducible synthesis of spherical particles in the 12-28 nm size regime as determined by transmission electron microscopy (TEM). The presence of the functional groups is supported by a combination of spectroscopic methods including DRUV-vis, DRIFTS, and NMR spectroscopy. Comonomer dispersity within the nanospheres was probed using elemental mapping techniques, and these support a homogeneous distribution of functional groups within the particles. Palladium(0) immobilization on phosphine-substituted nanospheres also results in a random distribution of the transition metal throughout the particles. Nanospheres containing multiple acid/base functionalities were also prepared, and these demonstrate functional group cooperativity based on enhanced conversions in the base-catalyzed Henry reaction, relative to nanosphere catalysts containing only basic groups. The diversity of functional groups that may be incorporated into the spheres suggests that these materials hold considerable promise as ligand supports and catalysts.Graphene nanoribbons (GNRs) have been suggested as a promising material for its use as nanoelectromechanical reasonators for highly sensitive force, mass, and charge detection. Therefore the accurate determination of the size-dependent elastic properties of GNRs is desirable for the design of graphene-based nanoelectromechanical devices. In this study we determine the size-dependent YoungÆs modulus and carbon-carbon binding energy in a homologous series of GNRs, C4n2+6n+2H6n+4 (n=2¢12), with the use of all electron first principles computations. An unexpected linearity between the binding energy and YoungÆs modulus is observed, making possible the prediction of the size-dependent YoungÆs modulus of GNRs through a single point energy calculation of the GNR ground state. A quantitative-structure-property relationship is derived, which correlates YoungÆs modulus to the total energy and the number of carbon atoms within the ribbon. In the limit of extended graphene sheets we determine the value of YoungÆs modulus to be 1.09 TPa, in excellent agreement with experimental estimates derived for graphite and suspended grapheme sheets. microscopy (TEM). The presence of the functional groups is supported by a combination of spectroscopic methods including DRUV-vis, DRIFTS, and NMR spectroscopy. Comonomer dispersity within the nanospheres was probed using elemental mapping techniques, and these support a homogeneous distribution of functional groups within the particles. Palladium(0) immobilization on phosphine-substituted nanospheres also results in a random distribution of the transition metal throughout the particles. Nanospheres containing multiple acid/base functionalities were also prepared, and these demonstrate functional group cooperativity based on enhanced conversions in the base-catalyzed Henry reaction, relative to nanosphere catalysts containing only basic groups. The diversity of functional groups that may be incorporated into the spheres suggests that these materials hold considerable promise as ligand supports and catalysts.
LETTER TO THE EDITOR: Structural phase transition accompanied by metal-insulator transition in PrRu4P12
C. H. Lee; H. Matsuhata; A. Yamamoto; T. Ohta; H. Takazawa; K. Ueno; C. Sekine; I. Shirotani; T. Hirayama
A structural phase transition has been found using electron diffraction technique in PrRu4P12 accompanied by a metal-insulator (M-I) transition (TMI = 60 K). Weak superlattice spots appeared at (H,K,L) (H + K + L = odd) position at a temperature of T = 12 K and 40 K. Above T = 70 K, the spots completely vanished. The space group
Electronic and geometric structure of transition-metal nanoclusters
Jennison, D.R.; Schultz, P.A.; Sears, M.P.; Klitsner, T.
A massively-parallel ab initio computer code, which uses Gaussian bases, pseudopotentials, and the local density approximation, permits the study of transition-metal systems with literally hundreds of atoms. We present total energies and relaxed geometries for Ru, Pd, and Ag clusters with N = 55, 135, and 140 atoms; we also used the DMOL code to study 13-atom Pd and Cu clusters, with and without hydrogen. The N = 55 and 135 clusters were chosen because of simultaneous cubo-octahedral (fcc) and icosahedral (icos) sub-shell closings, and we find icos geometries are preferred. Remarkably large compressions of the central atoms are observed for the icos structures (up to 6% compared with bulk interatomic spacings), while small core compressions ({approx} 1 %) are found for the fcc geometry. In contrast, large surface compressive relaxations are found for the fcc clusters ({approx} 2-3% in average nearest neighbor spacing), while the icos surface displays small compressions ({approx} 1%). Energy differences between icos and fcc are smallest for Pd, and for all systems the single-particle densities of states closely resembles bulk results. Calculations with N = 134 suggest slow changes in relative energy with N. Noting that the 135-atom fcc has a much more open surface than the icos, we also compare N = 140 icos and fcc, the latter forming an octahedron with close packed facets. These icos and fcc clusters have identical average coordinations and the octahedron is found to be preferred for Ru and Pd but not for Ag. Finally, we compare Harris functional and LDA energy differences on the N = 140 clusters, and find fair agreement only for Ag.
The Use of Soluble Polyolefins as Supports for Transition Metal CatalystsĀ
Hobbs, Christopher Eugene
lab has had interest in the development of soluble polymer-supports for transition metal catalysts. We have developed several separation methods for these soluble polymer-bound catalysts. These include thermomorphic liquid/liquid and solid...
First-principles density functional theory study of sulfur oxide chemistry on transition metal surfaces
Lin, Xi, 1973-
In this thesis, the chemistry of sulfur oxides on transition metals is studied extensively via first-principles density functional theory (DFT) computations, focusing on the chemical reactivity and selectivity in sulfur ...
Activation of methane by transition metal-substituted aluminophosphate molecular sieves
Iton, Lennox E. (Downers Grove, IL); Maroni, Victor A. (Naperville, IL)
Aluminophosphate molecular sieves substituted with cobalt, manganese or iron and having the AlPO.sub.4 -34 or AlPO.sub.4 -5, or related AlPO.sub.4 structure activate methane starting at approximately 350.degree. C. Between 400.degree. and 500.degree. C. and at methane pressures .ltoreq.1 atmosphere the rate of methane conversion increases steadily with typical conversion efficiencies at 500.degree. C. approaching 50% and selectivity to the production of C.sub.2+ hydrocarbons approaching 100%. The activation mechanism is based on reduction of the transition metal(III) form of the molecular sieve to the transition metal(II) form with accompanying oxidative dehydrogenation of the methane. Reoxidation of the - transition metal(II) form to the transition metal(III) form can be done either chemically (e.g., using O.sub.2) or electrochemically.
Cyanide Bridged Molecular Magnetic Materials with Anisotropic Transition Metal Ions: Investigation of Bistable Magnetic Phenomena
Avendano, Carolina
II PB derivatives of OsIII were found to exhibit photomagnetic and charge transfer induced spin transition (CTIST) behavior and a study of alkali metal cation dependence revealed marked differences in both the photomagnetic and CTIST properties...
Pressure Induced Metal Insulator Phase Transition in Eu2Ir2O7
Fazel Fallah Tafti; Jun Ishikawa; Yo Machida; Alix McCollam; Satoru Nakatsuji; Stephen Julian
The metal to insulator phase transition of the pyrochlore iridate Eu2Ir2O7 has been studied by means of resistivity measurements under pressure in the range 2 to 12 GPa. At ambient pressure, the system is a \\
Reactivity of halide and pseudohalide ligands in transition-metal complexes
Kukushkin, Yu.N.; Kukushkin, V.Yu.
The experimental material on the reactions of coordinated halide ligands, as well as cyanide, azido, thiocyanato, and cyanato ligands, in transition-metal complexes has been generalized in this review.
Zwitterionic late transition metal alkene polymerisation catalysts containing aminofulvene-aldiminate (AFA) ligandsĀ
Rahman, Mohammed Mahmudur
Over recent years significant progress has been made in the design and development of late transition metal cationic catalysts for olefin polymerisation. Never-the-less, the activation of catalyst precursors and generation ...
