Improved Thermoelectric Performance through Double Substitution in Shandite-Type Mixed-Metal Sulfides

Substitution of tin by indium in shandite-type phases, A₃Sn₂S₂ with mixed Co/Fe occupancy of the A-sites is used to tune the Fermi level within a region of the density of states in which there are sharp, narrow bands of predominantly metal d-character. Materials of general formula Co_2.5+ Fe_0.5- Sn_2-In S₂ (x = 0, 0.167; 0.0 ≤ y ≤ 0.7) have been prepared by solid-state reaction and the products characterized by powder X-ray diffraction. Electrical-transport property data reveal that the progressive depopulation of the upper conduction band as tin is replaced by indium increases the electrical resistivity, and the weakly temperature-dependent ρ(T) becomes more semiconducting in character. Concomitant changes in the negative Seebeck coefficient, the temperature dependence of which becomes increasingly linear, suggests the more highly substituted materials are n-type degenerate semiconductors. The power factors of the substituted phases, while increased, exhibit a weak temperature dependence. The observed reductions in thermal conductivity are principally due to reductions in the charge-carrier contribution on hole doping. A maximum figure-of-merit of (ZT)_max = 0.29 is obtained for the composition Co_2.667Fe_0.333Sn_1.6In_0.4S₂ at 573 K: among the highest values for an n-type sulfide at this temperature.