High-pressure characterization of multifunctional CrVO₄

Abstract

The structural stability and physical properties of CrVO4 under compression were studied by x-ray diffraction, Raman spectroscopy, optical absorption, resistivity measurements, and ab initio calculations up to 10 GPa. High-pressure x-ray diffraction and Raman measurements show that CrVO4 undergoes a phase transition from the ambient pressure orthorhombic CrVO4-type structure (Cmcm space group, phase III) to the high-pressure monoclinic CrVO4-V phase, which is proposed to be isomorphic to the wolframite structure. Such a phase transition (CrVO4-type → wolframite), driven by pressure, also was previously observed in indium vanadate. The crystal structure of both phases and the pressure dependence in unit-cell parameters, Raman-active modes, resistivity, and electronic band gap, are reported. Vanadium atoms are sixth-fold coordinated in the wolframite phase, which is related to the collapse in the volume at the phase transition. Besides, we also observed drastic changes in the phonon spectrum, a drop of the band-gap, and a sharp decrease of resistivity. All the observed phenomena are explained with the help of first-principles calculations.

High pressure crystal structures of orthovanadates and their properties

“…However, after the phase transition, due to a huge volume collapse; a relative volume change of 20%, the density of the compounds is considerably increased and the bulk modulus becomes comparable to that of zircon-type vanadates. Analogous results to those above described for InVO4 has been also reported for CrVO4 [27]. The changes induced by pressure in the crystal structure drive important changes in the electronic properties which will be discussed in the section devoted to electronic properties.…”

“…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]. 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.…”

“…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].…”

“…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.…”

“…In addition to rare-earth vanadates, another group of materials studied under HP are InVO4 [26], CrVO4 [27], FeVO4 [28], and BiVO4 [29,30]. The main interest in these compounds comes from the fact that they have an optical band gap, which has an optimum value for the non-contaminant production of hydrogen, a zero-emission fuel, from solar energy via the photocatalytic splitting of water [31].…”