FAST DIFFUSION OF THE OFF-CENTER IMPURITIES CU+ AND LI+ IN THE KCL LATTICE

In this work, we perform a study of the off-center effect: on the diff usion coefficient and we propose a new mechanism for fast diffusion. T he model assumes that the equilibrium positions of the cationic impuri ties are noncentral and that the diffusion proceeds by hopping across the potential barrier along the nonlinear paths with the highest proba bility [1]. The nonlinear paths between two consecutive positions char acterize even the selfdiffusion and furthermore, in the off-center dif fusion this path is appropriate. The main results is that the off-cent er position considerably enhances the diffusion. The theoretical diffu sion coefficients have been obtained by modeling the potential barrier . Changes of the configuration entropy and the vibration spectra due t o the presence of the noncentral impurity are included in the model. W e emphasize the good agreement of the model with the experimental data for KCl:Cu+ and show that if the impurity is placed close to the cent ral site, the appropriate diffusion coefficient is close to that for t he cationic self-diffusion: We proceeded in the Li+ cation case as in the case of Cu+ cation.