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The ammonium halides are an interesting systems because of their polymorphism and the possible internal rotation of the ammonium ion. The static properties of the mixed ionic crystal NH 4 Cl 1−x Br x have been recently investigated, using the three-body potential model (TDPM) by applying Vegard's law. Here, by using a simple theoretical model, we estimate the bulk modulus of the alloys NH 4 Cl 1−x Br x , in terms of the bulk modulus of the end members alone. The calculated values are comparable to those deduced from the three-body potential model (TDPM) by applying Vegard's law.

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The growth of multiphased binary and ternary mixed crystals by the melt method using the miscible alkali halides, viz., NaBr and KCl, along with the experimental determination of their compressibility, have been recently reported. These materials are particularly useful for the construction of laser optical windows. Here, we suggest a theoretical model that enables the estimation of the compressibility of the multiphased mixed crystals in terms of the elastic data of the end members alone. The calculated values are in reasonable agreement with the experimental ones. The model makes use of an early suggestion that interconnects the Gibbs energy for the formation and migration of defects in solids, with bulk properties, and has been found of value in a large variety of solids.