Diffusion of Gold and Silver in Thallium

Abstract

The diffusivities D of gold and silver in the bcc phase of thallium and in the c and a directions of hcp thallium have been measured by a radioactive tracer technique. The diffusivities of gold are described by $$\begin{array}{c}{\mathrm{\hspace{0.17em}D}}_{\mathrm{bcc}}{\text{=5.2×10}}^{\text{−4}}\exp \text{(−6000/RT),}\\ D_{\mathrm{hcp}}^a{\text{=5.3×10}}^{\text{−4}}\exp \text{(−5200/RT)}\end{array}$$ , and $$D_{\mathrm{hcp}}^c{\text{=2.0×10}}^{\text{−5}}\exp \text{(−2800/RT)\hspace{0.17em}}{\mathrm{cm}}^2/\sec$$ ; and those of silver by $$\begin{array}{c}{\mathrm{\hspace{0.17em}D}}_{\mathrm{bcc}}{\text{=4.2×10}}^{\text{−2}}\exp \text{(−11\hspace{0.17em}900/RT),}\\ D_{\mathrm{hcp}}^a{\text{=3.8×10}}^{\text{−2}}\exp \text{(−11\hspace{0.17em}800/RT)}\end{array}$$ , and $$D_{\mathrm{hcp}}^c{\text{=2.7×10}}^{\text{−2}}\exp \text{(−11\hspace{0.17em}200/RT)\hspace{0.17em}}{\mathrm{cm}}^2/\sec$$ . The diffusivities of gold and silver are thus about two orders of magnitude higher than the thallium self‐diffusion in the bcc phase and four to eight orders of magnitude higher than the thallium self‐diffusion in the hcp phase. These results suggest that gold and silver are dissolved at least partially in the interstitial state in both phases in agreement with a theory which predicts that d‐d correlation forces should cause dissolved noble metals to favor interstitial sites in thallium. Moreover, the difference in diffusivities of the noble metals in the two thallium phases indicates that the interstitial solute population is higher in the hcp phase, which has larger interstitial sites, than the bcc phase.