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