Selective exchange of divalent transition metal ions in cryptomelane-type manganic acid with tunnel structure
Tsuji, M. (Department of Chemistry, Faculty of Science, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152 (Japan)); Komarneni, S. (Materials Research Laboratory and Department of Agronomy, The Pennsylvania State University, University Park, Pennsylvania 16802-4801 (United States))
The ion-exchange selectivity of divalent transition metal ions on cryptomelane-type manganic acid (CMA) with tunnel structure has been studied using the distribution coefficients ([ital K][sub [ital d
Hydrothermal syntheses, structures and magnetic properties of two transition metal coordination polymers with a square
Gao, Song
bonding, resulting in a 3-D supramolecular architecture. Magnetic susceptibility studies reveal reserved. Keywords: Hydrothermal synthesis; Coordination polymer; Magnetic property; Transition metal analysis was performed with a ZRY-2P Thermal Analyzer. The magnetic susceptibility measurements were
Multi-body forces and the energetics of transition metals, alloys, and semiconductors
Carlsson, A.E.
Progress over the past year is divided into 3 areas: potential-energy functions for transition-metal aluminides; electronic structure and energetics of complex structures and quasicrystals; and ceramic materials (PdO, PtO).
Tuning Magnetic Order in Transition Metal Oxide Thin Films
NASA Astrophysics Data System (ADS)
Grutter, Alexander John
In recent decades, one of the most active and promising areas of condensed matter research has been that of complex oxides. With the advent of new growth techniques such as pulsed laser deposition and molecular beam epitaxy, a wealth of new magnetic and electronic ground states have emerged in complex oxide heterostructures. The wide variety of ground states in complex oxides is well known and generally attributed to the unprecedented variety of valence, structure, and bonding available in these systems. The tunability of this already diverse playground of states and interactions is greatly multiplied in thin films and heterostructures by the addition of parameters such as substrate induced strain and interfacial electronic reconstruction. Thus, recent studies have shown emergent properties such as the stabilization of ferromagnetism in a paramagnetic system, conductivity at the interface of two insulators, and even exchange bias at the interface between a paramagnet and a ferromagnet. Despite these steps forward, there remains remarkable disagreement on the mechanisms by which these emergent phenomena are stabilized. The contributions of strain, stoichiometry, defects, intermixing, and electronic reconstruction are often very difficult to isolate in thin films and superlattices. This thesis will present model systems for isolating the effects of strain and interfacial electronic interactions on the magnetic state of complex oxides from alternative contributions. We will focus first on SrRuO3, an ideal system in which to isolate substrate induced strain effects. We explore the effects of structural distortions in the simplest case of growth on (100) oriented substrates. We find that parameters including saturated magnetic moment and Curie temperature are all highly tunable through substrate induced lattice distortions. We also report the stabilization of a nonmagnetic spin-zero configuration of Ru4+ in tetragonally distorted films under tensile strain. Through growth on (110) and (111) oriented substrates we explore the effects of different distortion symmetries on SrRuO3 and demonstrate the first reported strain induced transition to a high-spin state of Ru 4+. Finally, we examine the effects of strain on SrRuO3 thin films and demonstrate a completely reversible universal out-of-plane magnetic easy axis on films grown on different substrate orientations. Having demonstrated the ability to tune nearly every magnetic parameter of SrRuO 3 through strain, we turn to magnetic properties at interfaces. We study the emergent interfacial ferromagnetism in superlattices of the paramagnetic metal CaRuO3 and the antiferromagnetic insulator CaMnO3 and demonstrate that the interfacial ferromagnetic layer in this system is confined to a single unit cell of CaMnO3 at the interface. We discuss the remarkable oscillatory dependence of the saturated magnetic moment on the thickness of the CaMnO3 layers and explore mechanisms by which this oscillation may be stabilized. We find long range coherence of the antiferromagnetism of the CaMnO3 layers across intervening layers of paramagnetic CaRuO3. Finally, we utilize the system of LaNiO3/CaMnO3 to separate the effects of intermixing and interfacial electronic reconstruction and conclusively demonstrate intrinsic interfacial ferromagnetism at the interface between a paramagnetic metal and an antiferromagnetic insulator. We find that the emergent ferromagnetism is stabilized through interfacial double exchange and that the leakage of conduction electrons from the paramagnetic metal to the antiferromagnetic insulator is critical to establishing the ferromagnetic ground state.
Conversion and displacement reaction types of transition metal compounds for sodium ion battery
NASA Astrophysics Data System (ADS)
Chen, Guo-Ying; Sun, Qian; Yue, Ji-Li; Shadike, Zulipiya; Yang, Yin; Ding, Fei; Sang, Lin; Fu, Zheng-Wen
Transition metal compounds of FeSe and CuWO4 thin films have been successfully fabricated by using R.F. sputtering method. Although two kinds of transition metal compounds of FeSe and CuWO4 thin films can react with sodium electrochemically, they exhibit different electrochemical features. The nanosized metal Fe is highly dispersed into Na2Se matrix and metal Cu is extruded from Na2WO4 mixture after the FeSe/Na and CuWO4/Na cells are discharged, respectively. The conversion reaction mechanism between FeSe and Na2Se is proposed for the FeSe/Na cell. While the displacement reaction mechanism for CuWO4/Na cell is proposed for the first time based on the transmission electron microscopy (TEM) and selected area electron diffraction (SAED) data. These various mechanisms make transition metal compounds interesting materials for rechargeable sodium ion batteries.
Bridged transition-metal complexes and uses thereof for hydrogen separation, storage and hydrogenation
Lilga, M.A.; Hallen, R.T.
The present invention constitutes a class of organometallic complexes which reversibly react with hydrogen to form dihydrides and processes by which these compounds can be utilized. The class includes bimetallic complexes in which two cyclopentadienyl rings are bridged together and also separately [pi]-bonded to two transition metal atoms. The transition metals are believed to bond with the hydrogen in forming the dihydride. Transition metals such as Fe, Mn or Co may be employed in the complexes although Cr constitutes the preferred metal. A multiple number of ancillary ligands such as CO are bonded to the metal atoms in the complexes. Alkyl groups and the like may be substituted on the cyclopentadienyl rings. These organometallic compounds may be used in absorption/desorption systems and in facilitated transport membrane systems for storing and separating out H[sub 2] from mixed gas streams such as the producer gas from coal gasification processes. 3 figs.
Preparation of transition metal nanoparticles and surfaces modified with (CO)polymers synthesized by RAFT
McCormick, III., Charles L.; Lowe, Andrew B.; Sumerlin, Brent S.
A new, facile, general one-phase method of generating thio-functionalized transition metal nanoparticles and surfaces modified by (co)polymers synthesized by the RAFT method is described. The method includes the stops of forming a (co)polymer in aqueous solution using the RAFT methodology, forming a colloidal transition metal precursor solution from an appropriate transition metal; adding the metal precursor solution or surface to the (co)polymer solution, adding a reducing agent into the solution to reduce the metal colloid in situ to produce the stabilized nanoparticles or surface, and isolating the stabilized nanoparticles or surface in a manner such that aggregation is minimized. The functionalized surfaces generated using these methods can further undergo planar surface modifications, such as functionalization with a variety of different chemical groups, expanding their utility and application.
