Effect of mixed occupancies on the thermoelectric properties of BaCuSeTe polychalcogenides.

The title materials have been reported earlier to be p-type thermoelectrics when x = 0.1 and y = 0. Here, we studied the properties after varying the Cu and the Se/Te concentrations. At first, materials with the same nominal Cu concentration, 5.9 Cu per formula unit, and different Se/Te ratios were prepared. The different thermoelectric properties indicated that the Se/Te ratio strongly affected the Cu deficiency, which is directly responsible for the charge carrier concentration. Single crystal structure data revealed the Cu amount to be less than 5.8 per formula unit when y = 0.4; therefore a sample of nominal composition "BaCu5.74Se0.46Te6.54" was also studied. This sample exhibited an electrical conductivity of 685 Ω-1 cm-1 at room temperature, which is almost three times larger than in case of "BaCu5.9SeTe6", in accord with the lower Cu amount causing a larger hole concentration. The larger mass fluctuation on the Se/Te site resulted in a lower lattice thermal conductivity, but the decreased Seebeck coefficient mitigated a performance increase in form of a higher figure-of-merit. In contrast to other Cu chalcogenides, the data are reproducible under the measurement conditions.