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

Abstract

We have studied the high-pressure behavior of FeVO 4 by means of single-crystal X-ray diffraction (XRD) and density functional theory (DFT) calculations. We have found that the structural sequence of FeVO 4 is different from that previously assumed. In particular, we have discovered a new high-pressure phase at 2.11(4) GPa (FeVO 4 -I′), which was not detected by previous powder XRD studies. We have determined that FeVO 4 , under compression (at room temperature), first transforms at 2.11(4) GPa from the ambient-pressure triclinic structure (FeVO 4 -I) to a second previously unknown triclinic structure (FeVO 4 -I′), which experiences a subsequent phase transition at 4.80(4) GPa to a monoclinic structure (FeVO 4 -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 FeVO 4 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 FeVO 4 -I and FeVO 4 -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.

High pressure crystal structures of orthovanadates and their properties

“…Interestingly, the third HP phase is isomorphic to wolframite as the HP polymorphs of InVO4 [26] and CrVO4 [27]. Thus, systemically all this group of compounds systematically transforms into a wolframite-type structure in which V is in six-coordination, in which are expected to have a much smaller electronic band-gap, which is supported by color changes observed in the samples at the phase transitions [4,26,27] and by optical and resistivity measurements that we will discuss in the next section. Based upon crystal chemistry arguments and the high-pressure systematic behavior of related oxides [10] it would not be surprising that also AlVO4 and TlVO4 transform under compression (directly or by means of successive transitions) into a wolframite-type polymorph.…”

“…GPa being the structure of the HP polymorph also triclinic, but having both Fe and V cations in an octahedral coordination. In addition, below 10 GPa two additional phase transitions have been reported [4,10]. Interestingly, the third HP phase is isomorphic to wolframite as the HP polymorphs of InVO4 [26] and CrVO4 [27].…”

“…The existence of structures with V atoms in six-coordination occurs at much lower pressure than in lanthanide vanadates. Other finger print of InVO4 and related oxides is their larger compressibility, with bulk moduli below 70 GPa [4,11,26,27,28,105,106].…”

“…In the case of triclinic FeVO4, the high-pressure behavior than in InVO4 and CrVO4, but still some similarities can be found with these compounds. A combination of high-pressure single-crystal x-ray diffraction measurements and density functionaltheory calculations [4] have shown that FeVO4 has an extremely reach polymorphism under compression. In particular, in this vanadate the first transition takes place at 2.1…”

“…There are even compounds like FeVO4 [4], undergoing two successive phase transitions below 5 GPa. This feature makes the study of orthovanadates under controlled and quasi-hydrostatic high-pressure conditions quite affordable now-a-days.…”