The optimization of thermoelectric parameters when introducing impurities with variable valence

The thermoelectric properties (the electrical and thermal conductivity, the Seebeck and Hall coefficients) of SnTe-based solid solutions in the Sn-In-Te system along In and Te isoconcentrates (1-5 at.% In and 50.0-51 at.% Te) and along radial sections SnTe-InTe and SnTe-In₂Te₃ were studied in the temperature range 300-800 K. The ratio of In atoms in different charge states (In³⁺ and In¹⁺) was controlled by changing the degree of deviation from stoichiometry at a fixed In concentration. It was established that the maximum value of the figure of merit ZT (∼1 at 800 K) can be attained through doping SnTe with In₂Te₃, which corresponds to the simultaneous introduction of indium in the In³⁺ charge state and cation vacancies. The experimental results were interpreted on the basis of a model for the energy spectrum of SnTe doped with In, taking into account the deviation from stoichiometry in SnTe and a variable valence of indium. It is shown that the introduction of impurities with a variable valence creates new opportunities for controlling the thermoelectric properties.