The diffusion rate of H and D in Nb is calculated with avoidance of the usual adiabatic, linear-coupling, and Condon approximations. The method employed is a generalization of the standard small-polaron theory with explicit account taken of the coupled interstitial-host vibrations. It is applicable in the medium temperature regime, $50 \mathrm{K}\le T\le 300 \mathrm{K}$, and is not limited to light interstitials. Whereas a harmonic calculation exhibits the qualitative features, anharmonic terms have to be included to get quantitative results.

• Received 19 January 1988

DOI:https://doi.org/10.1103/PhysRevLett.60.2307