Solid-state solubility and its limits. The alkali halide case

The group of binary common-ion alkali halide systems correspond to an excel lent vehicle for discussing and demonstrating the influence of mismatch in size - between the building units of the: host crystal and the units of the guest - on the formation of mixed crystals and the limits of solid-stare s olubility. The numerical values of just four constants are needed to formul ate a uniform thermodynamic description of the mixed crystalline state of t he group of systems. In terms of deviation from ideal mixing behavior, in t erms of the excess Gibbs energy, two constants are needed to relate the mag nitude of the function to mismatch in size. One constant is to account for the asymmetry of the function, and one to express the dependence on tempera ture. The values of the constants have been assessed with the help of data available for 13 systems showing complete miscibility. The uniform descript ion allows one to calculate, fur every system of the group, the solubilitie s of the components in one another at any temperature.