Preparation of transition metal nanoparticles and surfaces modified with (co)polymers synthesized by RAFT
McCormick, III, Charles L. (Hattiesburg, MS); Lowe, Andrew B. (Hattiesburg, MS); Sumerlin, Brent S. (Pittsburgh, PA)
A new, facile, general one-phase method of generating thiol-functionalized transition metal nanoparticles and surfaces modified by (co)polymers synthesized by the RAFT method is described. The method includes the steps of forming a (co)polymer in aqueous solution using the RAFT methodology, forming a colloidal transition metal precursor solution from an appropriate transition metal; adding the metal precursor solution or surface to the (co)polymer solution, adding a reducing agent into the solution to reduce the metal colloid in situ to produce the stabilized nanoparticles or surface, and isolating the stabilized nanoparticles or surface in a manner such that aggregation is minimized. The functionalized surfaces generated using these methods can further undergo planar surface modifications, such as functionalization with a variety of different chemical groups, expanding their utility and application.
Bridged transition-metal complexes and uses thereof for hydrogen separation, storage and hydrogenation
Lilga, Michael A. (Richland, WA); Hallen, Richard T. (Richland, WA)
The present invention constitutes a class of organometallic complexes which reversibly react with hydrogen to form dihydrides and processes by which these compounds can be utilized. The class includes bimetallic complexes in which two cyclopentadienyl rings are bridged together and also separately .pi.-bonded to two transition metal atoms. The transition metals are believed to bond with the hydrogen in forming the dihydride. Transition metals such as Fe, Mn or Co may be employed in the complexes although Cr constitutes the preferred metal. A multiple number of ancilliary ligands such as CO are bonded to the metal atoms in the complexes. Alkyl groups and the like may be substituted on the cyclopentadienyl rings. These organometallic compounds may be used in absorption/desorption systems and in facilitated transport membrane systems for storing and separating out H.sub.2 from mixed gas streams such as the product gas from coal gasification processes.
Bridged transition-metal complexes and uses thereof for hydrogen separation, storage and hydrogenation
Lilga, Michael A. (Richland, WA); Hallen, Richard T. (Richland, WA)
The present invention constitutes a class of organometallic complexes which reversibly react with hydrogen to form dihydrides and processes by which these compounds can be utilized. The class includes bimetallic complexes in which two cyclopentadienyl rings are bridged together and also separately .pi.-bonded to two transition metal atoms. The transition metals are believed to bond with the hydrogen in forming the dihydride. Transition metals such as Fe, Mn or Co may be employed in the complexes although Cr constitutes the preferred metal. A multiple number of ancilliary ligands such as CO are bonded to the metal atoms in the complexes. Alkyl groups and the like may be substituted on the cyclopentadienyl rings. These organometallic compounds may be used in absorption/desorption systems and in facilitated transport membrane systems for storing and separating out H.sub.2 from mixed gas streams such as the produce gas from coal gasification processes.
Quantum-to-classical transition for fluctuations in the early Universe
Claus Kiefer; David Polarski; A. A. Starobinsky
According to the inflationary scenario for the very early Universe, all\\u000ainhomogeneities in the Universe are of genuine quantum origin. On the other\\u000ahand, looking at these inhomogeneities and measuring them, clearly no specific\\u000aquantum mechanical properties are observed. We show how the transition from\\u000atheir inherent quantum gravitational nature to classical behaviour comes about\\u000a-- a transition whereby none
New solid state routes to lithium transition metal oxides via reactions with lithium oxide
Marco D. Aguas; Graham C. Coombe; Ivan P. Parkin
Thermolysis of a mixture of lithium oxide and transition metal halides MXn (M = Ti, Zr, Hf, V, Nb, Ta, Mo, W, Fe, Mn) at 800░C for 2¢12 h produces crystalline lithium transition metal oxides Li2MO3 (M = Ti, Zr, Hf, Mn), Li3MO4 (M = V, Nb, Ta), Li2MnO3 Li2Fe3O4 and Li2MO4 (M = Mo, W). For M = Cr,
The emergence of sulfoxides as efficient ligands in transition metal catalysis.
Sipos, Gellķrt; Drinkel, Emma E; Dorta, Reto
Sulfoxides are capable of forming stable complexes with transition metals and there have been many comprehensive studies into their binding properties. However, the use of sulfoxides, particularly chiral sulfoxides, as ligands in transition metal catalysis is rather less well developed. This review aims to describe these catalytic studies and covers new developments that are showing very promising results and that have led to a renewed interest in this field. PMID:25954773
X-Ray absorption spectroscopy of transition metal¢magnesium hydride thin films
T. J. Richardson; B. Farangis; J. L. Slack; P. Nachimuthu; R. Perera; N. Tamura; M. Rubin
Mixed metal thin films containing magnesium and a first-row transition element exhibit very large changes in both reflectance and transmittance on exposure to hydrogen gas. Changes in electronic structure and coordination of the magnesium and transition metal atoms during hydrogen absorption were studied using dynamic in situ transmission mode X-ray absorption spectroscopy. Mg K-edge and Ni, Co, and Ti L-edge
Molecular orbital calculations on transition metal dimers containing a linear dinitrogen bridge
Powell, Mary Cynthia
MOLECULAR ORBITAL CALCULATIONS ON TRANSITION METAL DIMERS CONTAINING A LINEAR DINITROGEN BRIDGE A Thesis by MARY CYNTHIA POWELL Submitted to the Graduate College of Texas A&M University in partial fulfillment of the requirements... for the degree of MASTER OF SCIENCE May 1984 Major Subject: Chemistry MOLECULAR ORBITAL CALCULATIONS ON TRANSITION METAL DIMERS CONTAINING A LINEAR DINITROGEN BRIDGE A Thesis by MARY CYNTHIA POWELL Approved as to style and content by: Michael B. Hall...
Electronic Structure of Transition Metal Clusters, Actinide Complexes and Their Reactivities
Krishnan Balasubramanian
This is a continuing DOE-BES funded project on transition metal and actinide containing species, aimed at the electronic structure and spectroscopy of transition metal and actinide containing species. While a long term connection of these species is to catalysis and environmental management of high-level nuclear wastes, the immediate relevance is directly to other DOE-BES funded experimental projects at DOE-National labs
Controlled synthesis of transition metal/conducting polymer nanocomposites.
Liu, Zhen; Liu, Yang; Zhang, Lin; Poyraz, Selcuk; Lu, Ning; Kim, Moon; Smith, James; Wang, Xiaolong; Yu, Yajiao; Zhang, Xinyu
A novel displacement reaction has been observed to occur between conducting polymers (CP) and metal salts which can be used to fabricate nanostructured CP-metal composites in a one-pot manner. Vanadium pentoxide (V(2)O(5)) nanofiber is used during the synthesis as the reactive seeds to induce the nanofibril CP-metal network formation. The CP-metal nanocomposites exhibit excellent sensory properties for hydrogen peroxide (H(2)O(2)) detection, where both high sensitivity and a low detection limit can be obtained. The sensory performance of the CP-metal composite can be further enhanced by a facile microwave treatment. It is believed that the CP-metal nanofibril network can be converted to a carbon-metal network by a microwave-induced carbonization process and result in the sensory enhancement. PMID:22842608
Gettering of transition metal impurities during phosphorus emitter diffusion in multicrystalline silicon solar cell processing
Bentzen, A.; Holt, A.; Kopecek, R.; Stokkan, G.; Christensen, J.S.; Svensson, B.G. [Section for Renewable Energy, Institute for Energy Technology, P.O. Box 40, NO-2027 Kjeller (Norway); Department of Physics, University of Konstanz, Jakob-Burckhardt-Strasse 27, D-78464 Constance (Germany); Department of Materials Technology, Norwegian University of Science and Technology, Alfred Getz vei 2, NO-7491 Trondheim (Norway); Department of Physics, University of Oslo, P.O. Box 1048 Blindern, No-0316 Oslo, Norway and Centre for Materials Science and Nanotechnology, University of Oslo, P.O. Box 1048 Blindern, NO-0316 Oslo (Norway)
We have investigated the gettering of transition metals in multicrystalline silicon wafers during a phosphorus emitter diffusion for solar cell processing. The results show that mainly regions of high initial recombination lifetime exhibit a significant lifetime enhancement upon phosphorus diffusion gettering. Nevertheless, transition metal profiles extracted by secondary ion mass spectrometry in a region of low initial lifetime reveal significant gradients in Cr, Fe, and Cu concentrations towards the surface after the emitter diffusion, without exhibiting a significant enhancement in the lifetime. In a region of higher initial lifetime, however, diminutive concentration gradients of the transition metal impurities are revealed, indicating a significantly lower initial concentration in these regions. From spatial maps of the dislocation density in the wafers, we find that lifetime enhancements mainly occur in regions of low dislocation density. Thus, it is believed that a generally higher concentration of transition metals combined with an impurity decoration of dislocations in regions of high dislocation density limit the initial lifetime and the lifetime after the phosphorus diffusion, in spite of the notable gettering of transition metal impurities towards the surface in these regions. Furthermore, after a hydrogen release from overlying silicon nitride layers, we observe that only regions of low dislocation density experience a significant lifetime enhancement. This is attributed to impurity decoration of the dislocations in the regions of both high dislocation density and high transition metal impurity concentration, reducing the ability of hydrogen to passivate dislocations in these regions.
