Phases and thermoelectric properties of SnTe with (Ge, Mn) co-doping

A lead-free SnTe compound shows good electrical properties but also high thermal conductivity, resulting in a low figure of merit ZT. We demonstrate a significant enhancement of the thermoelectric properties of SnTe by (Ge, Mn) co-doping. (Ge, Mn) co-doped samples (Sn_0.8Ge_0.2)_1-xMn_xTe with x = 0, 0.03, 0.06, 0.09, 0.12, 0.15, 0.18 and 0.2 were prepared for this investigation. The substitution of Ge for Sn in SnTe promotes the solubility of Mn in a SnTe-based phase up to 20 at%, which further enlarges the band gap and gives rise to enhanced valence band convergence as compared with Mn doping, leading to a notably increased Seebeck coefficient and a power factor. All alloys retain p-type conduction and hole carrier concentration increases with increasing Mn content. The solute Ge and Mn atoms as well as the second phase of Ge in a SnTe-based system enhance phonon scattering and thus reduce thermal conductivity. The synergistic role that Ge and Mn play in regulating the electron and phonon transport of SnTe yields a maximum figure of merit ZT of 1.22 at 873 K for the sample (Sn_0.8Ge_0.2)_0.85Mn_0.15Te.