Thermodynamic Studies of Mineral-Like Cesium Compounds as Materials for Radiochemical Technologies

Interest in mineral-like cesium compounds is attributed to the development of environmentally friendly sources of Cs137 for medicine and materials for immobilizing cesium from nuclear waste. Phosphates Cs2Zr1.5Mg0.5(PO4)3, CsZr2(PO4)3, and CsMgPO4 are synthesized by the sol–gel method and characterized by powder X-ray diffraction and electron microprobe analysis. The temperature dependence of the heat capacity of Cs2Zr1.5Mg0.5(PO4)3 in a range of 79–1140 K is studied by precision adiabatic vacuum and differential scanning calorimetry. The heat capacity of CsZr2(PO4)3 and CsMgPO4 in a range of 320–940 K is studied by differential scanning calorimetry. Based on the data obtained, standard thermodynamic functions Cₚ⁰ (T), S0(T), H0(T) – H0(0), and G0(T) – H0(0) are calculated in a temperature range of (T → 0 to 1140 K) for Cs2Zr1.5Mg0.5(PO4)3 and in a range of 320–950 K for CsZr2(PO4)3 and CsMgPO4 taking into account the literature data. Using experimental and literature data, the standard Gibbs functions of formation of all the studied phosphates from elements and oxides at 298.15 K are calculated; it is shown that they exhibit high energetic stability (confirming the possibility of applying them in radiochemistry). The thermodynamics of the synthesis reactions for cesium–zirconium and cesium–magnesium–zirconium phosphates is analyzed; the probability of formation of these phosphates in the interior of the earth is explored.