Thermoelectric Power of TlCl Containing PbCl2

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Abstract
Thermoelectric power of pure TlCl and TlBr was measured as a function of temperature between 275° and 400°C. Thallium metal electrodes were used. Measurements were also made on TlCl doped with PbCl2 in concentrations between 0.01 and 0.5 mole %, and on TlCl doped with Tl2S. The thermoelectric power of pure TlCl is about −0.2 mV/deg, and it becomes more negative as a result of adding PbCl2. The data for TlCl:PbCl2 are in excellent agreement with the thermodynamical theory of Howard and Lidiard for Schottky defects, if it assumed that there is no association between the cation vacancies and Pb+ + ions. With this assumption, the data are analyzed to give the following values for defect parameters in TlCl: enthalpy of formation of Schottky pair, 1.3 eV; entropy of formation of cation vacancy, 0.3 meV/deg (3.7 k); entropy of formation of anion vacancy, 0.6 meV/deg (6.6 k); heat of transport of cation vacancy, −0.9 eV; heat of transport of anion vacancy, −0.5 eV; activation energy for cation mobility, 0.5 eV; activation energy for anion mobility, 0.2 eV. The Tl2S impurity apparently does not enter the crystal in the form of unassociated anion vacancies and S− — ions.

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