A simple theory of thermal effects in ionic crystals

Abstract

An equation of state is obtained for ionic crystals based on the Born theory by assuming that the thermal motions of ions are uncorrelated and that the mean vibration energy of each ion is kT. The important feature of the theory is that the " thermal pressure " term is written entirely in terms of the interaction potentials. This is an advance over current born theory equations of state which require acknowledge of the experimental coefficient of thermal expansion. The calculated volume thermal expansivities of the alkali halides at room temperature and atmospheric pressure are in good agreement with experiment (r.m.s. error 12.5%). ' For many purposes, the present theory appears to be an adequate approximation for ionic crystals at finite temperatures.