Intrinsically low lattice thermal conductivity and multivalley band structure induced promising high thermoelectric performance in Pb3Bi2S6

Exploring novel material with lower thermal conductivity and excellent electrical property is beneficial to the application of thermoelectrics. The recently developed Pb₃Bi₂S₆ is regarded as promising thermoelectric material since its intrinsically low thermal conductivity. However, the mechanism of phonon-glass behavior is unclear, and the intrinsic thermoelectric performance is relative lower. In this study, the mechanism of lower thermal conductivity and the thermoelectric transport properties are evaluated by first-principles calculations and Boltzmann transport theory. Our findings indicate that the hierarchical chemical bonding present in BiS₆, PbS₆, and PbS₈ polyhedral structures, arising from the dual 6s² lone pair electrons of Pb and Bi atoms, along with rattler-like behavior of Pb atoms, contributes to an intrinsically low lattice thermal conductivity in Pb₃Bi₂S₆. Obviously multivalley in valence band edge leads to excellent electrical properties and resulting in promising thermoelectric performance under p-type doping. A maximum ZT ∼1.25 can be obtained at 700 K with carrier concentration of ∼8.07 × 10¹⁹ cm⁻³. This work reveals the mechanism for intrinsic low lattice thermal conductivity and provides useful guidance for achieving the promising performance in Pb₃Bi₂S₆.