The p-T phase diagram of ammonium hexafluoroaluminate

Mikhail V. Gorev (1) , Igor N. Flerov (1) , Alain Tressaud (2) , Etienne Durand (2)

1 L.V. Kirensky Institute of Physics
2 ICMCB - Institut de Chimie de la Matière Condensée de Bordeaux

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.

Mots clés: Ammonium hexafluoroaluminate; Phase diagrams

Domaines: Matériaux

Mikhail V. Gorev, Igor N. Flerov, Alain Tressaud, Etienne Durand. The p-T phase diagram of ammonium hexafluoroaluminate. Journal of Physics: Condensed Matter, 2002, 14 (25), pp.6447-6453. ⟨10.1088/0953-8984/14/25/312⟩. ⟨hal-00818777⟩