Point-defect mobility in thallous chloride doped with divalent cation and anion impurities

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Abstract
A detailed study of the temperature dependence of the ionic conductance of single crystals of thallous chloride has been made. Twenty crystals were grown from the melt and detailed results are presented for ten of these: three doped with divalent cations, four doped with divalent anions and three which were nominally pure. Precautions were taken during the conductivity measurements to minimise the effects of sublimation at elevated temperatures. An analysis of the data using computer fitting techniques has provided values for the thermodynamic parameters which govern the formation, migration and interaction of the point defects in the host lattice. The conventional Schottky model with transport on both sublattices and taking account of both nearest-neighbour and long-range (Debye-Huckel) defect interactions was employed. The value for the enthalpy of formation of a Schottky defect was found to be 1.247 eV and that for the migration of the cation and anion vacancies to be 0.379 eV and 0.097 eV respectively. The defect parameters derived are compared with previous experimental and theoretical results and the effect of the changes of the dopant species on the overall conductivity, particularly at lower temperatures, is discussed. The anion vacancy is found to be more mobile than the cation vacancy with the transport number of the former being virtually unity at low temperatures except in the case of the cation-doped crystals. The overall conductivity is interpreted in terms of the concentrations and mobilities of the defects and the extent of the vacancy-impurity association.