Slowly Diffusing Impurities in Lithium Metal

The diffusion of Sn, Pb, Sb, and Bi tracers in Li metal has been investigated, using a thin-film evaporating-sectioning technique. The results can be expressed by the Arrhenius relations $D_{\mathrm{Sn}}=0.62\mathrm{}{e}^{-\frac{(15.0\mathrm{}\pm 1.2)}{\mathrm{RT}}}$, ${\mathrm{D}}_{\mathrm{Pb}}=1.6\mathrm{}\times {10}^2{e}^{-\frac{(25.2\mathrm{}\pm 1.2)}{\mathrm{RT}}}$, $D_{\mathrm{Sb}}=1.6\mathrm{}\times {10}^{10}{e}^{-\frac{(41.5\mathrm{}\pm 4.5)}{\mathrm{RT}}}$, and $D_{\mathrm{Bi}}=5.3\mathrm{}\times {10}^{13}{e}^{-\frac{(47.3\mathrm{}\pm 3.4)}{\mathrm{RT}}}$. The data are not in agreement with established diffusion models based on attractions between vacancies and excess valency ions. Instead, diffusivity appears to decrease as the valency of the impurity increases. However, on the assumption that it is not probable for an impurity in a highly electropositive matrix to be more than singly ionized, the diffusive behavior of impurities in Li can be at least qualitatively understood in terms of two competing mechanisms, to the effect that relatively large and electropositive impurities should diffuse mainly via vacancies, small and electronegative impurities as interstitials.