Isotope effect for anomalous fast diffusion

Abstract

The very small isotope effect is characteristic of the so-called anomalous fast diffusion of metallic solutes in metals. Consistency is critically re-examined between the small isotope effect and the interstitial (or interstitial-type) mechanism which is most popularly accepted. It is shown that the isotope-effect coefficient E for the interstitial diffusion can be regarded as a kind of average of the ‘kinetic factors’ for all kinds of jumps. The origin of the small ‘kinetic factors’ is discussed on the basis of the Vineyard rate theory. It is pointed out that there are two possible types of the saddle-point configuration: ‘stable’ and ‘unstable’. If interstitial solutes move via the ‘stable’ configuration, the interstitial mechanism is consistent with the small isotope effect. A few comments are made on previous arguments on the isotope effect.