On the Phase Transitions in Ammonium Hexafluoroaluminate(III)

Abstract

Abstract The heat capacities of (NH4)3[AlF6] were measured between 11 and 300 K with an adiabatic calorimeter. Two anomalies were found at (193.0±0.3) K and (220.79±0.05) K. The enthalpy and entropy of the phase transitions are (790±150) J mol−1 and (4.2±0.8) J K−1 mol−1 for the former, and (4030±150) J mol−1 and (18.5±0.7) J K−1 mol−1 for the latter, respectively. The total anomalous entropy, (22.7±1.5) J K−1 mol−1, is in agreement with R In 16=23.05 J K−1 mol−1 predicted by a model involving orientational disorder of the hexafluoroaluminate(III) and the ammonium ions. Discontinuities in fluorine T1 and T1ρ at 221 K by pulse NMR measurement also support this model. The potential barrier height of the hindered rotation of the NH4+ ion was estimated to be 7 kJ mol−1 from the heat capacity data. The intermediate phase undercooled and coexisted with the low temperature phase in a limited range of temperature. The upper phase transition is of the first order but the anomalous heat capacity follows the Landau theory of the phase transitions of the second kind. The high symmetry of the crystal structure of the present crystal at room temperatures in comparison with that of the alkali cryolites was discussed in terms of the hydrogen bonding, molecular disorder and packing consideration.