Electromigration in Niobium

Effective valences of niobium, iron, cobalt, and chromium in niobium have been measured by a radioactive tracer method. The effective valence of self-diffusion is small and positive and is explained by a driving force caused essentially by positive holes. Fe, Co, and Cr show small negative effective valences which are related to a negative coupling of solvent and solute fluxes. A dissociative mechanism involving interstitial--vacancy pairs, similar to that already proposed by Ablitzer (ibid. , 1977, 36, 391), is analysed by the pair-association method, and is shown to account correctly for the negative coupling of solvent and solute fluxes observed in electromigraton of Fe and Co tracers. This model is also shown to be consistent with measurements of the isotope effect of iron and self-diffusion enhancement in Nb--Fe dilute alloys published by Ablitzer (loc. cit. ). 30 ref.--AA.