Point Defects in Ionic Crystals

Abstract

In recent years there have been many calculations of the properties of point defects in ionic crystals, particularly calculations of the energies of vacancies, interstitials and impurity ions. Apart from those on colour centres where one is obliged to solve for the quantum mechanical states and energy levels of the centre, these calculations have been almost entirely made using classical ionic models derived from the original work of Born. Even in the case of colour centres questions of the relaxation of the crystal about the centre and of electron-lattice coupling are answered by using such models to describe the lattice response. The importance of the long-range Coulomb forces in all these models clearly differentiates them from those for other types of solid. It is this fact also which in the long run allows us to make quantitative predictions for ionic crystals with greater confidence than for other materials.