Coherency stresses and diffusion in solids: neutron spectroscopic versus Gorsky effect measurements of deuterium diffusion in niobium

Published neutron scattering and Gorsky effect results on the diffusion and spatial fluctuations of D interstitials in NbD_x (x0.5) reveal striking differences. It is shown that these differences are caused by coherency stresses which raise the elastic energy of the short-wavelength D fluctuations studied by coherent neutron scattering. Gorsky effect measurements, on the other hand, probe long-range fluctuations which were coherencystress free in the experiments above. The observed differences can quantitatively be explained within the elasticity-theoretical concepts of Cahn, Krivoglaz, and Wagner and Horner. This represents the first successful application of these concepts to a diffusion coefficient. The correlation factor for D diffusion is, finally, determined from a comparison of the Gorsky effect and neutron scattering data.