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 pressurevolume equation of state, axial compressibilities, Debye temperature (264(2) K), and thermal expansion coefficients are all determined for the trigonal Ca3V2O8 structure

Study of Structure of Defect Centers in Europium Vanadate Nanoparticles with Heterovalent Dopants

The EuVO$_4$ nanoparticles doped with Ca$^{2+}$ impurities were synthesized by citrate-nitrate sol-gel method and investigated. Diffuse reflection spectra of the EuVO$_4$:Ca samples contain two additional bands around 400 and 470 nm those were not observed for the undoped EuVO$_4$ . These bands were ascribed to Ca-induced defects in the vanadate crystal lattice. Origin of these defects is studied taking into account results of high resolution XRD measurements. The additive XRD peaks were found for the Eu$_{0.8}$Ca$_{0.2}$VO$_4$ , sample. These peaks were identified as traces of the Ca$_2$ V$_2$ O$_7$ second crystal phase in the synthesized nanoparticles. The maximal concentrations of the Ca$^{2+}$ heterovalent impurity permits to keep single phase EuVO$_4$:Ca sol-gel nanoparticles were estimated