Stability of FeVO4 under Pressure: An X-ray Diffraction and First-Principles Study.

The high-pressure behavior of the crystalline structure FeVO4 has been studied by means of X-ray diffraction using a diamond-anvil cell and first-principles calculations. The experiments were carried out up to a pressure of 12.3 GPa, until now the highest pressure reached to study an FeVO4 compound. High-pressure X-ray diffraction measurements show that the triclinic P1̅ (FeVO4-I) phase remains stable up to ≈3 GPa; then a first-order phase transition to a new monoclinic polymorph of FeVO4 (FeVO4-II') with space group C2/ m is observed, having an α-MnMoO4-type structure. A second first-order phase transition is observed around 5 GPa toward the monoclinic ( P2/ c) wolframite-type FeVO4-IV structure, which is stable up to 12.3 GPa in coexistence with FeVO4-II'. The unit cell volume reductions for the first and second phase transitions are Δ V = -8.5% and -13.1%. It was observed that phase transitions are irreversible and both high-pressure phases remain stable once the pressure is released. Calculations were performed at the level of the generalized gradient approximation plus Hubbard correction (GGA+ U) and with the hybrid Heyd-Scuseria-Ernzerhof (HSE06) exchange-correlation functional in order to have a good representation of the pressure behavior of FeVO4. We found that theoretical results follow the pressure evolution of structural parameters of FeVO4, in good agreement with the experimental results. Also, we analyze FeVO4-II (orthorhombic Cmcm, CrVO4-type structure) and -III (orthorhombic Pbcn, α-PbO2-type structure) phases and compare our results with the literature. Going beyond the experimental results, we study some possible post-wolframite phases reported for other compounds and we found a phase transition for FeVO4-IV to raspite (monoclinic P21/ c) type structure (FeVO4-V) at 36 GPa (Δ V = -8.1%) and a further phase transition to the AgMnO4-type (monoclinic P21/ c) structure (FeVO4-VI) at 66.5 GPa (Δ V = -3.7%), similar to the phase transition sequence reported for InVO4.

High-Pressure Crystal Structure, Lattice Vibrations, and Band Structure of BiSbO4.

The high-pressure crystal structure, lattice-vibrations, and electronic band structure of BiSbO4 were studied by ab initio simulations. We also performed Raman spectroscopy, infrared spectroscopy, and diffuse-reflectance measurements, as well as synchrotron powder X-ray diffraction. High-pressure X-ray diffraction measurements show that the crystal structure of BiSbO4 remains stable up to at least 70 GPa, unlike other known MTO4-type ternary oxides. These experiments also give information on the pressure dependence of the unit-cell parameters. Calculations properly describe the crystal structure of BiSbO4 and the changes induced by pressure on it. They also predict a possible high-pressure phase. A room-temperature pressure-volume equation of state is determined, and the effect of pressure on the coordination polyhedron of Bi and Sb is discussed. Raman- and infrared-active phonons were measured and calculated. In particular, calculations provide assignments for all the vibrational modes as well as their pressure dependence. In addition, the band structure and electronic density of states under pressure were also calculated. The calculations combined with the optical measurements allow us to conclude that BiSbO4 is an indirect-gap semiconductor, with an electronic band gap of 2.9(1) eV. Finally, the isothermal compressibility tensor for BiSbO4 is given at 1.8 GPa. The experimental (theoretical) data revealed that the direction of maximum compressibility is in the (0 1 0) plane at ∼33° (38°) to the c-axis and 47° (42°) to the a-axis. The reliability of the reported results is supported by the consistency between experiments and calculations.