Phase Transitions in Ammonium Fluoride

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Abstract
The temperature and pressure dependence of the transitions to two high‐pressure modifications of NH4F, one of which is reported for the first time, have been studied. By analogy with NH4I, it would appear that these transitions occur at room temperature from the normal wurtzite structure to an fcc (NaCl) structure at 3.8 kilobars to a bcc (CsCl) structure at 11 kilobars. The respective volume changes, roughly 28 and 11% of the zero pressure molar volume, can be understood quantitatively in terms of a simple hard‐sphere ionic picture with reasonable values for the ionic radii. Molar volumes and compressibilities have been obtained for these phases, as well as the transition parameters over a range of temperatures from 200° to 400°K.

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