Li self-diffusion in pure and doped β-LiAl using pulsed-field gradient NMR

The self-diffusion coefficient of ⁷Li in pure and doped β-LiAl has been measured over the temperature range 297-478 K using a pulsed-field-gradient NMR spin-echo technique. The lithium self diffusion coefficients in these alloys show Arrhenius behavior, D (Li) = D₀ (Li) exp( -(E) k_BT), within this observed temperature range. The diffusion constant D₀ (Li) and the associated activation energy 〈E〉 decrease with increasing lithium content. The results can be understood if two types of vacancy point defects are considered: vacancies partially bound to, or associated with, immobile antistructure defects, and vacancies which otherwise are free. A vacancy diffusion mechanism based on two mean jump times for these two types of vacancies yields a quantitative description of the Li diffusion process. The diffusion coefficients of Li in LiAl alloys doped with Ag and In have also been determined, and it was found that the results are closely related to the corresponding defect states.