Physical Properties and Structural Stability of Cobalt Pyrovanadate Co₂V₂O₇ under High-Pressure Conditions

Metal vanadates MxV2O5-x, (with M divalent metal) are compounds of technological interest from the point of view of energy storage technology and photocatalysis. This notwithstanding, studies on their structural stability are scarce. Here, we report our findings on the high-pressure behavior of Co2V2O7. Cobalt pyrovanadate is found to be structurally stable up to at least 6 GPa, as single crystal and powder X-ray diffraction experiments confirm the absence of phase transitions in this pressure range. The stability of Co2V2O7 under compression is attributed to the 6-fold coordination of the Co2+ atom, as opposed, for instance, to the extreme instability observed in its Zn2V2O7 counterpart, where Zn2+ has an effective coordination number of less than 5. Regarding electronic and optical properties, Co2V2O7 is found to have an indirect band gap, which decreases under compression due to the shortening of the Co-O bonds. No collapses of the band gap have been observed up to 10 GPa. Ab initio simulations, up to above 10 GPa, support and extend our findings, which are discussed by comparison with analogue compounds. They also provide a description of vibrational and elastic properties, including Raman and IR modes and their symmetry assignment.