Abstract
Abstract The self-diffusion of chloride ions in MgCl2- and CoCl2-crystallites with the CdCl2-type structure was studied, the results being compared with previous data on MnCl2 and NiCl2. The diffusion rate was measured through gas/solid-isotopic exchange reactions in a carefully cleaned vacuum system. The results were analyzed by the use of a model of semi-infinite crystal. The Arrhenius plots of apparent diffusion coefficient consist of two segments of line, from whose slopes the activation heat of migration E and the apparent heat of formation of vacancies Hv⁄nv are determined. [Hv denotes the total heat of formation of two anion vacancies and one catoin vacancy with nv=2 or 3.] The values for E and Hv were 0.55 and 1.1 eV in MgCl2 and 1.3 and 2.0 eV in CoCl2, respectively. A linear relationship holds between the logarithms of pre-exponential factor of the diffusion coefficient and E or (E+Hv⁄nv); viz., the theta-rule holds in this case. In the initial stage of the exchange reaction, ions or atoms in the surface region of crystals were observed to easily migrate. Sodium ions doped deliberately as impurity enhance the diffusion of chloride ions. It is concluded that the diffusion takes place through the vacancy mechanism.
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