Calorimetric, dilatometric and DTA under pressure studies of the phase transitions in elpasolite (NH4)(2)KZrF7

Heat capacity, thermal expansion, and sensitivity to the hydrostatic pressure of (NH₄)₂KZrF₇ elpasolite are studied in a wide temperature range. The changes in deformation and entropy during successive phase transitions are determined: Δ(ΔV/V) = 3·10⁻⁴; ΔS = 8 J/mol K The temperatures and entropies of phase transitions turned out to be slightly sensitive to pressure changes. An analysis of the entropy of phase transformations was performed in the framework of the model of the cubic phase structure Fm-3 m. In the low temperature phase, an anomalous behavior of thermodynamic properties, which is not characteristic of phase transitions, was observed, accompanied by a significant change in the crystal lattice entropy.