Early warnings and missed alarms for abrupt monsoon transitions
NASA Astrophysics Data System (ADS)
Thomas, Zoe; Kwasniok, Frank; Boulton, Chris; Cox, Peter; Jones, Richard; Lenton, Timothy; Turney, Chris
Palaeo-records from China demonstrate that over millennial timescales the East Asian Summer Monsoon (EASM) is dominated by abrupt and large magnitude monsoon shifts, switching between periods of high and weak monsoon rains. It has been hypothesised that over these timescales, the EASM exhibits two stable states with bifurcation-type tipping points between them. Here we test this hypothesis by looking for early warning signals of past bifurcations in a speleothem record from Sanbao Cave, China, spanning the penultimate glacial cycle, and in multiple simulations of a model derived from the data. We detect critical slowing down prior to an abrupt monsoon shift at the penultimate deglaciation, but such signals are only detectable when the change in system stability is sufficiently slow to be detected by the sampling resolution of the dataset.
Stellar populations of early-type galaxies in different environments II. Ages and metallicities
P. Sanchez-Blazquez; J. Gorgas; N. Cardiel; J. J. Gonzalez
This is the second paper of a series devoted to study the stellar content of early-type galaxies. The goal of the series is to set constraints on the evolutionary status of these objects. We use a new set of models which include an improved stellar library (MILES) to derive simple stellar population (SSP)-equivalent parameters in a sample of 98 early-type galaxies. The sample contains galaxies in the field, poor groups, and galaxies in the Virgo and Coma clusters.We find that low-density environment galaxies span a larger range in SSP age and metallicity than their counterparts in high density environments, with a tendency for lower sigma galaxies to be younger. Early-type galaxies in low-density environments appear on average ~1.5 Gyr younger and more metal rich than their counterparts in high density environments. The sample of low-density environment galaxies shows an age metallicity relation in which younger galaxies are found to be more metal rich, but only when metallicity is measured with a Fe-sensitive index. Conversely, there is no age-metallicity relation when the metallicity is measured with a Mg sensitive index. The mass-metallicity relation is only appreciable for the low-density environment galaxies when the metallicity is measured with a Mg-sensitive index and not when the metallicity is measured with other indicators. On the contrary, this relation exists for the high-density environment galaxies independently of the indicator used to measure the metallicity. This suggests a dependence of the mass-metallicity relation on the environment of the galaxies. Our data favour a scenario in which galaxies in low density environments have suffered a more extended star formation history than the galaxies in the Coma cluster, which appear to host more homogenous stellar populations.
Pressure effect on phonon frequencies in some transition metals: A molecular dynamics study
S. Kazanc; S. Ozgen
It is important to determine the atomic lattice vibrations of metallic materials, under high-pressure conditions, due to its effects on material properties such as thermal, electrical and optical conductions. In this work, we have investigated the changes of acoustic phonon frequencies with hydrostatic pressure for Cu, Ni, Al, Ag and Au transition metals, using molecular dynamics (MD) simulations based on
K. A. Gesheva; T. Ivanov; F. Hamelmann
The paper presents the recent study on the technology and investigation of thin metal oxide films based on transition metals, such as W and Mo. Defined is the application aspect of the research, by describing the optical systems based on these films, namely the electrochromic device and the X-ray mirror. Results on the optical absorption in the films are presented,
A new nonhydrolytic single-precursor approach to surfactant-capped nanocrystals of transition metal oxides
J÷rg Rockenberger; Erik C. Scher; A. Paul Alivisatos
The advent of new methods to prepare semiconductor and metal nanocrystals, specifically the injection of molecular precursors into hot organic surfactants, has yielded markedly improved samples with good size control, narrow size distribution, and good crystallinity of individual and dispersable nanocrystals. It is of considerable interest to apply these methods to the synthesis of transition metal oxide nanoparticles, which typically
The metal-insulator transition in VO2 studied using terahertz apertureless near-field microscopy
Mittleman, Daniel
studied the metal-insulator transition in a vanadium dioxide VO2 thin film using terahertz apertureless.1063/1.2801359 Vanadium dioxide VO2 is prototype metal oxide with a complex phase behavior dictated by strong electron as a process of domain growth and eventual coalescence.6 Recent experiments suggest that a destabiliza- tion
Transition metal complexes with Schiff-base ligands: 4-aminoantipyrine based derivatives¢a review
N. Raman; S. Johnson Raja; A. Sakthivel
The survey highlights structural properties and biological studies of transition metal complexes derived from 4-aminoantipyrine. The most important results of extensive studies (syntheses, spectral, magnetic, redox, structural characteristics, antimicrobial and DNA cleavage) of the metal complexes with heterocyclic Schiff bases of 4-aminoantipyrine with some aldehydes and oximes are reviewed.
Complex transition metal hydrides: linear correlation of countercation electronegativity versus T-D bond lengths.
Humphries, T D; Sheppard, D A; Buckley, C E
For homoleptic 18-electron complex hydrides, an inverse linear correlation has been established between the T-deuterium bond length (T = Fe, Co, Ni) and the average electronegativity of the metal countercations. This relationship can be further employed towards aiding structural solutions and predicting physical properties of novel complex transition metal hydrides. PMID:26077621
Hybrid Car-Parrinello\\/Molecular Mechanics Modelling of Transition Metal Complexes: Structure, Dynamics and Reactivity
Leonardo Guidoni; Patrick Maurer; Stefano Piana; Ursula Rothlisbergera
The theoretical modelling of chemically active transition metal (TM) centres is a notoriously difficult task. The metal-ligand interactions in these complexes are often highly directional and the concoction of suitable analytic interaction potentials can be far from trivial. The situation is rendered even more difficult by the fact that at finite temperature, the system might switch dynamically be- tween different
Temperature dependence of the electrical conductivity of selenous acid and its transition metal selenites
B. A. El-Sayed; A. A. A. Emara; F. S. M. Abd El-Hameed; S. M. Shaaban
The electric conductivity has been measured over a wide range of temperatures including phase transition temperatures of selenous acid and its transition metal selenites. The effect of hydrogen bonding formation of the crystal structure, as well as, the ion pair formation on the conduction mechanism in the low and high temperature regions was visualized using infrared spectra. The values of
The electronic structure of lithium transition metal oxides
NASA Astrophysics Data System (ADS)
Kocher, Michael P.
