Activation Energy for the Diffusion of Coupled Pairs of Vacancies in Alkali Halide Crystals

Abstract

The theoretical activation energy for the diffusion of neutral combined pairs of positive- and negative-ion vacancies in alkali halides has been determined. Changes in Coulombic, repulsive, and polarization energies were taken into account with appropriate corrections for second-order effects. Calculations for the rigid and distorted lattice showed that distortion effects during diffusion are of appreciable magnitude. This energy contribution has been estimated as accurately as possible near the saddle point. An activation energy of 0.375 ev was obtained for diffusion in the KCl lattice. This activation energy value shows that migration of neutral combined pairs of positive- and negative-ion vacancies, rather than diffusion via single vacancies, may well be the predominant process. The experimental evidence for this type of migration is discussed. It is concluded that diffusion by means of neutral combined pairs of vacancies is supported rather than prohibited by the theoretical results.