The thermodynamics of mixed crystals of ammonium chloride and ammonium bromide. - III. The heat capacity from 8 to 300 K of solid solutions of composition NH4Br0.20Cl0.80and NH4Br0.74Cl0.26

Abstract

The heat capacity of the mixed crystals NH4Br0.20CI0.80and NH4Br0.74CI0.26has been measured fromca. 8 to 300 K. The first of these solids has, as expected, two gradual transitions, the lower having an entropy of transition ∆Sλof 3.79 J K–1mol–1, and the upper a ∆Sλof 4.51 J K–1mol–1. The heat capacity between these two transitions is always considerably above the ‘normal’ or baseline value, and it appears that the orientational disordering of the ammonium ions, while completed in theγ→ β transition, commences in the δ-phase. The other, bromide-rich mixed crystal shows no sign of a δ →γtransition, but has aλ-typeγ→ β transition which culminates at 262.5 K, with ∆Sλ= 7.31 J K–1mol–1. The heat capacity of both mixed crystals in the β-phase, like that of the pure components, is ‘anomalously’ high, the additional heat capacity for the solid solutions being larger than that for the pure salts. The phase diagram for the ammonium-chloride–ammonium-bromide system is shown; it combines previous information with that provided by the heat-capacity results reported in this and previous papers.