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 P (1) over bar (FeVO4-I) phase remains stable up to approximate 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 alpha-MnMoO4-type structure. A second first-order phase transition is observed around 5 GPa toward the monoclinic (P2/c) wolframite-type FeVO4-W structure, which is stable up to 12.3 GPa in coexistence with FeVO4-Ir. The unit cell volume reductions for the first and second phase transitions are Delta 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, alpha-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 P2(1)/c) type structure (FeVO4-V) at 36 GPa (Delta V = 8.1%) and a further phase transition to the AgMnO4-type (monoclinic P2(1)/c) structure (FeVO4-VI) at 66.5 GPa (Delta V = 3.7%), similar to the phase transition sequence reported for InVO4.