The nature of ammonium ion disorder in ammonium tetrafluoroaluminate, NH4AlF4.

Abstract

To clarify previous conflicting reports concerning the existence of an order-disorder phase transition in NH4AlF4 at T ?150 K, the heat capacity of this compd. was measured from T = 15 to 305 K, by adiabatic calorimetry. The calorimetric results show a small anomaly in Cp centered at T = 157 K (?S = 0.02 R), and this is attributed to a soft mode phase transition assocd. with slight changes in the (AlF4)nn- layers. Anal. of the contribution of NH4 ion disorder to the heat capacity indicates that the NH4 ion disorder is gradually thermally excited, and can be adequately described by a 3-dimensional Einstein oscillator for which ?E ?400 K. There is no evidence of an order-disorder phase transition assocn. with the NH4 ions.