We studied the electronic structures of Cu2TryTi4–yS8 (Tr = Sc, Co) thiospinels by combining thermoelectric measurements and first-principles calculations. The thiospinels showed n-type and metallic properties governed by a conductive network composed of edge-shared (Ti/Tr)S6 octahedra. The substitutions of Sc and Co for Ti decreased the electron carrier concentration in a similar manner, leading to increases in both the electrical resistivity and Seebeck coefficient. Although the electrical resistivity was equivalent for Tr = Sc and Tr = Co, the Seebeck coefficient was remarkably enhanced for Tr = Co as compared to that for Tr = Sc. The enhanced Seebeck coefficient in the Co-substituted system was ascribed to the increased density of states of conduction band near the Fermi level due to the partial replacement of Ti-3d orbitals by Co-3d orbitals. Owing to the electronic structure modification, a higher dimensionless thermoelectric figure of merit (ZT = 0.2 at y = 1–1.5) was achieved for the Co-substituted samples at 673 K.
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