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Network structure of Mo-oxide glasses

The structure of molybdate glasses have been investigated by neutron and high-energy X-ray diffraction coupled with Reverse Monte Carlo (RMC) simulation technique. From the modelling the partial atomic correlation functions gij(r), the coordination number distributions CNij and bond angle distributions have been revealed. For binary 90MoO3-10Nd2O3 glass composition the fraction of MoO4/MoO6 was 0.55/0.25. Three type of ternary system have been studied, where the most important structural units was authenticated. For MoO3-Nd2O3-B2O3 sample mixed MoO4-BO4 and MoO4-BO3 linkages form pronounced intermediate-range order. In case of MoO3-ZnO-B2O3 series the BO3 and BO4 units are linked to MoO4 and/or ZnO4, forming mixed MoO4-BO4(BO3), MoO4-ZnO4 and ZnO4-BO4(BO3) bond-linkages.


Introduction
Rare-earth molybdates display a variety of crystal and amorphous structures and possess useful electro-physical, optical, high ion and electron conductivity properties derived from the parent oxide La 2 Mo 2 O 9 [1,2]. The molybdates LiLn(MoO 4 ) 2 , Ln=Nd, Pr, Ho are excellent LN-doped laser hosts [3].
Zinc boromolybdate materials are known as low melting dielectric materials and they have high potential in several applications, due to their stable and unique structure with favourable optical properties. In contrast to the most molybdates with relatively well-characterised crystalline and magnetic structures, e.g. [4][5][6][7], the preparation and structural information on amorphous molybdate systems is not ample. Glass formation in molybdate systems, containing different kind of oxides, has been examined by our team as a part of long term academic projects, concerning non-traditional glass forming systems [8][9][10]. The phase diagram for several oxides, MoO 3 -Nd(La) 2 O 3 -B 2 O 3 was set up [9] and the basic structural and optical characteristics were investigated by XRD, DTA, SEM, UV-VS spectra and IR spectroscopy [10].
The purpose of this work is to verify the competition for the glass network formation between nontraditional network formers -MoO 3 , ZnO -and to get a deeper insight into the structural network characteristics of new series of molybdates containing modifiers Nd 2  In this paper, the structure of molybdate glasses is investigated using neutron and high-energy Xray diffraction experiments, and for structure modelling the Reverse Monte Carlo (RMC) computer simulation technique [11] was applied. We expect to obtain new structural data, especially on the characteristics of the partial atomic pair correlation functions, the first neighbour atomic distances, coordination numbers and network former units and their linkages.

Samples
The samples with the nominal composition 90MoO 3 -10Nd 2 O 3 (hereafter referred to as Mo90Nd10), ternary  11 B isotope (99.6%) has been used in order to avoid the high neutron absorption cross section of the 10 B isotope. The melting temperature was limited to 1300 o C taking into account the volatility of the components. The glasses were obtained by pouring the melts onto an iron plate and by pressing with another iron plate.

