The nature of ammonium ion disorder in ammonium tetrafluoroaluminate, NH4AlF4

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Abstract
In order 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 compound has been measured from T=15 to 305 K, by adiabatic calorimetry. The calorimetric results show a small anomaly in Cp centered at T=157 K (ΔS=0.02 R), and this is attributed to a soft mode phase transition associated with slight changes in the (AlF4)n−n layers. Analysis of the contribution of ammonium ion disorder to the heat capacity indicates that the ammonium ion disorder is gradually thermally excited, and can be adequately described by a three‐dimensional Einstein oscillator for which θE ∼400 K. There is no evidence of an order–disorder phase transition association with the ammonium ions.