Anisotropic Diffusion of Cadmium and Gold Tracers in Zinc Single Crystals

Abstract

The anisotropic diffusion coefficients of the radioactive tracers cadmium ($115m$) and gold (198) in single crystals of high-purity zinc, in directions parallel and perpendicular to the hexagonal axis, were measured by the standard sectioning technique. Cadmium diffused at a rate faster than that of self-diffusion and with ${D}_{\ensuremath{\perp}}g{D}_{\mathrm{II}}$. Gold diffused at a rate slower than that of self-diffusion and with ${D}_{\mathrm{II}}g{D}_{\ensuremath{\perp}}$. The results of the measurements are as follows: For cadimum ($115m$), ${D}_{\mathrm{II}}=(0.11\ifmmode\pm\else\textpm\fi{}0.01)\mathrm{exp}[\ensuremath{-}\frac{(20.54\ifmmode\pm\else\textpm\fi{}0.09)\ifmmode\times\else\texttimes\fi{}{10}^{3}}{\mathrm{RT}}]$ ${\mathrm{cm}}^{2}$/sec, ${D}_{\ensuremath{\perp}}=(0.12\ifmmode\pm\else\textpm\fi{}0.01)\mathrm{exp}[\ensuremath{-}\frac{(20.42\ifmmode\pm\else\textpm\fi{}0.03)\ifmmode\times\else\texttimes\fi{}{10}^{3}}{\mathrm{RT}}]$ ${\mathrm{cm}}^{2}$/sec; for gold (198), ${D}_{\mathrm{II}}=(0.97\ifmmode\pm\else\textpm\fi{}0.22)\ifmmode\times\else\texttimes\fi{}\mathrm{exp}[\ensuremath{-}\frac{(29.73\ifmmode\pm\else\textpm\fi{}0.26)\ifmmode\times\else\texttimes\fi{}{10}^{3}}{\mathrm{RT}}]$ ${\mathrm{cm}}^{2}$/sec, ${D}_{\ensuremath{\perp}}=(0.29\ifmmode\pm\else\textpm\fi{}0.12)\mathrm{exp}[\ensuremath{-}\frac{(29.72\ifmmode\pm\else\textpm\fi{}0.45)\ifmmode\times\else\texttimes\fi{}{10}^{3}}{\mathrm{RT}}]$ ${\mathrm{cm}}^{2}$/sec. On the basis of the vacancy mechanism one can interpret the fast diffusion of cadmium in terms of an appreciable binding between the vacancy and the diffusing ion; conversely the slow diffusion of gold in terms of a repulsion between the vacancy and the diffusing ion.