Observed Diffusion Coefficients Research Articles

Diffusion of Silver in Silver Bromide and Evidence for Interstitialcy Migration

Experimental results are presented for the diffusion of silver in silver bromide single crystals for temperatures from 140\ifmmode^\circ\else\textdegree\fi{}C to the melting point. The results are obtained by the sectioning technique, using radioactive tracers. The observed diffusion coefficients fall below the values that would normally be expected from the electrical conductivity by a factor of 0.46 to 0.66. The discrepancy is ascribed to the simultaneous occurrence of two types of interstitialcy jumps, with corrections introduced for differing displacements of interstitials and tracer atoms and for correlation effects according to Bardeen and Herring. The resulting mobilities can be represented as $\ensuremath{\mu}={\ensuremath{\mu}}_{0}\mathrm{exp}(\ensuremath{-}\frac{U}{\mathrm{kT}})$ over a wide range of temperatures with ${\ensuremath{\mu}}_{0}=6.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3} \mathrm{and} 1.57\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ ${\mathrm{cm}}^{2}$/volt-sec and $U=0.078 \mathrm{and} 0.225$ ev, respectively, for collinear and non-collinear jumps. These values are in reasonable agreement with theoretical calculations for silver chloride. The diffusion coefficient is observed to fall increasingly below expected values as the temperature approaches the melting point, and as a possible explanation it is surmised that the activation energy for collinear jumps drops gradually towards zero. A comparison is made to similar results which have been reported for diffusion of silver in silver chloride.

Self-Diffusion in Argon

The self-diffusion coefficient of argon was measured at five different temperatures between −183°C and 53.5°C by observing the diffusion of radioactive A41 into normal argon. The decrease in the observed diffusion coefficient caused by the larger mass of A41 was deduced and found to be 0.6 percent. The experimental values are compared with several theoretical calculations. In particular, it is shown that use for argon of an inverse power model (V ∝ r−ν), with different values of ν ascribed to different temperature ranges, is not permissible since widely different values of ν at the same temperature can be obtained from different experimental data. The experimental figures agree very well with numerical calculations by Hirschfelder, Bird, and Spotz based on an interaction potential of the form 4E[—(r0/r)6+(r0/r)12].