Columbite Single-Crystal CoV₂O₆ under High Pressure: An XRD and Raman Spectroscopy Study

The application of high pressure to orthorhombic columbite CoV₂O₆ resulted in two phase transitions, characterized by single-crystal X-ray diffraction (XRD) and Raman spectroscopy up to 54.9(1) GPa. The first phase transition, at 20.0(1) GPa, is isostructural, preserving the space group (<i>Pbcn</i>) but implies a sudden volume reduction (4.7%) and a symmetrization of VO₆ octahedra, as well as a group shift of the vibrational modes toward higher wavenumbers. The named columbite-II structure remains stable up to 45.6(1) GPa. Beyond this pressure, a different monoclinic polymorph (space group <i>P</i>2₁/<i>c</i>) was deduced from the high-quality diffraction data. The two observed phase transitions are reversible, but below 4.9(3) GPa, the sample mainly transforms into the previously known brannerite polymorph. The anisotropic compressibility of the lattice axis and the Birch-Murnaghan equation of state were obtained from the XRD analysis. Our experimental results are supported by density functional theory calculations.