Complex Structural Evolution and Compressibility of β-Cu2V2O7 under Pressure

The structural response of β-Cu2V2O7 to compression was investigated by synchrotron single-crystal X-ray diffraction up to 12.6 GPa using helium as a highly hydrostatic pressure-transmitting medium and complemented by density-functional theory calculations. Compared with previous studies using 4:1 methanol–ethanol, the present experiments reveal a different pressure-induced structural evolution and significantly lower compressibility. Instead of the previously reported β → γ transition, β-Cu2V2O7 transforms from monoclinic β (C2/c) to a previously unreported monoclinic β′ (C2/m) phase at 0.3 GPa, followed by two triclinic polymorphs, γ′ (P1̅) at 3.3 GPa and γ″ (P1̅) at 8.4 GPa. These transitions involve increased Cu and V coordination numbers, substantial volume collapses, and enhanced framework connectivity. Accordingly, the bulk modulus increases from 54 GPa for the β phase to approximately 103 GPa for the highest-pressure polymorph. Experimental equations of state agree closely with ab initio calculations, which further show that the competing polymorphs have small enthalpy differences, helping explain the complex pressure-induced polymorphism. Differences between the present and the previously reported structural sequences are discussed in terms of the possible effects of experimental and intrinsic factors, although the available data do not establish a specific origin for the discrepancy.