SPECIFIC HEATS OF POTASSIUM IODIDE BY A MODIFIED SHELL MODEL

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Abstract
The shell model of Woods et al. has been modified to include the effect of the many-body interaction in the ionic crystals in a simple phenomenological way. This is based on the work of Lowdin, Lundqvist, and Verma and Dayal. In the new model which the authors refer to as the modified shell model, it is assumed that the volume-dependent potential is due to the interaction between the cores of the ions only. This introduces additional parameters in the model which have been derived from the elastic and dielectric properties of the crystal. The variation of the specific heat with temperature of potassium iodide has been studied by the original model of Woods et al. as well as by the new model. It is seen that the theoretical specific heats and dispersion curves given by both the models are in fair agreement with the experimental results. However, the shell model fails to account for the Cauchy discrepancy in this crystal, whereas the new model removes this difficulty.