Ammonium fluoride II and III have been recovered at 100°K and atmospheric pressure and examined by x-ray powder diffraction and by thermal analysis. The structure of phase III is closely related to that of cesium chloride. Phase II when heated from 100°K at atmospheric pressure transforms to a new phase, designated V, at about 120°K and V in turn transforms to I at about 230°K. Phase III under the same conditions transforms at about 120°K to a new phase, designated VI, and this in turn transforms to another new phase designated VII at about 200°K. On further heating to 230°K, phase VII transforms to I. Phases V and VII are probably stacking variants of the wurtzite-type structure of ammonium fluoride I. Phases II and VI have powder patterns too complicated to analyze unambiguously on the data available.
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Nabar et al. (1969) studied this question.