Phase transitions and thermodynamic properties of (NH_4)_3VO_2F_4 cryolite

摘要

Calorimetric measurements performed in a wide temperature range on (NH_4)_3VO_2F_4 have shown the presence of four heat capacity anomalies at T_1 =438 K, T_2 = 244 K, T_3 = 210.2 K, T_4 = 205.1 K associated with the first order phase transitions. In accordance with the permittivity behavior, the structural transformations are of nonferroelectric nature. Pressure dependence of the phase transition temperatures has been studied by DTA under pressure. The entropy of phase transitions is analyzed mainly in the framework of the orientational disordering of NH~-_4 and VO_2F~(3-)_4 ions in a cubic phase.