Charge Transport and Thermoelectric Properties of Ge-Doped Famatinites Cu3Sb1−yGeyS4

Ge-doped famatinites Cu 3 Sb 1− y Ge y S 4 (0 ≤ y ≤ 0.1) were prepared by mechanical alloying and hot pressing. The phase transition, microstructure, charge transport properties and thermoelectric properties were examined in accordance with the Ge content. The famatinites were maintained as a single phase with a tetragonal structure at temperatures below their melting point without secondary phases. The melting points of Cu 3 SbS 4 and Cu 3 Sb 0.92 Ge 0.08 S 4 were 817 and 819 K, respectively. The hot-pressed specimens exhibited high relative densities of 98.3–99.5%. The a -axis and c -axis were decreased from 0.5386 to 0.5378 nm and from 1.0744 to 1.0719 nm, respectively, by doping Sb sites with Ge. Both intrinsic and Ge-doped famatinites exhibited positive Hall and Seebeck coefficients. The carrier concentration and mobility of Cu 3 SbS 4 were 2.2 × 10 18 cm −3 and 1.6 cm 2 V −1 s −1 , but those of Ge-doped specimens increased to (0.4–3.3) × 10 19 cm −3 and 29–71 cm 2 V −1 s −1 , respectively. Cu 3 SbS 4 exhibited non-degenerate semiconductor characteristics and demonstrated a dimensionless figure of merit, ZT , of 0.1 at 623 K owing to a power factor of 0.14 m Wm −1 K −2 and a thermal conductivity of 0.62 Wm −1 K −1 . However, the Ge-doped specimens exhibited degenerate semiconductor behaviors, and Cu 3 Sb 0.92 Ge 0.08 S 4 exhibited the highest ZT of 0.55 at 623 K owing to a power factor of 0.64 m W m −1 K −2 and a thermal conductivity of 0.72 Wm −1 K −1 . Graphic abstract