Point Defects in Simple Ionic Solids

Apart from man's innate need to model and the satisfaction that a good model can bring, the real purpose of scientific modelling is to increase our understanding of a system. More importantly, it provides the basis from which to move forward to understand more complex systems and to design such new systems for specific applications by making predictions about their properties. Simple ionic solids, such as the alkali and silver halides, some fluoritestructured crystals and binary oxides, provide the most accessible and well-developed testing grounds for the study, both experimental and theoretical, of point defects in crystalline materials. This is because defects in these crystals typically carry a charge different from those on the ions that comprise the normal components of the matrix. Their presence, nature, interactions and movements can therefore be rather easily quantitatively determined through the measurement of readily observed macroscopic properties such as ionic conductivity. These charges can be present whether the defects are intrinsic or extrinsic. Intrinsic defects, such as Schottky or Frenkel defects, are equilibrium thermodynamic defects and exist in all materials because the balance between the enthalpy required for their formation in a perfect lattice and the resulting increase in the entropy of the system gives rise to a minimum in the Gibbs free energy. There is a corresponding equilibrium intrinsic defect concentration at each temperature.