Thermal properties and phase transitions in (NH4)3ZrF7

The heat capacity, thermal dilatation, permittivity, and T-p phase diagram of (NH4)3ZrF7 have been studied in wide temperature and pressure ranges. Two phase transitions were found in addition to previously known structural transformations. The stability of crystal phases to temperature and pressure has been examined and a possibility of transition between two cubic phases has been discussed. The total excess entropy change ΣΔSi = Rln6 is characteristic for the order–disorder processes, but its stepwise increase is not consistent with the structural model associated with the strong disorder assumed in the initial cubic phase.