Enhancement of diffusion of off-center impurities

We show 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 occurs 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 k_BT becomes comparable with 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.