ABSTRACT
The heat capacity, thermal expansion, and T-p phase diagrams of (NH4)3TiF7 and (NH4)3SnF7 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)3TiF7. The different sign of baric coefficients for phase transition between two cubic phases in (NH4)3TiF7 and (NH4)3SnF7 was supposed due to nonlinear phase boundary with pressure in the latter fluoride.
Acknowledgment
The reported study was partially supported by RFBR, research project No 15-02-02009-a.