It was shown that Brownian-type motion of off-center impurity atoms in a double-well potential, together with elastic and electrostatic long-range interactions between these atoms and self-atoms of the lattice, could result in an enhancement of their diffusion in the host. A cross-over occurred from Arrhenius behavior, at low temperatures, to non-activated motion at high temperatures. Deviations from the Arrhenius law were expected to start at temperatures where kT became comparable to the relatively low-energy barrier of the double-well potential for the off-center atom. This could explain the experimental fact that, at high temperatures, the diffusion coefficients of off-center atoms exceeded, by 3 or 4 orders of magnitude, the corresponding self-diffusion coefficients for motion through the comparatively high-energy barrier between interstitial positions.

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