Thermodynamic properties and p-T phase diagrams of (NH4)(3)M3+F6 cryolites(M3+: Ga, Sc)

Calorimetric and high pressure experiments are used to establish thermodyna mic features of (NH4)(3)M3+F6 cryolites with M3+: Ga, Sc as they undergo on e and three structural phase transitions, respectively. The heat capacity m easurements performed between 80 K and 370 K, using an adiabatic calorimete r, have shown that all phase transitions under study are first order with d ifferent closeness to the tricritical point. The total entropy change in th e Sc compound connected with the successive transformations is almost equal to the entropy change in the Ga cryolite. The effect of hydrostatic pressu re has been studied up to 0.6 GPa, using differential thermographic analysi s. The p-T phase diagrams of both cryolites were found to be rather complic ated, including triple points. The experimental results are considered as c onnected with order-disorder phase transitions.