Dual-phonon transport and mean-free-path filtering in PbBi₂S₄ thermoelectric compound

Symmetry breaking induced by lone-pair electrons (LPEs) contributes to the intrinsically low lattice thermal conductivity of the PbBi₂S₄ compound with a weakly disordered crystal structure. However, the limited understanding of the dual-phonon transport mechanisms and carrier mean-free-path filtering in the PbBi₂S₄ compound hinders the enhancement of its thermoelectric performance. In this work, we systematically elucidate the electron-phonon transport mechanisms in PbBi₂S₄ based on first-principles calculations combined with the Boltzmann transport equation. Our results demonstrate that the low lattice thermal conductivity and weakly disordered crystal structure in quasi-one-dimensional PbBi₂S₄ originate from the static off-center effect driven by the LPEs of Pb atoms. The electronic instability caused by the LPEs of Pb atoms induces disordered vibrations of S atoms and facilitates the transport of diffusons. These diffusons dominate the thermal transport process in PbBi₂S₄, resulting in a lattice thermal conductivity of 0.55 W m^-1 K^-1 at 300 K. Furthermore, heavy-light band degeneracy can be achieved by manipulating the carrier density, leading to an optimal power factor. Eventually, by combining dualphonon transport and carrier mean-free-path filtering, an optimal ZT value of 1.6 is obtained in n-type PbBi₂S₄ at 800 K. This work provides theoretical guidance for the experimental optimization of Pb-Bi-S-based thermoelectric materials.