On the energy of formation of defects

We present a formulation of the Gibbs free energy of defect formation in crystals which is formally based on a quasi-classical thermodynamical cluster expansion of the Gibbs free energy of the defect crystal. Thus a microscopic formulation of the Gibbs free energy of formation is achieved. The related quantities like formation volume per defect and defect susceptibility functions are then derived from the formation energy. The resulting expressions depend only on the defect concentration and the bulk properties of the crystal. Finally we discuss an approximate form of the formation energy which was introduced intuitively by Varotsos and Alexopoulos. As an example for the applicability of this formulation we present an explanation of the anomaly in the conductivity and diffusion of AgBr.