Impurity diffusion and vacancy–impurity binding energy associated with elements of the second transition series in niobium and molybdenum

Abstract

Impurity diffusion of ruthenium and palladium in niobium has been studied by radioactive–tracer and electron–microprobe techniques. The results show that the impurity diffusion coefficients are a little higher than the self–diffusion coefficients.

Analysis of data for impurity diffusion in niobium and molybdenum shows the following features : (i) the difference Δ Q between activation energies for impurity diffusion and self–diffusion is proportional to Δ Z, the difference of charge between the substitutional d impurity and the matrix ionic cores, when Δ Z is small; (ii) Δ Q is positive for the diffusion in noibium of an impurity at the right of niobium in the periodic table with small ΔZ, and for the diffusion in molybdenum of an impurity at the left of molybdenum; (iii) feature (i) breaks down for |Δ Z| higher than 2. These different points can be qualitatively explained if Δ Q is approximately given by the impurity–vacancy binding energy Δ E. This impurity–vacancy binding energy is estimated in the generalized double–scattering scheme of Gautier, Moraitis and Farlebas (1976).