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Exploring the coordination change of vanadium and structure transformation of metavanadate MgV2O6 under high pressure

Raman spectroscopy, synchrotron angle-dispersive X-ray diffraction (ADXRD), first-principles calculations, and electrical resistivity measurements were carried out under high pressure to investigate the structural stability and electrical transport properties of metavanadate MgV2O6. The results have revealed the coordination change of vanadium ions (from 5+1 to 6) at around 4 GPa. In addition, a pressure-induced structure transformation from the C2/m phase to the C2 phase in MgV2O6 was detected above 20 GPa, and both phases coexisted up to the highest pressure. This structural phase transition was induced by the enhanced distortions of MgO6 octahedra and VO6 octahedra under high pressure. Furthermore, the electrical resistivity decreased with pressure but exhibited different slope for these two phases, indicating that the pressure-induced structural phase transitions of MgV2O6 was also accompanied by the obvious changes in its electrical transport behavior.

as an efficient catalyst, monoclinic MgV 2 O 6 has been well investigated at ambient condition. In addition, different methods to synthesize MgV 2 O 6 and its structure evolution under high temperature have been reported 12,[34][35][36][37] . However, structural information and electrical transport properties of MgV 2 O 6 under high pressure have not been studied.
At ambient condition, MgV 2 O 6 adopts brannerite-like structure (space group C2/m). As shown in Fig. 1, the Mg 2+ cations lie in octahedral sites, and the vanadium atoms occupy distorted VO 6 octahedral sites, where it can be described as 5 + 1 coordination since the sixth oxygen atom is only weakly bonded at distances of 2.4-2.8 Å whereas the other oxygen atoms bonding below 2.4 Å 38,39 . The distorted VO 6 octahedra are joined by sharing opposite corners forming chains along b axis of the unit cell. Pairs of octahedra in adjacent chains share edges on side of the chain.
In the present paper, we report a comprehensive investigation on the high pressure behavior of MgV 2 O 6 at room temperature using Raman spectroscopy, ADXRD, first-principles calculations, and electrical resistivity measurements. A coordination change of vanadium ions from 5+ 1 to 6 and a structural phase transition (space group from C2/m to C2) have been found around 4 GPa and 20 GPa, respectively. The origins of this structural phase transition and subtle differences between these two structures have been analyzed in details. Significant changes in electrical resistivity have also been detected when the structural phase transitions occurred.

