Pressure induced structural transitions, coordination crossover, and optoelectronic response in divalent metavanadate CaV${}_2$O${}_6$

Abstract

High-pressure synchrotron X-ray diffraction, Raman spectroscopy, photoluminescence (PL), and UV-Vis absorption measurements were carried out on the divalent metavanadate CaV${}_2$O${}_6$ up to 31 GPa at room temperature. Ambient CaV${}_2$O${}_6$ crystallizes in a centrosymmetric pseudo-brannerite monoclinic structure ($C2/m$), where vanadium atoms reside in fivefold-coordinated VO${}_5$ trigonal bipyramids. Under hydrostatic compression, the system undergoes a structural phase transition starting at $\sim$1 GPa to a lower-symmetry, non-centrosymmetric monoclinic phase ($C2$), which becomes the single stable phase above 5.2 GPa. This transition involves an increase in vanadium coordination from VO${}_5$ bipyramids to edge-sharing distorted VO${}_6$ octahedra. High-pressure Raman scattering corroborates the structural phase boundaries, exhibiting mode softening (${\gamma }_3=-0.42$, ${\gamma }_9=-0.46$) within the mixed-phase regime (1.0 - 4.4 GPa) and sharp anomalies in mode frequencies and linewidths across 5.2 GPa. The optical measurements show complete PL quenching near 1 GPa due to non-radiative recombination pathways introduced by local polyhedral distortions, alongside a non-monotonic evolution of the indirect optical band gap. Above 20 GPa, an anomaly in polyhedral distortion and internal strain drives an isosymmetric structural modification within the $C2$ phase, marked by an alteration in bulk compressibility and stabilization of the optical gap. Comparative high-pressure band-gap measurements on the sister compound MgV${}_2$O${}_6$ validate the explicit coupling between cation coordination changes and optoelectronic response in divalent metavanadates.