Polymorphic Transitions in Metallic Halides

Abstract

An x-ray powder analysis of RbI made at 4500 kg/${\mathrm{cm}}^{2}$ pressure and room temperature shows the crystal form of RbI to be body-centered cubic under these conditions. The lattice constant is 4.33$_{5}\mathrm{A}$. The volume change from the face-centered lattice which is stable below 4000 atmospheres is 0.037 ${\mathrm{cm}}^{3}$ per gram. This is about 0.009 ${\mathrm{cm}}^{3}$ per gram greater than Bridgman's piston displacement value and the difference may be significant.With the exception of the fluorides, all of the rubidium and potassium halides undergo polymorphic transformations under pressure, which in the light of the present knowledge of RbI are assumed to be of the face-centered to body-centered type. The method of Born and Mayer is used to calculate the lattice constants for both face- and body-centered types of each halide at the experimentally known temperature and pressure of equilibrium. From these the volume changes of the transformation are computed. They are found to agree quantitatively with the experimental values. Theoretical values for the transition pressure are also given. These are confirmed by experiment only in a qualitative manner, but the method may be used to explain, for instance, the nonoccurrence of transitions in the sodium halides up to 50,000 atmospheres. Similar calculations are made for AgI, the crystal form of which has been previously shown by the author to be face-centered cubic at pressures beyond 3000 kg/${\mathrm{cm}}^{2}$.