It was recalled that concentration fluctuations, such as those of H in metals, were associated with coherency stresses if the impurity atoms caused the lattice to expand or contract. The coherency stresses increased the elastic energy, accelerated diffusive decay of the fluctuations and therefore increased the value of the diffusion coefficient according to Fick’s law. A study was made here of the effect of coherency stresses upon D diffusion in NbD0.33. Neutron spectroscopy was used to separate coherent and incoherent scattering, via neutron spin analysis. The diffusion coefficient of D-interstitials, as deduced from the coherent scattering intensity, was found to be up to 30 times larger than the diffusion coefficient which was deduced from Gorsky effect measurements of the same system. This was so in spite of the fact that both diffusion coefficients described D diffusion according to Fick’s law. It was demonstrated that the large differences between the 2 diffusion coefficients reflected a differing effect of coherency stresses upon them. These differences could be quantitatively described in terms of the Wagner-Homer elasticity concept.

Coherency Stresses and Deuterium Diffusion in NbD0.33. U.Stuhr, K.Cornell, H.Wipf: Journal of Alloys and Compounds, 1999, 293-295, 289-91