Precise Characterization of the Rich Structural Landscape Induced by Pressure in Multifunctional FeVO₄

We have studied the high-pressure behavior of FeVO₄ by means of single-crystal X-ray diffraction (XRD) and density functional theory (DFT) calculations. We have found that the structural sequence of FeVO₄ is different from that previously assumed. In particular, we have discovered a new high-pressure phase at 2.11(4) GPa (FeVO₄-I'), which was not detected by previous powder XRD studies. We have determined that FeVO₄, under compression (at room temperature), first transforms at 2.11(4) GPa from the ambient-pressure triclinic structure (FeVO₄-I) to a second previously unknown triclinic structure (FeVO₄-I'), which experiences a subsequent phase transition at 4.80(4) GPa to a monoclinic structure (FeVO₄-II'), which was also previously detected in powder XRD experiments. Single-crystal XRD has enabled these novel findings as well as an accurate determination of the crystal structure of FeVO₄ polymorphs under high-pressure conditions. The crystal structure of all polymorphs has been accurately solved at all measured pressures. The pressure dependence of the unit-cell parameters and polyhedral coordination have been obtained and are discussed. The room-temperature equation of state and the principal axes of the isothermal compressibility tensor of FeVO₄-I and FeVO₄-I' have also been determined. The structural phase transition observed here between these two triclinic structures at 2.11(4) GPa implies abrupt coordination polyhedra modifications, including coordination number changes. DFT calculations support the conclusions extracted from our experiments.