Thermoelectric properties of the off-stoichiometric tetrahedrites Cu12+mSb4S13

Tetrahedrite Cu12Sb4S13 is attracting attention as a thermoelectric material owing to its inherently low thermal conductivity. Various studies have been conducted to improve the thermoelectric performance of tetrahedrite by doping transition elements at Cu sites or incorporating nanostructures and nano-inclusions. In this study, off-stoichiometric (Cu-poor and Cu-rich) tetrahedrites, Cu12+mSb4S13 (- 0.3 <= m <= 0.3) were synthesized and their thermoelectric properties were investigated. All the stoichiometric and off-stoichiometric specimens contained a single tetrahedrite phase, and as the Cu content increased, their lattice constant increased. The electrical conductivity exhibited a positive temperature dependence (non-degenerate semiconductor behavior), while it decreased with the Cu content. The Seebeck coefficient increased with the temperature as well as the Cu content. The thermal conductivity did not exhibit significant temperature dependence, but it decreased as the Cu content increased at a constant temperature. Therefore, Cu deficiency improved the electrical transport in the tetrahedrite but had a negative effect on enhancing the thermal transport. The Cu-poor composition increased the power factor, rather than the stoichiometric composition; Cu11.7Sb4S13 exhibited the highest power factor of 1.08 mWm(-1) K-2 at 723 K. However, the Cu-rich composition reduced the thermal conductivity compared to the stoichiometric tetrahedrite; Cu12.3Sb4S13 exhibited the lowest thermal conductivity of 0.54 Wm(-1) K-1 at 323 K and 0.65 Wm(-1) K-1 at 723 K. A maximum dimensionless figure-of-merit of 0.91 was achieved at 723 K for Cu11.9Sb4S13.