Vacancy-antistructure defect interaction diffusion in β-LiAl and β-LiIn

The lithium mass transport mechanisms of the NaTl-type intermetallic compounds β-LiAl and β-LiIn were studied by using the pulsed-field-gradient (PFG) nuclear magnetic resonance (NMR) spin-echo technique over a temperature range of 297-485 K. The lithium self-diffusion coefficients in β-LiAl alloys were found to follow Arrhenius behavior. The lithium diffusion coefficient and the associated diffusion constant D₀ (Li) and activation energy 〈E〉 decrease with increasing lithium content. The results can be interpreted if two types of vacancy point defects are considered: vacancies associated with immobile antistructure defects and vacancies which are otherwise free. A vacancy-diffusion mechanism based on two mean effective jump times, τ_A and τ_Bτ_B/τ_A = 9.0), for these two types of vacancies yields a quantitative description of the dependence of the lithium diffusion rate on defect concentrations under a V_Li-Li_Al attractive interaction at a nearest neighbor distance. Very high values of lithium diffusivities (approximately 10^-6 cm² s^-1 at room temperature) which increase with decreasing lithium content were observed in β-LiIn alloys. The lithium self-diffusion coefficient obeys the Arrhenius relation over the temperature range 300-400 K. The diffusion constant D₀ (Li) and activation energy 〈E〉 determined from the Arrhenius relation decrease with increasing lithium content. The results suggest that there is a repulsive interaction between the vacancy V_Li and the antistructure atom Li_In. Theoretical analyses based on a three-effective-jump-times diffusion model indicate that this V_Li-Li_In repulsive interaction is so strong that it extends well into the second nearest-neighbor distance.