Abstract
Starting from the HartreeโFock wave functions of free ions different lattice mechanical properties of a lowโpolarisability ionic solid, namely, the RbF crystal are calculated within the framework of the perturbationโtheoreticalโmodel approach of ionic solids developed by Basu and Sengupta. In a recent investigation it is shown that the dipolar polarisation in an ionic solid may be additively decomposed into three parts, two short range polarisation effects and a long range one apart from the displacement polarisation. From an analysis of the Szigeti relations it is found that both the short range polarisation effects in the low polarisability ionic crystals are small compared to those in high polarisability crystals. This fact leads to a major simplification in calculation for this group of solids. The overall agreement obtained broadly justifies the approximations. Since no crystal property is used to obtain the parameters the discrepancies clearly indicate the regions where the omitted interactions are important. Finally a critical analysis of the results is presented and the difficulties of the existing phenomenological models for these solids are also discussed.