Experiments on metals typically showed an abrupt change in the Arrhenius behavior of surface self-diffusion at temperatures which were equal to 60 to 75% of the bulk melting point. In order to explain this phenomenon, it was proposed - based upon correlational evidence - that the most common mechanism for surface self-diffusion was one in which adatoms dominated low-temperature transport, while surface vacancies dominated at high temperatures. The high-temperature dominance of vacancies resulted from their substantially higher diffusion entropy; which was a consequence of the large vibrational displacements of surface atoms relative to the bulk. This phenomenon could also explained Arrhenius behavior on some non-metal surfaces.

An Improved Theory for Temperature-Dependent Arrhenius Parameters in Mesoscale Surface Diffusion. A.S.Dalton, E.G.Seebauer: Surface Science, 2007, 601[3], 728-34