Phase transitions and thermodynamic properties of (NH4)3VO2F4 cryolite

Calorimetric measurements performed in a wide temperature range on (NH4)3VO2F4 have shown the presence of four heat capacity anomalies at T1 = 438 K, T2 = 244 K, T3 = 210.2 K, T4 = 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 NH4+ and VO2F43− ions in a cubic phase.

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