Currently, LiCoO2 is the cathode in the majority of the batteries used in cellular phones and laptop computers. Due to the low abundance of cobalt, thermal instability and environmental concerns, there has been a strong effort to find an alternative material. This dissertation focuses on ab initio calculations of the electronic structure of several lithium transition metal oxides used as cathode material in Li-ion batteries, especially layered LiMn1/2Ni1/2O2 and LiMn1/3,Ni 1/3,Co1/3O2, and olivine structure LiFePO 4 and FePO4. These materials offer substantial increases in energy density and cycle life, and could be used in electric vehicles. Density Functional Theory (DFT) was used to calculate the electronic structure of LixMn1/2Ni1/2O2 and LixMn1/3,Ni1/3,Co1/3O2. To understand the fundamental characteristics of these materials, the spherical integrated charge and spin density, and angular momentum projected density of states was calculated to investigate the effect of insertion of Li. The observed changes of the integrated spin density suggest Ni is changing valence state as Li is removed. However, the integrated charge density shows no dependence on the Li concentration, which suggests that Ni remains in the same charge state. The electronic density of states reveals that the hybridized O p near the Fermi level are key component to charge compensation mechanism. This provides evidence that the O has a key role in the charge regulation during delithiation/lithiation and Ni is not changing from Ni2+ to Ni4+. The calculated projected density of states was compared with EELS measurements to further validate these findings. The electronic structure of LiFePO4 and FePO4 was calculated using DFT and DFT+U. The spherically integrated spin and charge densities show a small dependency of the Li concentration, but do not suggest a change from Fealpha to Fealpha+1. The projected density of states shows an increase in the hybridization of the O p and Fe d states as Li is removed, suggesting the covalent bonding within the material is a key component to the charge compensation mechanism.
Oxygen 1s x-ray-absorption edges of transition-metal oxides
F. M. F. de Groot; M. Grioni; J. C. Fuggle; J. Ghijsen; G. A. Sawatzky; H. Petersen
The oxygen 1s x-ray-absorption edges of a series of 3d-transition-metal oxides have been measured. The structures at the edge arise from covalent mixing of the metal and oxygen states, which introduces oxygen p character in unoccupied states of mainly metal character. The spectra can be divided into two regions: The first is a double-peaked sharp structure near threshold, which can
The involvement of transition metal ions on iron-dependent lipid peroxidation
Marisa G. RepettoNidia; Nidia F. Ferrarotti; Alberto Boveris
The metals iron (Fe) and copper (Cu) are considered trace elements, and the metals cobalt (Co) and nickel (Ni) are known as\\u000a ultra-trace elements, considering their presence in low to very low quantity in humans. The biologic activity of these transition\\u000a metals is associated with the presence of unpaired electrons that favor their participation in redox reactions. They are part
Percolation Aspects of the Metal-Insulator Transition in Expanded Cs
T. Arai; R. L. McGreevy
An æatom removalÆ method has been used to produce models of the structure of expanded Cs along the coexistence curve, to points above the critical point, based on a model of the structure at the triple point. A percolation transition in the metallic bond network is found at p\\/pc ' 0.21, very close to the experimental value for the metal-non-metal
Complexation of transition metals by 3-azidopropionitrile. An electrospray ionization mass spectrometry study
Narciso Couto; M. Filomena Duarte; M. Tereza Fernandez; Paula Rodrigues; M. Teresa Barros; M. Lourdes Costa; Benedito J. Costa Cabral
Most complexes of azides and transition metals involve the N3? azide anion as a ligand other than an organic azide. Complexes of organic azides with metals are involved in biological applications\\u000a and in the deposition of nitrenes on metal surfaces, producing nitride layers for semi-conductors preparation; this makes\\u000a the study of these interactions an important issue. This work describes a
CVD - technology of transition metal oxides and their impact on solar energy utilization
The paper presents results concerning CVD thin films of transition metals (W and Mo) and their composite structures W02:W and MoOZ:Mo, as well as WO films. The composite structure materials 3 consisting of a suspension of metallic molybdenum or tungsten grains in a host matrix of metal dioxide demonstrate significant solar absorptance coupled with a high infrared reflectance, which is
Trace Metals in the Early Ocean, Biological Implications, and Evolving Biospheric Redox
NASA Astrophysics Data System (ADS)
Lyons, T. W.; Scott, C.; Gill, B. C.; Anbar, A. D.
Dissolved molybdenum abounds in modern, oxygen-rich seawater as the highly soluble molybdate ion. But this trace metal, also a nutrient essential to phytoplankton productivity, was not always so readily available. Several periods of the geologic past are noted for being euxinic (i.e., oxygen-poor and sulfide-rich) at regional or perhaps even global scales. During the Proterozoic in particular, numerous euxinic settings may have persisted in the ocean for a billion years or more following the initial rise of oxygen in the atmosphere. Under such conditions, Mo is sequestered efficiently, and corresponding limitation in the oceanic inventory may have impacted nitrogen fixation by prokaryotes, as well as uptake of fixed nitrogen by early eukaryotes. Mo is essential for nitrogen fixation and nitrate assimilation as a cofactor for nitrogenase and assimilatory nitrate reductase enzymes, respectively. Evidence for analogous limitations is emerging for portions of the Paleozoic and Mesozoic, but the potential impacts have not yet been considered. In each case, the critical arguments hang on our ability (1) to estimate oceanic extents of oxygen-poor, sulfide-rich settings for the time slice of interest and (2) to discriminate Mo drawdown in the global ocean from that occurring locally within individual basins. Traditional and emerging approaches for distinguishing local from global ancient anoxia/euxinia will be highlighted with an eye toward estimating the impact on availability of Mo and other bioessential, redox-sensitive transition metals. The context for this exploration will be the long history of evolving redox in the ocean and atmosphere from the Archean forward. The Phanerozoic examples are also associated with major biotic extinction events, and our understanding of the relationships among the primary drivers of extinction, widespread euxinic conditions, and the impact of micronutrient availability during extinction and recovery remains nascent.
Foundation Degree to Honours Degree: The Transition Experiences of Students on an Early Years Programme
ERIC Educational Resources Information Center
Morgan, Julia
This article focuses on an exploratory study, undertaken in 2009-2012, which explored student transitions from a foundation degree (level 5) into the third year of a BA honours degree (level 6). Direct entry students and staff from an early years programme at a post-1992 British university and second-year foundation degree students and staff fromģ
Mexican American Birthweight and Child Overweight: Unraveling a Possible Early Life Course Health Transition
ERIC Educational Resources Information Center
Hamilton, Erin R.; Teitler, Julien O.; Reichman, Nancy E.
Mexican American children have a weight distribution that categorizes them as relatively healthy at birth but relatively unhealthy by age 3. This early life course transition in health based on weight raises the question of whether Mexican American children "outgrow" the epidemiologic paradox of favorable birth outcomes despite social disadvantageģ
Transition metal catalyzed nucleophilic allylic substitution: activation of allylic alcohols via ?-allylic species.
