High-temperature phase transformation and thermodynamic properties of Ca3(VO4)2

Ca-3(VO4)(2) is a promising candidate for applications in ferroelectric, laser host, and optical materials owing to its unique whitlockite structure and excellent physicochemical properties. In this study, the Ca-3(VO4)(2) powder was fabricated via a conventional solid-phase route in air, using V2O5 and CaO as precursors. The trigonal Ca-3(VO4)(2) belongs to the space group R3c, having unit cell parameters of a = 10.8074 & Aring;, b = 10.8074 & Aring;, and c = 37.98871 & Aring;, respectively. Ca-3(VO4)(2) melts congruently at 1680 K, as determined from differential scanning calorimetry measurements of the thermal profile of the second heating test. Enthalpy changes in Ca-3(VO4)(2) were experimentally determined for the first time across temperatures between 573 and 1623 K by drop calorimetry. The temperature-dependent molar heat capacity of Ca-3(VO4)(2) was calculated from the enthalpy changes at elevated temperatures. Thermodynamic properties (entropy, enthalpy, and Gibbs free energy changes) were also calculated. The thermodynamic properties measured in this study were further applied to evaluate the appropriate precursors for fabricating Ca-3(VO4)(2) via solid-state calcination. The results strongly recommend the use of V2O5 and Ca(OH)(2) precursors for preparing the Ca-3(VO4)(2) samples because of their thermodynamic advantages.