Effects of Sn and In Double-Doping on the Thermoelectric Performance of Cu3Sb1-x-ySnxInyS4 Famatinites

Famatinite (Cu₃SbS₄ ) is a promising p-type thermoelectric material because of its low lattice thermal conductivity and high Seebeck coefficient. In this study, famatinite powders, double-doped with Sn (a B^IV group element) and In (a B^III group element) to give Cu₃Sb_1-x-ySn_xIn_yS₄ (0.02 ≤ x ≤ 0.12 and 0.06 ≤ y ≤ 0.10), were synthesized by mechanical alloying and then consolidated by hot pressing. Phase analysis and microstructure observations were conducted over a range of doping levels, and the charge transport parameters and thermoelectric properties were evaluated. Except for the specimen with y = 0.10, in which the secondary phase CuInS₂ was found, all the specimens exhibited a tetragonal famatinite phase without secondary phases. The Sn/In double doping increased the unit cell a-axis to 0.5387–0.5389 nm and changed the c-axis to 1.0744–1.0752 nm. As the temperature increased, the electrical conductivity decreased while the Seebeck coefficient increased, which indicates that the Sn/In double-doped famatinites have degenerate semiconductor characteristics. With increasing Sn and In content, the carrier concentration increased, so that the electrical conductivity increased and the Seebeck coefficient decreased. Cu₃Sb_0.80Sn_0.12In_0.08S₄ exhibited the highest power factor, 0.87 mW m^-1 K^-2 at 623 K, with greatly increased thermal conductivity. Cu₃Sb_0.86Sn_0.08In_0.06S₄ showed the highest value for the dimensionless figure of merit, ZT = 0.53 at 623 K, with a power factor of 0.78 mW m^-1 K^-2 and thermal conductivity of 0.90 W m^-1 K^-1.