It was shown that the Brownian-type motion of the off-center impurity atoms in a double-well potential together with elastic and electrostatic long-range interactions between these atoms and the self-atoms of the lattice may result in enhancement of their diffusion in the host. The crossover occurred from Arrhenius behavior at low temperatures to non-activated motion at high temperatures. Deviations from the Arrhenius law temperature dependence should start at the temperatures when kT became comparable to the relatively low-energy barrier of the double-well potential for the off-center atom. This may explain the experimental fact that at high temperatures the diffusion coefficients for off-center atoms exceed by three or four orders of magnitude, the corresponding self-diffusion coefficients for the motion through the comparatively high-energy barrier between the interstitial positions.

Enhancement of Diffusion of Off-Center Impurities. A.Gordon, S.Dorfman, D.Fuks: Physica B, 2003, 328[3-4], 336-44