THE OFF-CENTER EFFECT ON THE DIFFUSION-COEFFICIENT OF CU+ AND LI+ IN THE KCL LATTICE

It 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 tempera ture regions. This fast diffusion of the Cu+ has been explained in man y papers as an interstitial diffusion although the optical spectra do not confirm the existence of interstitial Cu+. In this paper a new mec hanism for fast diffusion is proposed. The model assumes that the equi librium positions of the cationic impurities are noncentral and that t he diffusion proceeds by hopping across the potential barrier along th e nonlinear paths with the highest probability. The main result shows that the off-center position considerably enhances the diffusion. Theo retical diffusion coefficients are obtained by modelling the potential barrier. Changes of the configuration entropy and the vibration spect ra 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 t hat if the impurity is placed dose to the central site the due diffusi on coefficient is dose to that for the cationic self-diffusion. The Li + cation case is dealt with as the case of the Cu+ cation.