THE EFFECT OF 3-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 f orces. Generalizations of the shell model with three-body forces is us ually used to describe the physical properties depending on lattice vi brations in these crystals. In the shell model, the electronic polariz ability is given by the sum of the free-state polarizabilities and a p ure 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 depen dence of the electronic polarizability. The obtained results are compa red with available experimental data, and good agreement is found. For completeness, we also compare our results with other theoretical pred ictions. According to the present model, the sign of the Cauchy relati on is the result of competition between the three-body and breathing t erms. (C) 1997 Published by Elsevier Science Ltd.