Neutron and X-ray diffraction experiments
Neutron diffraction (ND) data have been obtained on the two-axis 'PSD' diffractometer at the 10 MW Budapest research reactor ( 0 =1.068Å) [12], on the time-of-flight 'HIPD' instrument [13] at the LANSCE pulsed neutron source (for Mo-Nd-Mg-O samples) and on the '7C2' diffractometer at the Orphée reactor of the LLB-CEA-Saclay ( 0 =0.726 Å) [14] (for the Mo-Nd-B-O and Mo-Zn-B-O samples series). Glass powders were contained in a cylindrical vanadium sample holder of 8 mm (PSD, 7C2) and 10 mm (HIPD) diameter for the three types of experiments. The structure factors, S(Q)s, were evaluated from the raw experimental date using the programme packages available at the facilities. The S(Q)s were obtained in the Q=0.45-9.8Å -1 (PSD), Q=0.2-15.5Å -1 (7C2) and Q=1.5-35Å -1 (HIPD) experiments. The agreement between the corresponding S(Q) data was within 1% in the overlapping Q range. The statistics of the PSD data is excellent at relatively low Q values, while the statistics and the information content of the 7C2 and HIPD data is unique above 8Å -1 . Therefore the S(Q) patterns were combined by normalizing the PSD data to the 7C2 and HIPD data sets in the 4-8Å -1 interval by the least-squares method, and the average values of the two spectra were used for further data treatment, while for Q<4Å -1 the PSD data and for Q>8Å -1 the 7C2 and HIPD data were used.
The high-energy X-ray diffraction (XRD) measurements were carried out at the BW5 experimental station [15] at HASYLAB, DESY for Mo-Nd-O and Mo-Nd-Mg-O samples. The powdered samples were filled into quartz capillary of 2 mm in diameter. The energy of the radiation was 109.5 keV. Raw data were corrected for detector dead time, background, absorption, and variations in detector solid angle. The synchrotron X-ray radiation makes it possible to reach diffraction data up to high-Q values. For the Mo-Nd-B-O samples X-ray diffraction studies were performed at the beam line ID22 at ESRF. The fine powdered samples were filled into kapton capillary tubes of 1 mm in diameter and measured at room temperature. The energy of the radiation was 70.8 keV. The X-ray S(Q) data were obtained with a good signal-to-noise ratio up to Q max =25Å -1 at BW5 station and up to Q max =22 Å -1 at ID22, for higher Q-values the experimental data proved to be noisy.
The overall run of the curves for the two radiations is fairly different, because of the differences in the weighting factors, w ij of the partial structure factors, S ij (Q): where c i , c j are the molar fractions of the components, b i , b j the coherent neutron and f i (Q), f j (Q) the Xray scattering amplitudes, and k is the number of elements in the sample. Note, that the neutron scattering amplitude is constant [16], while the X-ray scattering amplitude is Q-dependent [17].

Reverse Monte Carlo simulations
Reverse Monte Carlo modelling have been performed using the RMC ++ code [11], starting with a disordered atomic configuration. The RMC minimizes the squared difference between the experimental S(Q) and the calculated one from a 3D atomic configuration. The RMC algorithm calculates the one-dimensional partial atomic pair correlation functions g ij (r), and they are inverse Fourier transformed to calculate the partial structure factors, S ij (Q). In order to generate a realistic structural model, minimum atom-atom distances were chosen to avoid an unphysical overlapping of the atoms and B atoms were imposed to be trigonally and tetrahedrally coordinated. It is reasonable to suppose that boron atoms are surrounded both by threecoordinated and four-coordinated oxygen atoms, forming trigonal BO 3 and tetrahedral BO 4 units as reported for binary neodymium borate glasses [20], for ternary rare-earth boromolybdate       figure 3. This is important, as far as the analysis of these results may give useful information on the linkage of the basic structural units forming the network structure. The second neighbour partial distributions are collected in figure 3.

