Ionic Transport of Aliovalent Solutes in Silver Chloride and Silver Bromide

The objectives of this work are to obtain values for thermodynamic parameters and determine the transport mechanism for defects in silver chloride and silver bromide. This is accomplished by analyzing the results obtained from diffusion and conductivity experiments on pure and doped crystals. The work includes the results of four different diffusion experiments and two conductivity experiments. Three experiments consist of Hg^ {2+} diffusion in AgCl, Ce^ {3+} diffusion in AgCl, and S ^{2-} diffusion in AgBr. All of these obey Arrhenius relations with respective activation enthalpies of 0.74 eV, 0.62 eV and 1.71 eV. The first of these is a remarkably small value for a divalent cation, which suggests diffusion by an interstitial mechanism. The behavior of Ce^{3+} differs markedly from literature reports for two other trivalent impurities with respect to its high diffusivity and its linear Arrhenius relation. The sulfur diffusion represents the first divalent anion which has been studied in these materials. The value of the Schottky formation enthalpy, which is useful in the analysis of the sulfur diffusion experiments, is obtained from its correlation to the melting temperature. Another set of experiments consists of a conductivity and self-diffusion study in both pure and sulfur-doped AgBr. The curvature in the Arrhenius plots suggest the presence of more than one transport mechanism. The combined results enable the separation of these mechanisms to be accomplished. In both the diffusion and conductivity studies, the presence of the impurity suppresses the vacancy component and enhances the interstitial components. The final experiment is a conductivity study in cerium-doped AgCl. The curved Arrhenius plot again suggests the presence of more than one mechanism; however, in this case the cation doping suppresses the interstitial contribution in favor of the vacancy contribution.