The nature of ammonium ion disorder in ammonium tetrafluoroaluminate, NH4AlF4.
White, Mary Anne
Wagner, Brian D.
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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.
Reproduced from White, Mary Anne, and Brian D. Wagner. 1985. "The nature of ammonium ion disorder in ammonium tetrafluoroaluminate, NH4AlF4.." Journal of Chemical Physics 83(11): 5844-5848, with the permission of AIP Publishing.