The effect of three-body forces on the volume dependence of the electronic polarizability of alkali halides

Violation of the Cauchy relation between the elastic constants for the alkali halide crystals clearly indicates the presence of multi-body forces. Generalizations of the shell model with three-body forces is usually used to describe the physical properties depending on lattice vibrations in these crystals. In the shell model, the electronic polarizability is given by the sum of the free-state polarizabilities and a pure crystalline term which depends on the potential. In this work, we propose a model for the three-body potential. This model is used with the breathing version of the shell model to calculate the volume dependence of the electronic polarizability. The obtained results are compared with available experimental data, and good agreement is found. For completeness, we also compare our results with other theoretical predictions. According to the present model, the sign of the Cauchy relation is the result of competition between the three-body and breathing terms. (C) 1997 Published by Elsevier Science Ltd.