Vanadium-free colusites Cu₂₆A₂Sn₆S₃₂ (A = Nb, Ta) for environmentally friendly thermoelectrics

The thermoelectric properties of colusites Cu₂₆A₂Sn₆S₃₂ with A = V, Nb, and Ta have been investigated on high-density sintered compacts over the temperature range of 300-670 K. The ingots melted in evacuated and sealed quartz tubes were found to be composed of two colusite phases with slightly different lattice parameters. The successive hot-pressing converted the two-phase samples to single-phase ones of high density. Scanning electron microscopic analysis revealed the presence of CuS- and Cu₂S-based secondary phases in only the sintered compact with A = Ta. For A = V, Nb, and Ta samples, the heavy hole effective mass of 3m₀-4m₀ (m₀: free electron mass) caused a high Seebeck coefficient, leading to a high thermoelectric power factor of 0.6-0.7 mW K^-2 m^-1 at 660 K. The electrical resistivity and lattice thermal conductivity for the A = Ta sample are lower than those for A = V and A = Nb samples, because the secondary phases in the A = Ta sample provide charge carriers and scatter effectively the phonons. As a result, a ZT of 0.8 at 670 K is achieved for the A = Ta system.