It was recalled that it was well-known that the diffusivity of the Cu+ cation in the NaCl and KCl lattices exceeded, by 3 or 4 orders of magnitude, the corresponding self-diffusivity in the intrinsic temperature region. This fast diffusion had previously been explained in terms of interstitial diffusion, although the optical spectra did not confirm the presence of interstitial Cu+. A new mechanism for the rapid diffusion was proposed here. This model assumed that the equilibrium positions of the cationic impurities were non-central, and that diffusion occurred via hopping across the potential barrier along non-linear paths with the highest probability. The main conclusion was that the off-center position considerably enhanced diffusion. Changes in the configurational entropy and vibration spectra, due to the presence of the non-central impurity, were treated by the model. Numerical predictions of the diffusion coefficient agreed with experimental data, and it was shown that, if the purity were placed close to the central site, the resultant diffusion coefficient was close to that for cationic self-diffusion. The same conclusions applied to the Li+ cation.

F.Despa: Physica Status Solidi B, 1995, 191[1], 31-43