Solid Solutions of the Alkali Halides

Abstract

X‐ray studies of solid solutions of alkali halide pairs with a common ion at room temperature and at 550°C show that these pairs can be divided into three groups depending on the percent deviation δ of their lattice parameters. For values of δ less than 6 percent miscibility is complete at room temperature. For values of δ between 6 and 13 percent miscibility is complete at 550°C. For values of δ larger than about 13 percent miscibility is not complete at 550°C. In terms of the Born theory of ionic lattices, the free energy of mixing, and Vegard's law, an approximate theory for the phase curve of these solid solutions is derived. For a typical alkali halide with an electrostatic energy of 180 kcal. at the absolute zero of temperature the equation of the phase curve is $${\text{9δ}}^2\text{/T(1–2x)+}\ln \mathrm{x-}\ln \text{\hspace{0.17em}(1−x)=0,}$$ where x is the mole fraction of one component dissolved in the other. The temperature above which miscibility is complete is given by T=4.5δ². The heat of mixing is approximately −9δ² cal. The predictions of this theory are in good agreement with the above‐mentioned experimental observations.