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.