Sundararaju, Basker; Achard, Mathieu; Bruneau, Christian
Modern organic synthesis now requires efficient atom economical synthetic methods operating under greener pathways to achieve C-C and C-heteroatom bond formation. Direct activation of allylic alcohols in the presence of transition metal catalysts leading to electrophilic ?-allyl metal intermediates represents such a promising target in the field of nucleophilic allylation reactions. During the last decade, this topic of recognized importance has become an emerging area, and selected transition metals, sometimes associated with alcohol activators, have brought elegant solutions for performing allylic substitution directly from alcohols in a regio, stereo and enantioselective manner. PMID:22576362
Insulator-metal transition of highly compressed carbon disulfide
Dias, Ranga P.; Yoo, Choong-Shik; Kim, Minseob; Tse, John S. (WSU); (Saskatchewan)
We present integrated spectral, structural, resistance, and theoretical evidences for simple molecular CS{sub 2} transformations to an insulating black polymer with threefold carbon atoms at 9 GPa, then to a semiconducting polymer above 30 GPa, and finally to a metallic solid above 50 GPa. The metallic phase is a highly disordered three-dimensional network structure with fourfold carbon atoms at the carbon-sulfur distance of {approx}1.70 {angstrom}. Based on first-principles calculations, we present two plausible structures for the metallic phase: {alpha}-chalcopyrite and tridymite, both of which exhibit metallic ground states and disordered diffraction features similar to that measured. We also present the phase and chemical transformation diagram for carbon disulfide, showing a large stability field of the metallic phase to 100 GPa and 800 K.
Phase diagram of the ultrafast photoinduced insulator-metal transition in vanadium dioxide
NASA Astrophysics Data System (ADS)
Cocker, T. L.; Titova, L. V.; Fourmaux, S.; Holloway, G.; Bandulet, H.-C.; Brassard, D.; Kieffer, J.-C.; El Khakani, M. A.; Hegmann, F. A.
We use time-resolved terahertz spectroscopy to probe the ultrafast dynamics of the insulator-metal phase transition induced by femtosecond laser pulses in a nanogranular vanadium dioxide (VO2) film. Based on the observed thresholds for characteristic transient terahertz dynamics, a phase diagram of critical pump fluence versus temperature for the insulator-metal phase transition in VO2 is established for the first time over a broad range of temperatures down to 17 K. We find that both Mott and Peierls mechanisms are present in the insulating state and that the photoinduced transition is nonthermal. We propose a critical-threshold model for the ultrafast photoinduced transition based on a critical density of electrons and a critical density of coherently excited phonons necessary for the structural transition to the metallic state. As a result, evidence is found at low temperatures for an intermediate metallic state wherein the Mott state is melted but the Peierls distortion remains intact, consistent with recent theoretical predictions. Finally, the observed terahertz conductivity dynamics above the photoinduced transition threshold reveal nucleation and growth of metallic nanodomains over picosecond time scales.
ParentsÆ Marital Distress, Divorce, and Remarriage: Links with DaughtersÆ Early Family Formation Transitions
Amato, Paul R.; Kane, Jennifer B.
We used data from the Add Health study to estimate the effects of parentsÆ marital status and relationship distress on daughtersÆ early family formation transitions. Outcomes included traditional transitions (marriage and marital births) and nontraditional transitions (cohabitation and nonmarital births). Relationship distress among continuously married parents was not related to any outcome. Offspring with single parents and remarried parents had an elevated risk of nonmarital births and nonmarital cohabitation. Offspring with remarried parents with a high-distress relationship had an elevated risk of early marriages and marital births. These results, combined with analyses of mediating variables, provide the strongest support for a modeling perspective, although some support also was found for a perspective based on escape from stress. PMID:21785523
Gas-phase activation of methane by ligated transition-metal cations
Schr÷der, Detlef; Schwarz, Helmut
Motivated by the search for ways of a more efficient usage of the large, unexploited resources of methane, recent progress in the gas-phase activation of methane by ligated transition-metal ions is discussed. Mass spectrometric experiments demonstrate that the ligands can crucially influence both reactivity and selectivity of transition-metal cations in bond-activation processes, and the most reactive species derive from combinations of transition metals with the electronegative elements fluorine, oxygen, and chlorine. Furthermore, the collected knowledge about intramolecular kinetic isotope effects associated with the activation of C¢H(D) bonds of methane can be used to distinguish the nature of the bond activation as a mere hydrogen-abstraction, a metal-assisted mechanism or more complex reactions such as formation of insertion intermediates or ?-bond metathesis. PMID:18955709
Correlating magnetotransport and diamagnetism of sp2-bonded carbon networks through the metal-insulator transition
NASA Astrophysics Data System (ADS)
Vora, P. M.; Gopu, P.; Rosario-Canales, M.; Pķrez, C. R.; Gogotsi, Y.; Santiago-Avilķs, J. J.; Kikkawa, J. M.
Titanium-carbide-derived carbons (TiC-CDCs) are porous sp2-bonded networks synthesized by exposing TiC to chlorine gas at an elevated temperature. The latter ōchlorination temperatureö adjusts the size of the pores and the sp2-bonded carbon domains within this material. We perform magnetoresistance, electronic transport, and superconducting quantum interference device magnetization measurements on TiC-CDC samples prepared at different chlorination temperatures. Transport reveals a metal-insulator transition where high (low) chlorination temperature samples are on the metallic (insulating) side of the transition. Magnetoresistance measurements are consistent with transport in the weak and strong localization regimes for metallic and insulating samples, respectively. Changes in diamagnetism, electronic transport, and magnetoresistance data across the metal-insulator transition are coordinated, suggesting that all three properties are controlled by a single parameter, likely the expansion of sp2-bonded domains.
Correlating Magnetotransport and Diamagnetism of sp2-Bonded Carbon Networks Through the Metal-Insulator Transition
NASA Astrophysics Data System (ADS)
Vora, P. M.; Kikkawa, J. M.; Gopu, P.; Rosario-Canales, M.; Santiago-Aviles, J. J.; Perez, C. R.; Gogotsi, Y.
Titanium carbide-derived carbons (TiC-CDCs) are porous sp2-bonded networks synthesized by exposing TiC to chlorine gas at an elevated temperature. The latter ``chlorination temperature'' adjusts the size of graphitic domains within this material. We perform magnetoresistance, temperature dependent resistance, and SQUID magnetization measurements on TiC-CDC samples prepared at different chlorination temperatures. Transport reveals a metal-insulator transition where high (low) chlorination temperature samples are on the metallic (insulating) side of the transition. Magnetoresistance measurements are consistent with electronic transport in the weak and strong localization regimes for metallic and insulating samples, respectively. The diamagnetic contribution to the total magnetization increases with chlorination temperature, suggesting that the metal-insulator transition is associated with the expansion of graphitic domains. We also discuss a magnetoresistance anomaly observed in insulating samples. This work supported by NSF DMR-0907266 and NSF MRSEC DMR-05-20020.
Photoluminescence response of colloidal quantum dots on VO2 film across metal to insulator transition
We have proposed a method to probe metal to insulator transition in VO2 measuring photoluminescence response of colloidal quantum dots deposited on the VO2 film. In addition to linear luminescence intensity decrease with temperature that is well known for quantum dots, temperature ranges with enhanced photoluminescence changes have been found during phase transition in the oxide. Corresponding temperature derived from luminescence dependence on temperature closely correlates with that from resistance measurement during heating. The supporting reflectance data point out that photoluminescence response mimics a reflectance change in VO2 across metal to insulator transition. Time-resolved photoluminescence study did not reveal any significant change of luminescence lifetime of deposited quantum dots under metal to insulator transition. It is a strong argument in favor of the proposed explanation based on the reflectance data. PACS 71.30. + h; 73.21.La; 78.47.jd PMID:25404877
Electronic spectroscopy and electronic structure of the smallest metal clusters: the diatomic 3D transition metal aluminides
NASA Astrophysics Data System (ADS)
Behm, Jane M.; Morse, Michael D.
