Charge transport and thermoelectric properties of Cu3Sb1−xGexSe4−ySy

Cu₃Sb_1−xGe_xSe_4−yS_y (0.04 ≤ x ≤ 0.12 and 0.10 ≤ y ≤ 0.25) permingeatite compounds were synthesized via dual doping of Ge and S at the Sb and Se sites, respectively. The structural, charge transport, and thermoelectric properties of these materials were systematically investigated. All samples exhibited high relative densities, ranging from 95.8% to 97.3%, and predominantly consisted of the tetragonal permingeatite phase. However, minor secondary phases, such as Se or Cu₈GeS₆, were detected depending on the S content. The introduction of Ge and S dopants caused a contraction in the lattice parameters of permingeatite. The electrical conductivity exhibited characteristics of a degenerate semiconductor, either remaining stable or slightly decreasing with increasing temperature. An increase in Ge content enhanced electrical conductivity, whereas an increase in S content reduced it. The Seebeck coefficient exhibited p-type behavior with positive values and decreased with increasing Ge content and decreasing S content. Dual doping with Ge and S significantly improved the power factor, with Cu₃Sb_0.96Ge_0.04Se_3.90S_0.10 achieving 0.63 mWm⁻¹ K⁻² at 623 K. In addition, the power factor showed a positive dependence on temperature, indicating the absence of intrinsic transition within the investigated temperature range. The thermal conductivity exhibited an inverse relationship with temperature, influenced by both the doping concentration and the temperature-dependent electronic and lattice components. Consequently, the thermoelectric performance was significantly enhanced by the dual doping strategy, achieving a maximum ZT of 0.37 at 623 K for both Cu₃Sb_0.96Ge_0.04Se_3.90S_0.10 and Cu₃Sb_0.92Ge_0.08Se_3.90S_0.10.