Copper impurity diffusion in sodium iodide

Abstract

A thin-layer method was used to study the diffusion of copper in NaI. Final concentration-distance profiles were measured by an optical-absorption method. All plots of ${log}_{10}C$ vs ${z}^{2}$ are linear for temperatures below 265\ifmmode^\circ\else\textdegree\fi{}C and above 415\ifmmode^\circ\else\textdegree\fi{}C, and they are curved for temperatures in between. The Arrhenius plot shows two straight segments from which activation energies are calculated to be 1.00 eV for the low-temperature region and 0.65 eV for the high-temperature region. The diffusion coefficient for copper in sodium iodide exceeds the self-diffusion coefficient of sodium in sodium iodide by factors of ${10}^{3}$-${10}^{4}$. Thus the diffusion of copper in sodium iodide cannot be explained by the vacancy mechanism, which is the usually accepted one for the alkali halides. The experimental data strongly suggest that above 415\ifmmode^\circ\else\textdegree\fi{}C most of the copper impurity will be interstitial and the diffusion is by a simple interstitial mechanism.