Thermoelectric Characteristics of Permingeatite Compounds Double-Doped with Sn and S

Sn/S double-doped permingeatites, Cu₃Sb_1-xSn_xSe_4-yS_y (0.02 ≤ x ≤ 0.08 and 0.25 ≤ y ≤ 0.50) were synthesized, and crystallographic parameters and thermoelectric characteristics were examined as a function of doping level. The lattice parameters of permingeatite were significantly modified by the dual doping of Sn and S, with S doping exerting a greater influence on lattice constants and variations in tetragonality compared to Sn doping. With an increase in the level of Sn doping and a decrease in S doping, the carrier concentration increased, leading to enhanced electrical conductivity, indicative of a degenerate semiconducting state. Conversely, an increase in S doping and a decrease in Sn doping led to a rise in the Seebeck coefficient, demonstrating p-type conductivity characteristics with positive temperature dependence. Additionally, the double doping of Sn and S substantially improved the power factor, with Cu₃Sb_0.98Sn_0.02Se_3.75S_0.25 exhibiting 1.12 mWm^-1K^-2 at 623 K, approximately 2.3 times higher than that of undoped permingeatite. The lattice thermal conductivity decreased with increasing temperature, while the electronic thermal conductivity exhibited minimal temperature dependence. Ultimately, the dimensionless figure of merit (ZT) was improved through the double doping of Sn and S, with Cu₃Sb_0.98Sn_0.02Se_3.50S_0.50 recording a ZT of 0.68 at 623 K, approximately 1.7 times higher than that of pure permingeatite.