Ionicity and the Structural Stability of Solids

The dielectric theory of Phillips and Van Vechten defines crystalline ionicity in terms of optical properties and predicts first-order structural phase transitions. Their approach, while very successful, is, nonetheless, empirical and has yet to be justified via an ab initio quantum mechanical approach. We present such an approach for examining the relationship between spectroscopic crystal ionicity and spatial structure. We demonstrate that the structural predictions based on ionicity scales derived from the dielectric theory are consistent with a microscopic point of view.