Raman Spectra at High Pressures.
Representative Raman spectra of MgV 2 O 6 at high pressures and the frequency shifts are shown in Fig. 2a and b, respectively. Factor group analysis gives the following irreducible representation of the optic modes Г opt = 8A g (R) + 4B g (R) + 4A u (IR) + 8B u (IR) and the acoustic modes (A u + 2B u ) of MgV 2 O 6 35 . Twelve phonon modes in total were discernible in the Raman spectrum at the ambient condition and the assignments of Raman bands, which are in good agreement with previous works 35,[40][41][42][43][44] , are listed in Supplementary Table S1. In addition, the frequencies, pressure coefficients, and Grüneissen parameters of the different modes are summarized in Supplementary Table S2. The strongest peak at 922 cm −1 is assigned to the stretching vibrations of V-O bonds. Peaks in the region of 500-750 cm −1 correspond to the stretching modes of V 3 O and (V 2 O 2 ) n . The 288, 441 and 837 cm −1 peaks come from symmetric and antisymmetric stretching vibration of the V-O-V bonds along the VO 6 octahedra chain, respectively. The band at 312 cm −1 is characteristic peak of V 3 O mode resulting from the edge-sharing VO 6 octahedra between neighbouring chains. Three peaks located at 208, 271 and 334 cm −1 represent the lattice modes and MgO 6 modes, respectively. Another two peaks located at 153 and 178 cm −1 corresponding to (V 2 O 2 ) n stretching modes generated by the edge-sharing between pairs of VO 6 octahedra.
As shown in Fig. 2, most of the Raman modes continuously move to higher wavenumbers with pressure increased up to 26.5 GPa. On the contrary, five modes located at 312, 523, 733, 837, and 922 cm −1 firstly shift toward low frequency and then move to high frequency above 3.9 GPa. This change in the pressure dependence of Raman frequency without appearance of new Raman modes or disappearance of existing Raman modes indicates that MgV 2 O 6 may undergo a subtle change in structure without altering symmetry at this pressure. With further increasing pressure, dramatic changes can be observed at 17.3 GPa and a set of new vibrational modes, which are marked by asterisks in Fig. 2a, appear at 132, 162, 214, 240, 384, 428, 469, 706, and 916 cm −1 . Another three modes located at 82, 856 and 1006 cm −1 delay their appearance to a higher pressure (20.1 GPa). All the new Raman modes shift to the higher frequencies linearly accompanied by an increase in intensity upon further compression. These noticeable spectral changes under high pressure are attributed to structural phase transition of MgV 2 O 6 . Upon decompression, the spectroscopic changes are partially reversible and the high-pressure phase doesn't fully revert to the original structure. In order to determine the structure of the high pressure phase of MgV 2 O 6 , in-situ ADXRD experiment was carried out and the structural evolution was also discussed in details in the subsequent section.
ADXRD at High Pressures. The ADXRD experiments were carried out up to 32.8 GPa and some selected patterns were shown in Fig. 3. All the reflections can be indexed by monoclinic structure MgV 2 O 6 with space group C2/m and no new feature is observed below 18.5 GPa. In the pattern at 20.2 GPa, a new peak begin to emerge as marked by a star. This obvious change in the diffraction pattern corresponds to the onset of a structural phase transition which is also suggested by the Raman measurements. The observed small pressure difference may be related to the fact that Raman spectroscopy is more sensitive to local structural change than ADXRD 45,46 . Upon further compression, another two new diffraction peaks appear at 25.2 GPa. These three new peaks become gradually stronger with pressure although the peaks belonging to the original structure are still dominant even at the highest pressure reached. This suggests that there is a wide pressure range where the lowand the high-pressure phases coexist and the completion of this structural phase transition may require much larger pressure overstep. When the pressure released, the diffraction pattern doesn't completely recover to the low pressure phase. It had been reported that some vanadates (NaVO 3 19 , LiVO 3 14 , SrV 2 O 6 , and BaV 2 O 6 33 ) can revert to their original phases from the high-pressure amorphous phases under high temperature. Therefore, in order to examine the reconstructing process of the structure, the pressure-quenched sample was heated in a muffle furnace at 723 K for 4 hours. Both Raman and XRD patterns indicate a recovery to the original phase after the heating, as shown in Fig. 4.
Rietveld structural refinements were employed for accurate phase analysis using GSAS software and the high-pressure phase can be validated as another monoclinic structure with space group C2, as shown in Supplementary Fig. S2. In addition, the information of the refined crystal structure was presented in Supplementary Table S3. The derived lattice parameters and volume as a function of pressure are plotted in Fig. 5. Both unit cell dimension and volume decrease with pressure except the β angle which increases with pressure below 20.2 GPa. An obvious slope change in all the plots can be recognized at 4.3 GPa, indicating the occurrence of a subtle change in the structure of MgV 2 O 6 which is also evidenced by Raman measurement.
The analyses of the V-O bond lengths as obtained from the structural refinement revealed that the longest V-O2(ii) bond length (2.6722 Å) drops dramatically under compression (down to 2.3733 Å at 4.3 GPa, as shown in Supplementary Fig. S3). This result is also reproduced by our calculations. At ambient pressure, six oxygen atoms are located around the vanadium atom and five of them have similar bond length (from 1.70 to 2.14 Å) and one has a noticeably longer bond length (2.61 Å). The longest V-O2(ii) bond length decreases rapidly under pressure (down to 2.4 Å at 4 GPa, as shown in Supplementary Fig. S3). Therefore, the abnormal change in compressibility and Raman vibration of MgV 2 O 6 may be attributed to the coordination number of vanadium ions changes from 5 + 1 to 6 at about 4 GPa (forming a more rigid network). This phenomenon is different from the increase in coordination number of vanadium, which is caused by the conversion from tetrahedral VO 4 to octahedral VO 6 as occurred in InVO 4 under pressure 47 . It is well known that the different catalytic properties of bulk and supported vanadate catalysts can usually be related to modifications in the coordination of the vanadium ions 48 . Therefore, the changes of V ions' coordination number from 5 + 1 to 6 in MgV 2 O 6 may lead to modify its redox properties.
The refined crystal structures of MgV 2 O 6 in C2/m phase at 1.9 GPa, 4.3 GPa, and C2 phase at 27.4 GPa based on the diffraction data are illustrated in Fig. 6. The derived Mg-O and V-O bond lengths of MgO 6 octahedra and VO 6 octahedra in different phases are listed in Table 1. It is worth noticing that the four identical Mg-O1 bonds in the MgO 6 octahedra of the C2/m phase split into two groups in the C2 phase, and so are the two V-O3 bonds in the VO 6 octahedra. Figure 6 shows a pressure enhanced distortion of the MgO 6 octahedra and VO 6 octahedra in the C2/m phase which eventually leads to the structural phase transition from C2/m to C2 phase. The C2 phase where B 0 is the bulk modulus and B′ is its pressure derivative. The bulk modulus is determined to be

