The pressure and temperature evolution of the Ca3V2O8 crystal structure using powder X-ray diffraction

Abstract

We present a comprehensive experimental study of the crystal structure of calcium vanadate (Ca₃V₂O₈) under systematic temperature and pressure conditions. The temperature evolution (4–1173 K) of the Ca₃V₂O₈ structural properties is investigated at ambient pressure. The pressure evolution (0–13.8 GPa) of the Ca₃V₂O₈ structural properties is investigated at ambient temperature. Across all pressures and temperatures used in the present work, the Ca₃V₂O₈ crystal structure was determined by Rietveld refinement of powder X-ray diffraction data. The experimental high-pressure data are also supported by density-functional theory calculations. According to the high-pressure results, Ca₃V₂O₈ undergoes a pressure-induced structural phase transition at a pressure of 9.8(1) GPa from the ambient pressure trigonal structure (space group R3c) to a monoclinic structure (space group Cc). The experimentally determined bulk moduli of the trigonal and monoclinic phases are, respectively, B₀ = 69(2) GPa and 105(12) GPa. The trigonal to monoclinic phase transition appears to be prompted by non-hydrostatic conditions. Whilst the trigonal and monoclincic space groups show a group/subgroup relationship, the discontinuity in the volume per formula unit observed at the transition indicates a first order phase transition. According to the high-temperature results, the trigonal Ca₃V₂O₈ structure persists over the entire range of studied temperatures. The pressure-volume equation of state, axial compressibilities, Debye temperature (264(2) K), and thermal expansion coefficients are all determined for the trigonal Ca₃V₂O₈ structure.