Thermochemistry of Framework Zirconium Phosphates A2Zr1.5Mg0.5(PO4)3 (A = K, Rb, Cs) with a Langbeinite Structure

The phosphates A2Zr1.5Mg0.5(PO4)3 (A = K, Rb, Cs) with a langbeinite structure are of interest as environmentally stable crystalline forms for radwaste immobilization. The phosphates were obtained by the sol-gel method and characterized by X-ray diffraction and electron microprobe analysis (EMPA). The enthalpies of drop-solution in a sodium molybdate melt (3Na2O⋅4MoO3) at 973 K were measured by high-temperature calorimetry. The standard enthalpies of formation from oxides (∆f,oxH°) and elements (∆f,elH°) at 298.15 K were calculated from the obtained experimental data. The energy stabilization of the zirconium–magnesium framework in a series of alkali cations was analyzed.