Shell Model Calculation of Thermal Expansion of Alkali Halides and Magnesium Oxide

The microscopic Grüneisen parameters and the temperature dependence of the thermal Grüneisen parameter have been calculated in the quasi-harmonic approximation from an anharmonic shell model for MgO, LiF, NaF, NaI, KCl, KBr, KI, RbCl, RbBr, and RbI. The shell model contains six harmonic and five anharmonic parameters which are determined from experimental elastic, dielectric, and optical data. Anharmonic effects are partly eliminated by referring all quantities to constant volume corresponding to 0 °K. The calculated microscopic Grüneisen parameters agree well with the available experimental data for NaI and RbI. For the thermal Grüneisen parameter the agreement with experimental data is in general within the joint experimental error of the thermal data and the empirical input data used to characterize the interatomic forces. The rigid ion model and the shell model with the parameters determined from room-temperature input data give much less satisfactory agreement. The results reproduce also the fact that the magnitude and shape of the Grüneisen function γ(T) are determined mainly by the cation.