A systematic study of the electronic spectroscopy, electronic structure, and chemical bonding has been initiated for the 3d series of diatomic transition metal aluminides. This report provides a review of the progress to date, with specific emphasis on AlCa, AlV, AlCr, AlMn, AlCo, AlNi, AlCu, and AlZn.
Vibronic spectroscopy of unsaturated transition metal complexes: CrC2H, CrCH3 , and NiCH3
Morse, Michael D.
to characterize diatomic transition metal oxides, nitrides, and carbides.8Ł22 In addition to these pure metallicVibronic spectroscopy of unsaturated transition metal complexes: CrC2H, CrCH3 , and NiCH3 Dale J investigation of small transition metal clusters and organo- metallic radicals is that these species serve
Bridging Early Services for Children with Special Needs and Their Families: A Practical Guide for Transition Planning.
ERIC Educational Resources Information Center
Rosenkoetter, Sharon E.; And Others
This volume presents a comprehensive plan for transition of children with special needs and their families between infant or early intervention services and preschool services, and between preschool services and kindergarten. It aims to help in bridging these services to minimize the stress and increase the promise of early childhood transitions.ģ
Ternary rare earth and actinoid transition metal carbides viewed as carbometalates
Dashjav, Enkhtsetseg [Max-Planck-Institut fuer Chemische Physik fester Stoffe, Noethnitzer Strasse 40, D-01187 Dresden (Germany); Kreiner, Guido [Max-Planck-Institut fuer Chemische Physik fester Stoffe, Noethnitzer Strasse 40, D-01187 Dresden (Germany); Schnelle, Walter [Max-Planck-Institut fuer Chemische Physik fester Stoffe, Noethnitzer Strasse 40, D-01187 Dresden (Germany); Wagner, Frank R. [Max-Planck-Institut fuer Chemische Physik fester Stoffe, Noethnitzer Strasse 40, D-01187 Dresden (Germany); Kniep, Ruediger [Max-Planck-Institut fuer Chemische Physik fester Stoffe, Noethnitzer Strasse 40, D-01187 Dresden (Germany)], E-mail: email@example.com; Jeitschko, Wolfgang [Institut fuer Anorganische und Analytische Chemie, Universitaet Muenster, Wilhelm-Klemm-Strasse 8, D-48149 Muenster (Germany)], E-mail: email@example.com
Ternary carbides A{sub x}T{sub y}C{sub z} (A=rare earth metals and actinoids; T=transition metals) with monoatomic species C{sup 4-} as structural entities are classified according to the criteria (i) metal to carbon ratio, (ii) coordination number of the transition metal by carbon atoms, and (iii) the dimensionality of the anionic network [T{sub y}C{sub z}]{sup n-}. Two groups are clearly distinguishable, depending on the metal to carbon ratio. Those where this ratio is equal to or smaller than 2 may be viewed as carbometalates, thus extending the sequence of complex anions from fluoro-, oxo-, and nitridometalates to carbometalates. The second group, metal-rich carbides with metal to carbon ratios equal to or larger than 4 is better viewed as typical intermetallics (''interstitial carbides''). The chemical bonding properties have been investigated by analyzing the Crystal Orbital Hamilton Population (COHP). The chemical bonding situation with respect to individual T-C bonds is similar in both classes. The main difference is the larger number of metal-metal bonds in the crystal structures of the metal-rich carbides.
Metal-insulator transition in Na{sub x}WO{sub 3}: Photoemission spectromicroscopy study
Paul, Sanhita, E-mail: email@example.com; Ghosh, Anirudha, E-mail: email@example.com; Raj, Satyabrata, E-mail: email@example.com [Department of Physical Sciences, Indian Institute of Science Education and Research - Kolkata, Mohanpur Campus, Nadia -741252, West Bengal (India)
We have investigated the validity of percolation model, which is quite often invoked to explain the metal-insulator transition in sodium tungsten bronzes, Na{sub x}WO{sub 3} by photoelectron spectromicroscopy. The spatially resolved direct spectromicroscopic probing on both the insulating and metallic phases of high quality single crystals of Na{sub x}WO{sub 3} reveals the absence of any microscopic inhomogeneities embedded in the system within the experimental limit. Neither any metallic domains in the insulating host nor any insulating domains in the metallic host have been found to support the validity of percolation model to explain the metal-insulator transition in Na{sub x}WO{sub 3}.
Maternal exposure to alkali, alkali earth, transition and other metals: Concentrations and predictors of exposure.
Hinwood, A L; Stasinska, A; Callan, A C; Heyworth, J; Ramalingam, M; Boyce, M; McCafferty, P; Odland, J ž
Most studies of metals exposure focus on the heavy metals. There are many other metals (the transition, alkali and alkaline earth metals in particular) in common use in electronics, defense industries, emitted via combustion and which are naturally present in the environment, that have received limited attention in terms of human exposure. We analysed samples of whole blood (172), urine (173) and drinking water (172) for antimony, beryllium, bismuth, cesium, gallium, rubidium, silver, strontium, thallium, thorium and vanadium using ICPMS. In general most metals concentrations were low and below the analytical limit of detection with some high concentrations observed. Few factors examined in regression models were shown to influence biological metals concentrations and explained little of the variation. Further study is required to establish the source of metals exposures at the high end of the ranges of concentrations measured and the potential for any adverse health impacts in children. PMID:25984984
Catalytic activities of Schiff base transition metal complexes
K. C. Gupta; Alekha Kumar Sutar
Many Schiff base complexes of metal ions show high catalytic activity. Chiral Schiff base complexes are more selective in various reactions such as oxidation, hydroxylation, aldol condensation and epoxidation. The catalytic activity of metal complexes of binaphthyl, binaphthol and their combinations with salen Schiff base is presented in this review. The pyridyl bis(imide) and pyridine bis(imine) complexes of cobalt(II), iron(II)
Differential tolerance of Sulfolobus strains to transition metals
Miller, K.W.; Sass, Risanico S.; Risatti, J.B.
Sulfolobus acidocaldarius strains 98-3 and B12, and S. solfataricus ATCC 35091 were evaluated for tolerance to Cd, Co, Cu, Ni, Zn and Mg. The tolerance of strains 98-3 and ATCC 35091 to these metals was Mg > Zn > Cd > Cu ??? Co > Ni. For B12, however, the order of tolerance was Mg > Cd > Zn = Co > Ni > Cu. Tolerance to these metals is also presented as a potentially useful taxonomic indicator.
Pinball liquid phase from Hund's coupling in frustrated transition-metal oxides
NASA Astrophysics Data System (ADS)
Ralko, Arnaud; Merino, Jaime; Fratini, Simone
The interplay of nonlocal Coulomb repulsion and Hund's coupling in the d -orbital manifold in frustrated triangular lattices is analyzed by a multiband extended Hubbard model. We find a rich phase diagram with several competing phases, including a robust pinball liquid phase, which is an unconventional metal characterized by threefold charge order, bad metallic behavior, and the emergence of high-spin local moments. Our results naturally explain the anomalous charge-ordered metallic state observed in the triangular layered compound AgNiO2. The potential relevance to other triangular transition-metal oxides is discussed.
High capacity hydrogen absorption in transition-metal ethylene complexes: consequences of nanoclustering.