Resistivity at High Pressures.
As an important technical parameter, electrical resistivity, which can influences the application of the materials, is closely related to crystal structure. In addition, electrical resistivity is a more sensitive symbol for the electronic structural phase transition 51,52 . Therefore, we conducted electrical resistivity measurements at room temperature up to 30.5 GPa and the result is shown in Fig. 7. At ambient condition, the electrical resistivity of MgV 2 O 6 is 4.66 × 10 5 Ω·cm. Upon compression, the electrical resistivity decreases monotonously with pressure. An obvious kink point at 3.8 GPa is observed in the resistivity-pressure curve, which is caused by the coordination change of vanadium ions (from 5 + 1 to 6). Under further compression, another more significant kink occurs at 19.1 GPa. The sample resistivity quickly drops two orders of magnitude in the pressure range from 19.1 GPa to 30.5 GPa, indicating that the electrical resistivity of the C2 high pressure phase is more sensitive to pressure. The qualitatively different behaviors of resistivity under compression are consistent with the stability of the two phases of MgV 2 O 6 determined from XRD and Raman spectroscopy and the two pressure regions determined from the band-gap behavior (see Supplementary Fig. S4). The C2/m phase is an indirect band-gap semiconductor (E g = 2.62 eV) which results a high resistivity at ambient condition. At 20 GPa a structural phase transition from C2/m to C2 phase occurred, which it was associated with a band-gap collapse (E g = 1.16 eV for C2 phase). In both C2/m and C2 phase, the band-gap always decreases under compression,  resulting in the decline of the resistivity. This phenomenon is familiar with previous study in PbCrO 4 under compression 53 . Upon decompression, the resistivity shows a reversible behavior across the C2/m-C2 phase boundary with about 5 GPa hysteresis. In addition, both spectral (16:3:1 methanol/ethanol/water mixture) and electrical (without any pressure medium) results gave similar phase transition pressures, indicating that the deviatoric stresses has no effect on the structural phase transition in MgV 2 O 6 . Unlike the situation occurred in BaWO 4 that the deviatoric stresses play an important role in its structural phase transition behavior 54 . The integrated study using Raman spectroscopy, ADXRD, first-principles calculations, and electrical resistivity measurement indicate that MgV 2 O 6 crystal structure experiences an effective coordination number change for vanadium cations from 5 + 1 to 6 at about 4 GPa and a structural phase transition from the C2/m phase to the C2 phase at about 20 GPa upon compression. Pressure enhances the distortions of MgO 6 octahedra and VO 6 octahedra in the C2/m phase and ultimately results in the occurrence of the structural phase transition to C2 phase. After the 5 + 1 to 6 coordination number change, the electrical resistivity of MgV 2 O 6 becomes slightly less sensitive to pressure whereas the C2 phase shows remarkable pressure induced decrease in its electrical resistivity. High pressure Raman spectroscopy and high pressure angle-dispersive X-ray diffraction. A diamond anvil cell was used for applying high pressures to the sample. T301 stainless steel foil was used as a gasket. A methanol/ethanol/water mixture (16:3:1) was used as a pressure medium in both Raman spectra and in-situ ADXRD measurements and no pressure medium was used for electrical resistance measurement. The pressure in the sample chamber was measured by ruby luminescence technique 55 . High-pressure Raman spectra of MgV 2 O 6 were carried out using a Jobin Yvon T64000 Raman microscope with a 633 nm He-Ne laser beam. High pressure ADXRD patterns were collected using MAR345 CCD detector at the 4W2 beam line of the Beijing Synchrotron Radiation Facility (BSRF) with wavelength of 0.6199 Å. Rietveld refinements were performed using the GSAS 56 program to obtain the lattice parameters under pressure.

Methods
High-pressure resistivity measurements. For the electrical resistivity measurements, a layer of mixture of cubic boron nitride (cBN) and epoxy were pressed onto the surface of the metal gasket for insulation. A hole of 120 μ m in diameter was drilled at the center of the gasket for containing the sample using laser. Four hand-cut platinum foil strips of 10 μ m thickness were directly attached to the sample as electrodes under a microscope (see Supplementary Fig. S5). The electrical insulation between the electrodes and the gasket was monitored during compression. The electrical resistivity of the sample was measured according Van der Pauw method 57 .
First-principles calculations. Our first-principles calculations were performed with the projector augmented wave (PAW) method as implemented in the Vienna ab initio simulation package (VASP) 58 . The PAW pseudopotential was adopted with 2p 6 3s 2 , 3p 6 3d 3 4s 2 , and 2s 2 2p 4 electrons as valence for Mg, V, and O atoms, respectively. The generalized gradient approximation add U (GGA + U) method with Perdew, Burke and Ernzerhof (PBE) type was used to describe the exchange and correlation interaction between the electrons. The Liechtenstein implementation with on-site Coulomb interaction U = 4.2 eV and on-site exchange interaction J = 0.8 eV was used to describe the localized d orbital of vanadium. The electronic wave functions were expanded in a plane-wave basis set with a cutoff energy of 700 eV. Monkhorst-Pack k-point meshes with a grid of 0.025 Å −1 for Brillouin zone sampling were chosen to achieve the total energy convergence of less than 1 meV/atom.