Schottky Defects in Alkali Halides

The method of lattice statics and the deformation-dipole model have been applied to Schottky defects in ionic crystals in approximations higher than zero order. For an isolated vacancy, all host-lattice-ion potential energy functions for short-ranged repulsive and Coulomb-defect forces were expanded to terms quadratic in the displacements and dipole moments, and the resulting forces were allowed to act on the first and/or second neighbors to the vacancy. The zero-order Coulomb force was allowed to act on the remainder of the host-lattice ions past first and/or second neighbors. The vacancy and Schottky-pair-formation energies in the 12 Na, K, Rb halides have been calculated. A comparison is made between the zero-order approximation and the one considered here. Fairly good agreement is obtained between our calculated values and the experimental values for the Schottky-pair-formation energies if the polarizations around the defect are properly treated with respect to deformation dipoles