The electronic structure of four ternary-metal oxides containing isolated vanadate ions is studied. Zircon-type YVO 4 , YbVO 4 , LuVO 4 , and NdVO 4 are investigated by high-pressure optical-absorption measurements up to 20 GPa. Firstprinciples calculations based on density-functional theory were also performed to analyze the electronic band structure as a function of pressure. The electronic structure near the Fermi level originates largely from molecular orbitals of the vanadate ion, but cation substitution influence these electronic states. The studied ortovanadates, with the exception of NdVO 4 , undergo a zircon-scheelite structural phase transition that causes a collapse of the band-gap energy. The pressure coefficient dE g /dP show positive values for the zircon phase and negative values for the scheelite phase. NdVO 4 undergoes a zircon-monazite-scheelite structural sequence with two associated band-gap collapses.

Using in-situ angle dispersive X-ray diffraction, we have shown that barium tungstate, which exists in scheelite phase at ambient conditions, transforms to a new phase at , 7^0.3 GPa. Analysis of our data based on Le Bail refinement suggests that this phase could be fergusonite and not HgMoO 4 type, which was proposed earlier from the Raman investigations. Beyond , 14 GPa, this compound undergoes another phase transformation to a significantly disordered structure. Both the phase transitions are found to be reversible. q

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: Raman-scattering measurements on NdVO 4 suggest a pressure-induced zircon to monazite phase transition beyond 5.9 GPa. The monazite phase undergoes a second phase transition to a yet unknown phase at 18.1 GPa. Lattice-dynamics calculations well support the experimental findings and predict a possible orthorhombic structure for the post-monazite structure of NdVO 4 .

X-ray diffraction and Raman-scattering measurements on cerium vanadate have been performed up to 12 and 16 GPa, respectively. Experiments reveal that at 5.3 GPa the onset of a pressure-induced irreversible phase transition from the zircon to the monazite structure. Beyond this pressure, diffraction peaks and Raman-active modes of the monazite phase are measured. The zircon to monazite transition in CeVO 4 is distinctive among the other rare-earth orthovanadates. We also observed softening of external translational T(E g ) and internal ν 2 (B 2g ) bending modes. We attributed it to mechanical instabilities of zircon phase against the pressure-induced distortion. We additionally report lattice-dynamical and total-energy calculations which are in agreement with the experimental results. Finally, the effect of non-hydrostatic stresses on the structural sequence is studied and the equations of state of different phases are reported.

X-ray diffraction and Raman scattering studies on the scheelite structured barium molybdate show that, at ~ 5.8 GPa, it undergoes a first order phase transition to the fergusonite structure (I2/a, Z = 4) -as also observed in iso-structural barium tungstate. At still higher pressures, barium molybdate transforms to another phase between ~ 7.2 -9.5GPa. On release of pressure from 15.8 GPa, the initial phase is recovered implying that the observed structural modifications are reversible. PACS : 61.10.-i, 62.50.+p, 61.50 Ks

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High-pressure Raman-scattering measurements and ab initio calculations on NdVO4 have been carried out up to 30 GPa. Our combined experimental and theoretical study confirms that beyond 5.9 GPa NdVO4 undergoes an irreversible zircon to monazite transition. The coexistence of zircon and monazite phases is experimentally observed up to ~ 8 GPa (which agrees with the theoretical transition pressure), stabilizing the monazite phase as a single phase around 10 GPa. Calculations additionally predict the existence of a second high-pressure phase transition at 12.4 GPa. This reversible phase transition has been experimentally observed beyond 18.1 GPa and remains stable up to 30 GPa. The post-monazite phase is predicted to have a monoclinic structure isomorphic to the BaWO4-II type structure. The calculated structure for the three polymorphs of NdVO4 is reported and the pressure dependence of their Raman modes is discussed.