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

[Image: see text] Substitution of tin by indium in shandite-type phases, A(3)Sn(2)S(2) 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+x)Fe(0.5–x)Sn(2–-y)In(y)S(2) (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(2.667)Fe(0.333)Sn(1.6)In(0.4)S(2) at 573 K: among the highest values for an n-type sulfide at this temperature.