The nature of ammonium ion disorder in ammonium tetrafluoroaluminate, NH4AlF4

Abstract

In order to clarify previous conflicting reports concerning the existence of an order-disorder phase transition in NH4AlF 4 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 (~ = 0.02 R), and this is attributed to a soft mode phase transition associated with slight changes in the (AlF 4): -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 (J E -400 K. There is no evidence of an order-disorder phase transition association with the ammonium ions.

Coexistence of metamagnetism and slow relaxation of magnetization in ammonium hexafluoridorhenate

“…Orientational disorder can occur in ammonium salts, and the disorder increases with temperature potentially changing the structure and Raman spectrum. [29][30][31] While the energetic ordering of the disordered structures appears unphysical as they would have been more apparent by XRD, they do indicate that sufficient thermalization of 1 will lead to dynamic distortions that emphasize hydrogen bonding.…”