The p$ndash$T phase diagram of ammonium hexafluoroaluminate

The heat capacity and the effect of hydrostatic pressure on structural phase transitions in ammonium hexafluoroaluminate, (NH4)3AlF6, have been studied. Two heat capacity anomalies have been found with maxima at T 1 = 218.5 ± 0.5 K and T 2 = 179 ± 2 K. Respective entropy changes of phase transitions are ΔS1 = 15.3 ± 0.5 J mol−1 K−1 and ΔS2 = 2.5 ± 0.5 J mol−1 K−1. It is shown that the p–T phase diagram of this compound is rather complex and contains a triple point (ptp = 0.12 GPa, Ttp = 221 K) and high-pressure phase. The mechanism of transformations and generalized p–T phase diagram for compounds of the (NH4)3MIIIF6 family are discussed within the framework of the rotational order–disorder model.