THE INFLUENCE OF N-IMPURITIES ON THE DIFFUSION COEFFICIENT OF H AND D IN Nb AND Ta

Gorsky-effect measurements of the diffusion coefficient of H and D in Nb and Ta in the presence of N interstitials are reported. The influence of the N interstitials on the diffusion coefficient is correlated with the low-temperature diffusion anomaly of H in the pure metals. For H in pure Nb and pure Ta the activation enthalpy decreases strongly below about 250 K which suggests a change of the microscopic diffusion mechanism. For these two systems the reduction of the diffusion coefficient due to N interstitials is particularly effective in the region of the low activation enthalpy leading to a suppression of the low-temperature diffusion process at the largest N concentrations. On the other hand, for D in Nb and Ta for which no decrease of the activation enthalpy is observed in the pure metals only a minor reduction of the low-temperature diffusivity by the N interstitials is found. The results cannot solely be explained by trapping effects. The low-temperature diffusion mechanism is particularly strongly influenced by the N impurities