Optical Dispersion and Ionicity

The theories of $A^N{B}^{8-N}$ semiconductors and salts due to Phillips and Van Vechten and to Wemple and DiDomenico are compared and their similarities discussed. The dispersion energy is found to be proportional to ionicity. A simple operational definition of ionicity, based on experimentally determined moments of ${\epsilon }_2\left(\mathcal{E}\right)$, is proposed. This definition can potentially be extended to provide a useful scaling parameter for other classes of materials such as metals and intermetallic compounds.