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

The high-pressure behavior of the crystalline structure FeVO 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 FeVO compound. High-pressure X-ray diffraction measurements show that the triclinic P1̅ (FeVO-I) phase remains stable up to ≈3 GPa; then a first-order phase transition to a new monoclinic polymorph of FeVO (FeVO-II') with space group C2/ m is observed, having an α-MnMoO-type structure. A second first-order phase transition is observed around 5 GPa toward the monoclinic ( P2/ c) wolframite-type FeVO-IV structure, which is stable up to 12.3 GPa in coexistence with FeVO-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 FeVO. We found that theoretical results follow the pressure evolution of structural parameters of FeVO, in good agreement with the experimental results. Also, we analyze FeVO-II (orthorhombic Cmcm, CrVO-type structure) and -III (orthorhombic Pbcn, α-PbO-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 FeVO-IV to raspite (monoclinic P2/ c) type structure (FeVO-V) at 36 GPa (Δ V = -8.1%) and a further phase transition to the AgMnO-type (monoclinic P2/ c) structure (FeVO-VI) at 66.5 GPa (Δ V = -3.7%), similar to the phase transition sequence reported for InVO.