The phase boundaries and the transition volumes of ammonium fluoride in the range 1 to 20 kbar and 20° to 340°C have been measured, and the transition entropies calculated. A new phase is reported, designated IV, that has an entropy about Rln6 greater than that of its neighbors I and II. This entropy difference suggests that the ammonium ions in phase IV are orientationally disordered among six equivalent positions. The liquid-I boundary shows that the liquid is mainly ionic.
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Kuriakose et al. (1968) studied this question.