Coherency stresses and deuterium diffusion in NbD0.33

Abstract

Concentration fluctuations of impurity atoms in solids, such as hydrogen in metals, are accompanied by coherency stresses if the impurity atoms expand or contract the lattice. The coherency stresses raise the elastic energy which accelerates the diffusive decay of the fluctuations and increases, therefore, the value of a diffusion coefficient defined according to Fick’s law. We studied the influence of coherency stresses on deuterium diffusion in NbD 0.33 by neutron spectroscopy, separating coherent and incoherent scattering by neutron spin analysis. The diffusion coefficient D bulk of the deuterium interstitials, obtained from the coherent scattering intensity, was found up to 30 times larger than the diffusion coefficient D chem reported from Gorsky effect measurements [H.C. Bauer, J. Völkl, T. Tretkowski, G. Alefeld, Z. Physik B29 (1978) 17] on the same system, in spite of the fact that both diffusion coefficients describe deuterium diffusion according to Fick’s law. We demonstrate that the large differences between D bulk and D chem reflect a dissimilar influence of coherency stresses on these two quantities, and that the differences can quantitatively be described within the elasticity-theoretical concept of Wagner and Horner.