Optic mode gruneisen parameters and their strain derivatives in alkali halides

Abstract

The Born-Mayer theory of interatomic forces and the Szigeti model of dielectric properties have been extended to predict the optic mode Gruneisen parameters and their volume derivatives for the alkali halides. A detailed analysis of the dielectric behaviour of Alkali halides under hydrostatic pressure is performed in the light of experimental data on first and second order strain derivatives of dielectric constants. To make the investigation critical and comprehensive we have adopted the inverse power law as well as the exponential law for the short range repulsive interactions. Three different sets of the Born repulsive parameters corresponding to compression data, ultrasonic data and dielectric data are used. The transverse optic mode Gruneisen parameters (γ TO) calculated, using short range repulsive parameters corresponding to compression and ultrasonic data, present good agreement with experimental values. The Lyddane, Sachs and Teller relation is used to predict the longitudinal optic mode Gruneisen parameters (γ LO) and their volume derivatives (∂γ LO/∂ V). The general features of the results obtained are that γ TO is larger than γ LO and that γ TO decreases with pressure whereas γ LO increases. A comparison of the average Gruneisen parameter obtained in the present study has been made with the macroscopic Gruneisen parameter deduced from the thermoelastic data. The volume dependence of the Szigeti effective charge parameter has also been predicted and discussed.