Phillips, A B; Shivaram, B S
We have recently shown that organo-metallic complexes formed by laser ablating transition metals in ethylene are high hydrogen absorbers at room temperature (Phillips and Shivaram 2008 Phys. Rev. Lett. 100 105505). Here we show that the absorption percentage depends strongly on the ethylene pressure. High ethylene pressures (>100 mTorr) result in a lowered hydrogen uptake. Transmission electron microscopy measurements reveal that while low pressure ablations result in metal atoms dispersed uniformly on a near atomic scale, high pressure ones yield distinct nanoparticles with electron energy-loss spectroscopy demonstrating that the metal atoms are confined solely to the nanoparticles. PMID:19420668
Reversible transition of deformation mode by structural rejuvenation and relaxation in bulk metallic glass
NASA Astrophysics Data System (ADS)
Meng, Fanqiang; Tsuchiya, Koichi; Seiichiro; Yokoyama, Yoshihiko
A transition of the deformation mode from heterogeneous, localized deformation to homogeneous deformation was observed in Zr50Cu40Al10 bulk metallic glass (BMG) by giant straining using the high-pressure torsion (HPT) method. The transition is accompanied by a pronounced decrease in hardness and elastic modulus as measured by nanoindentation. Annealing of the deformed BMG resulted in the restoration of the localized deformation, hardness, and elastic modulus; thus, the transition is reversible. The observed reversible transition can be attributed to a change in the local atomic environment in the rejuvenated volume and the relaxed one.
Multiphase segregation and metal-insulator transition in single crystal La
Kiryukhin, V.; Kim, B. G.; Podzorov, V.; Cheong, S.-W.; Koo, T. Y.; Hill, J. P.; Moon, I.; Jeong, Y. H.
The insulator-metal transition in single crystal La{sub 5/8-y}Pr{sub y}Ca{sub 3/8}MnO{sub 3} with y{approx}0.35 was studied using synchrotron x-ray diffraction, electric resistivity, magnetic susceptibility, and specific heat measurements. Despite the dramatic drop in the resistivity at the insulator-metal transition temperature T{sub MI}, the charge-ordering (CO) peaks exhibit no anomaly at this temperature and continue to grow below T{sub MI}. Our data suggest then, that in addition to the CO phase, another insulating phase is present below T{sub CO}. In this picture, the insulator-metal transition is due to the changes that occur within this latter phase. The CO phase does not appear to play a major role in this transition. We propose that a percolationlike insulator-metal transition occurs via the growth of ferromagnetic metallic domains within the parts of the sample that do not exhibit charge ordering. Finally, we find that the low-temperature phase-separated state is unstable against x-ray irradiation, which destroys the CO phase at low temperatures.
Barium distributions in teeth reveal early life dietary transitions in primates
Austin, Christine; Smith, Tanya M.; Bradman, Asa; Hinde, Katie; Joannes-Boyau, Renaud; Bishop, David; Hare, Dominic J.; Doble, Philip; Eskenazi, Brenda; Arora, Manish
Early life dietary transitions reflect fundamental aspects of primate evolution and are important determinants of health in contemporary human populations1,2. Weaning is critical to developmental and reproductive rates; early weaning can have detrimental health effects but enables shorter inter-birth intervals, which influences population growth3. Uncovering early life dietary history in fossils is hampered by the absence of prospectively-validated biomarkers that are not modified during fossilisation4. Here we show that major dietary shifts in early life manifest as compositional variations in dental tissues. Teeth from human children and captive macaques, with prospectively-recorded diet histories, demonstrate that barium (Ba) distributions accurately reflect dietary transitions from the introduction of motherÆs milk and through the weaning process. We also document transitions in a Middle Palaeolithic juvenile Neanderthal, which shows a pattern of exclusive breastfeeding for seven months, followed by seven months of supplementation. After this point, Ba levels in enamel returned to baseline prenatal levels, suggesting an abrupt cessation of breastfeeding at 1.2 years of age. Integration of Ba spatial distributions and histological mapping of tooth formation enables novel studies of the evolution of human life history, dietary ontogeny in wild primates, and human health investigations through accurate reconstructions of breastfeeding history. PMID:23698370
Barium distributions in teeth reveal early-life dietary transitions in primates.
Austin, Christine; Smith, Tanya M; Bradman, Asa; Hinde, Katie; Joannes-Boyau, Renaud; Bishop, David; Hare, Dominic J; Doble, Philip; Eskenazi, Brenda; Arora, Manish
Early-life dietary transitions reflect fundamental aspects of primate evolution and are important determinants of health in contemporary human populations. Weaning is critical to developmental and reproductive rates; early weaning can have detrimental health effects but enables shorter inter-birth intervals, which influences population growth. Uncovering early-life dietary history in fossils is hampered by the absence of prospectively validated biomarkers that are not modified during fossilization. Here we show that large dietary shifts in early life manifest as compositional variations in dental tissues. Teeth from human children and captive macaques, with prospectively recorded diet histories, demonstrate that barium (Ba) distributions accurately reflect dietary transitions from the introduction of mother's milk through the weaning process. We also document dietary transitions in a Middle Palaeolithic juvenile Neanderthal, which shows a pattern of exclusive breastfeeding for seven months, followed by seven months of supplementation. After this point, Ba levels in enamel returned to baseline prenatal levels, indicating an abrupt cessation of breastfeeding at 1.2?years of age. Integration of Ba spatial distributions and histological mapping of tooth formation enables novel studies of the evolution of human life history, dietary ontogeny in wild primates, and human health investigations through accurate reconstructions of breastfeeding history. PMID:23698370
Electronic and magnetic properties of NbSe2 monolayer doped vacancy and transition metal atoms
NASA Astrophysics Data System (ADS)
Manchanda, Priyanka; Sellmyer, David; Skomski, Ralph
Two-dimensional transition-metal dichalcogenides (2D TMDs) have attracted much attention recently due to potential applications including optoelectronic devices. Atomically thin layers of materials such as MoS2, WS2, NbS2, NbSe2, TaTe2 can easily be synthesized by exfoliation techniques and exhibit variety electronic phases such as metal, semiconductor, superconductor depending on the choice of metal. Most of the TMDs are nonmagnetic and various techniques have been proposed to induce or modulate magnetic properties that are essential for nanoelectronic device applications. We use DFT calculations to analyze the effect of strain, hydrogen adsorption, and doping. Emphasis is on the magnetic properties of NbSe2 monolayers containing vacancies and 3 d transition metal atoms. We find that magnetism can be induced by vacancy creation and transition metal-substitution in NbSe2, with effects similar to strain and hydrogen adsorption. The moment mainly arises from the localized nonbonding 3d electrons of the transition-metal atoms. Our findings contribute to the ongoing search ``for-better-than-graphene'' thin-film materials for novel electronic devices. This research is partially supported by DOE BES (DE-FG02-04ER46152).
Theoretical analysis of structural stability of TM5Si3 transition metal silicides
NASA Astrophysics Data System (ADS)
Chen, Yue; Kolmogorov, A. N.; Pettifor, D. G.; Shang, Jia-Xiang; Zhang, Yue
A combination of electronic-structure calculations from density-functional theory (DFT) through a tight-binding (TB) model to analytic bond-order potentials (BOPs) has been used to investigate the structural trend of the TM5Si3 compounds across the early transition metals (TM). First of all, the formation energies of TM5Si3 , whose ground states adopt the competing D88 , D8l , or D8m structure types, have been calculated by using DFT (TM is Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W). In agreement with experiments the DFT results predict the observed D88?D8m structural trend across the 3d series and the D88?D8l?D8m trend across the 4d and 5d series. A p-d canonical TB model is then shown to reproduce these trends, thereby providing a valid basis for the application of BOP theory. By performing a moment analysis within the BOP formalism, we conclude that up to the fifth moment of the density of states is required to explain the structural trend across the 3d series whereas up to the ninth moment is required for the 4d and 5d series.