QUANTUM DIFFUSION IN TRANSITION-METALS

The functions governing the temperature dependence of small-polaron diffusion are calculated by using the phonon dispersion relations with high accuracy for octahedral interstitial sites in copper, palladium, nickel and platinum f.c.c. hosts, as well as for tetrahedral and octahedral sites in tantalum, niobium and vanadium hosts. The current experimental data on the diffusion coefficient for hydrogen in b.c.c. tantalum and niobium, and for positive muons in copper are very well reproduced for all temperature ranges. By comparison with exact calculation, very accurate results are also obtained by using an approximate universal function calculated within the Debye approximation, for which we give a closed analytical formula. This allows manual calculations of the diffusion coefficient for light interstitial impurities in transition metals within the small-polaron model.