Thermal properties of (NH4)2MeF6·NH4F (Me: Ti, Sn) crystals undergoing transformation between two cubic phases

The heat capacity, thermal expansion, and T-p phase diagrams of (NH4)(3)TiF7 and (NH4)(3)SnF7 were studied in wide temperature and pressure ranges. The total excess entropies at successive Pa-3 P4/mnc 4/m (Ti) and single Pa-3 Pm-3m (Sn) phase transitions are close to each other and characteristic for the order-disorder processes. The Pm-3m cubic phase and direct Pa-3 Pm-3m transformation were found at high pressure in (NH4)(3)TiF7. The different sign of baric coefficients for phase transition between two cubic phases in (NH4)(3)TiF7 and (NH4)(3)SnF7 was supposed due to nonlinear phase boundary with pressure in the latter fluoride.