Isotope Inter‐Diffusion and Self‐Diffusion in Solid Lithium Metal

Abstract

Abstract The inter‐diffusion of Li isotopes in separated 7 Li and 6 Li metals has been measured in the temperature range 35 to 178° C. The results can be expressed as equation image where 7 D 6 denotes the diffusion of 6 Li tracer in pure 7 Li metal, and vice versa. The 7 D 6 ‐values exceed 6 D 7 by about 23% at the melting point and by about 37% at 0.7 T m (°K). There are indications that the striking isotope effect may be due to tunneling. Tunneling considerations founded on a square barrier approximation suggest a dumbbell interstitial mechanism as a likely self‐diffusion model for lithium. According to the calculation for the dumbbell, the contribution from the tunneling process to the effective energy of self‐diffusion is of the same order as, or smaller than, than, the potential barrier to activated motion. Without favoring any particular diffusion model, the following expressions can be computed for uncorrelated self‐diffusion in isotopically pure lithium metals: equation image (diffusion coefficients in cm 2 /S, energies in kcal/mol).