A study was made of the off-center effect upon diffusion, and a new mechanism was proposed for fast diffusion. The model assumed that the equilibrium positions of the cationic impurities were non-central, and that diffusion occurred via hopping across the potential barrier; along the non-linear paths with the highest probability. The non-linear paths between 2 consecutive positions characterized even self-diffusion. Such paths were also relevant to the off-center diffusion. The main conclusion was that off-center positioning considerably enhanced diffusion. The theoretical diffusion coefficients were obtained by modelling the potential barrier. Changes in the configurational entropy and vibration spectra, due to the presence of the non-central impurity, were incorporated into the model. Good agreement of the model with experimental data was found for Cu-doped KCl. This showed that, if the impurity was placed close to the central site, the appropriate diffusion coefficient was close to that for cationic self-diffusion.

F.Despa, V.Topa: Radiation Effects and Defects in Solids, 1995, 137[1-4], 299-303