Anisotropic Diffusion of Cadmium and Gold Tracers in Zinc Single Crystals

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 $D_{\perp }>D_{\mathrm{II}}$. Gold diffused at a rate slower than that of self-diffusion and with $D_{\mathrm{II}}>D_{\perp }$. The results of the measurements are as follows: For cadimum ($115m$), $D_{\mathrm{II}}=(0.11\mathrm{}\pm 0.01)\exp [-\frac{(20.54\mathrm{}\pm 0.09)\times {10}^3}{\mathrm{RT}}]$ ${\mathrm{cm}}^2$/sec, $D_{\perp }=(0.12\mathrm{}\pm 0.01)\exp [-\frac{(20.42\mathrm{}\pm 0.03)\times {10}^3}{\mathrm{RT}}]$ ${\mathrm{cm}}^2$/sec; for gold (198), $D_{\mathrm{II}}=(0.97\mathrm{}\pm 0.22)\times \exp [-\frac{(29.73\mathrm{}\pm 0.26)\times {10}^3}{\mathrm{RT}}]$ ${\mathrm{cm}}^2$/sec, $D_{\perp }=(0.29\mathrm{}\pm 0.12)\exp [-\frac{(29.72\mathrm{}\pm 0.45)\times {10}^3}{\mathrm{RT}}]$ ${\mathrm{cm}}^2$/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.