Study of the compressibility of alkali halides as a function of temperature

Abstract

In the present investigation, the shell-model technique has been applied to the study of the isothermal compressibility κT of five of the alkali halides, viz. NaCl, NaF, NaI, KCl, and KI. κT can be theoretically evaluated if the volume dependence of the potential energy Φ of the crystal lattice and the volume derivatives of the frequency spectra are known. Now Φ is taken to be of an exponential form, having two terms of which the first gives the energy due to Coulomb interactions between the ions, and the second one accounts for the overalp interaction energy between the ions in the nearest-neighbor approximation. The repulsive parameters and the volume derivatives of the frequencies are interdependent, and a correct estimate of them is made by using a process of iteration. The volume derivatives of the frequency spectra are obtained by solving the secular equations of the shell model at different volumes. κT's for all these solids are given in the temperature range 0-1000°K. The results show good agreement with experimental observations throughout the region. It is found, in agreement with experiments, that κT remains finite even at temperatures near the melting point, which is contrary to the findings of early workers. © 1976 The American Physical Society.