Discussion
In the present study for the Mo-O atom pair correlations a well-defined first neighbour distance is obtained at 1.80±0.05 Å, and the course of the curves is practically the same for all investigated samples. The observed Mo-O distance is somewhat greater than the 1.75(5)Å distance reported for glassy 90MO 3 -10Nd 2 O 3 from EXAFS measurements [23], and shows similarity to the Mo-O 1.73-1.82Å distances in crystalline LaBMoO 6 [24] and in Nd 2 O 3 -MoO 3 [5]. The Nd-modifier shows characteristic Nd-O first neighbour distance at 2.1Å, and a next one at 2.8 Å for the binary Mo90Nd10, while for the other compositions the first neighbour distance shifts to slightly higher distances 2.4-2.8(9) Å. The intensity decreases and the peak is less characteristic inspite of the increased Nd-concentration in the ternary samples. The reason for this result may be in the more complicated network due to the increased number of constituent components. The small peak at 2.85 Å seems to be reproducible, however, the peak position agrees with the second O-O distance, thus its origin may be artificial.  [5] and usually this is the case for molybdate crystals. For glassy molybdate network the presence of MoO 4 -2 units were reported [9,10,21], while the formation of MoO 6 units was also observed, they are present in the network. In case of binary system it would mean that 55% of the Mo atoms are 4-fold oxygen coordinated, while 25% are 6-fold coordinated, while the rest 20% are in 3-and 5-fold coordinated states, with an error about 5%. Taking into consideration the g Mo-O (r) distribution, we can establish that the shorter distance at 1. The average number of oxygen neighbours of the modifier Nd is 4.3 atoms for both the binary Mo90Nd10 and for the boron-containing ternary sample, while it is significantly smaller for the Mg containing samples, 3.1 atoms. The reason may be that Mg is also a modifier, which plays a similar role like Nd, increasing the number of non-bridging oxygen (NBO) atoms.
The B-O coordination number distributions contain both 3-and 4-fold oxygen coordinated boron atoms. The average coordination number slightly decreases from 3.3 to 3.1 atoms with increasing boron content in case of Mo-Nd-B-O system (see table 2). The fraction of BO 3 and BO 4 to the total number of BO 3 +BO 4  is an important structural characteristics. It was found that the relative number of 3-and 4-fold coordination depends on the boron concentration. With increasing boron content the relative number of BO 3 units is increasing, while the number of BO 4 is decreasing. For 50MoO 3 -25Nd 2 O 3 -25B 2 O 3 sample ~70% of B-atoms are 3-coordinated, while ~30% are 4-coordinated, the average coordination number is 3.3 atoms; for 40MoO 3  content. For the low boron content sample the relative number of 3-fold coordinated boron atoms is significantly higher than that for the higher boron content samples. In consequence, the average CN B-O slightly increases with increasing B 2 O 3 . We discussed above the relative number of BO 3 and BO 4 units, but we have to keep in mind, that complex borate glasses cannot be viewed as a simple network built only from BO 3 triangles and BO 4 tetrahedrals, but usually they consist of relatively large structural units, such as boroxol, pentaborate, triborate and diborate groups [29,30]. However, a possible analysis of such kind of superunits is out of the scope the present study. We have calculated the three-particle bond-angle distributions using the final atomic configuration of the RMC algorithm, plotted both as the function of cos( Θ ) (scale below) and Θ (upper scale), where Θ represents the actual bond angle.  For the Mo-O-B and Mo-O-Zn the peak positions are at 149±3° and 104±5°, respectively, which are very close values to the ideal tetrahedral configuration. B-O-Zn distribution also show a close link between the boron and zinc units. The addition of a modifier initially converts BO 3 triangles into BO 4 tetrahedra, increasing coordination number and strengthens the network. We suppose that the network is formed by BO 3 and BO 4 organized into superstructure units, and linked to MoO 4 or ZnO 4 , forming in this manner mixed bond-linkages.
In the these systems the oxygen centred coordination number distributions show that most of Mo atoms favour to be connected through an oxygen to Zn and B atoms, and the majority of the Zn also prefer to be linked to B or Mo forming in this way [4] Mo-O- [3], [4] B, [4] Mo-O- [4] Zn, [3], [4] B-O- [4] Zn linkages.

Conclusions
Molybdate glasses have been prepared by melt quenching technique, and the network structure was investigated by neutron and high-energy X-ray diffraction. For data evaluation the RMC simulation technique was applied to obtain a possible 3-dimensional network configuration, which is consistent with the experimental data. From the RMC modelling the partial atomic correlation functions g ij (r) and the coordination number distributions CN ij have been revealed. Formations of MoO 4 units were estabilished with 1.75/1.80 Å Mo-O distances. In the binary glass MoO 4 (55%) and MoO 6 (25%) structural units were revealed. In ternary systems mainly MoO 4 units are present, and with decreasing MoO 3 concentration, the ratio of MoO 6 units roughly decreases. For the B-containing ternary glasses B-O first neighbour distance was obtained at 1.40 Å, the B-O network is formed by BO 3 and BO 4 groups. ZnO proved to be a network former, not a modifier as it is often reported in the literature for similar systems. From the analyses of the obtained structural parameters we have concluded that the glassy network is formed by trigonal BO 3 and tetrahedral BO 4 , MoO 4 , ZnO 4 groups. Concentration dependence was found for the BO 4 /BO 3 fraction, it increases with increasing B 2 O 3 content. We have concluded that the network is formed by organization of BO 3 and BO 4 groups. The BO 3 and BO 4 units are linked to MoO 4 or ZnO 4 , forming mixed [4] Mo-O- [3], [4] B, [4] Mo-O- [4] Zn, [3], [4] B-O- [4] Zn bondlinkages. Significant medium-range order exists up to ~7 Å.