Relaxation and Activation Energies for an Interstitial Neutral Defect in an Alkali Halide Lattice

A method has been developed for calculating the relaxation energy for an interstitial neutral defect in an alkali halide lattice by expanding the electrostatic, polarization, and dipole-dipole energy contributions to second order in terms of the displacements of the ions from their regular positions. The repulsive energy contributions involving the defect atom are treated exactly, whereas the repulsive contributions involving the regular ions themselves are also expanded to second order. This method has been applied to the case of an interstitial chlorine atom in NaCl for positions where the defect atom is at the center of a cube of ions and at the center of a square of ions; the difference when related to the same standard configuration gives an activation energy of approximately 0.5 ev for the migration of a neutral interstitial chlorine atom in NaCl.