Model calculation of the pressure derivative of the low-frequency dielectric constant of alkali halide crystals

The mechanism of polarization of insulators in a static electric field is still not clearly understood. The current situation presents an anomaly. The model which describes the high-frequency dielectric behavior quite well is found to be inadequate for the static case. In fact, with the same set of parameters within any single phenomenological model, it has not been possible to describe satisfactorily both the dielectric constants and their pressure derivatives. In search of a suitable model which can achieve this we have, in the present work, tried to analyze the mechanism of polarization from first principles. The interesting finding of this analysis is that there are two types of short-range polarization effects having entirely different origins: One of which (the second-order exchange interaction) contributes both to the static and high-frequency dielectric properties and the other (the first-order exchange interaction) to the static one only. We have suggested a model based on this analysis. The present calculation of the pressure derivative of the static dielectric constant of alkali halide crystals within a simplified version of this model together with our earlier calculation of the same for the high-frequency dielectric constant provides a fair overall description of the entire dielectric behavior of insulators. In addition, the present investigation also discusses the approximations which yield the well-known models for ionic solids which include polarization.