The Off‐Center Effect on the Diffusion Coefficient of Cu+ and Li+ in the KCl Lattice

Abstract

AbstractIt is well‐known that the diffusion coefficients of the Cu+ cation in the NaCl and KCl lattices exceed by three or four orders of magnitude the corresponding self‐diffusion coefficients in the intrinsic temperature regions. This fast diffusion of the Cu+ has been explained in many papers as an interstitial diffusion although the optical spectra do not confirm the existence of interstitial Cu+. In this paper a new mechanism for fast diffusion is proposed. The model assumes that the equilibrium positions of the cationic impurities are noncentral and that the diffusion proceeds by hopping across the potential barrier along the nonlinear paths with the highest probability. The main result shows that the off‐center position considerably enhances the diffusion. Theoretical diffusion coefficients are obtained by modelling the potential barrier. Changes of the configuration entropy and the vibration spectra due to the presence of the noncentral impurity are included in the model. The numerical results of the diffusion coefficient computed in this way are in agreement with the experimental data and it is shown that if the impurity is placed close to the central site the due diffusion coefficient is close to that for the cationic self‐diffusion. The Li+ cation case is dealt with as the case of the Cu+ cation.