MASS-TRANSPORT BY CROWDIONS - A SIMPLE-MODEL APPROPRIATE TO BODY-CENTERED-CUBIC NA AND K

The relevance of self-interstitials to low-temperature diffusion in bo dy-centred-cubic Na and K metals is briefly reviewed. Previous studies have been based largely on numerical computations, with an assumed in ter-ionic force field. Therefore, a simple model is proposed here in w hich the formation and motion energies of crowdions, E(f) and E(m) res pectively, are related to a characteristic energy Ms(2) where M is the atomic mass and s an appropriate sound velocity, and to N, the number of atoms involved in the crowdion motion. This model leads to the con clusion that E(f) much greater than E(m). Isotope effects in Na metal find a natural explanation in such a crowdion model.