Abstract: Ternary Pb-Bi-S compounds emerge as potential thermoelectric materials owing to low thermal conductivity, but the origin of their intrinsic low lattice thermal conductivities lacks further investigation. Herein, a series of ternary Pb_mBi₂S_3+m (m = 1-10) compounds are synthesized and their crystal structure evolutions with increasing m values are clearly unclosed. The room-temperature lattice thermal conductivities in PbBi₂S₄, Pb₃Bi₂S₆ and Pb₆Bi₂S₉ can reach at 0.57, 0.56 and 0.80 W m^-1 K^-1, respectively, outperforming other ternary sulfur-based compounds. Theoretical calculations show that the low lattice thermal conductivities in Pb_mBi₂S_3+m (m = 1-10) mainly originate from soft phonon dispersion caused by strong lattice anharmonicity, and both asymmetric chemical bond and lone pair electrons (Pb 6s² and Bi 6s²) can favorably block phonon propagation. Furthermore, the elastic measurements also confirm relatively low sound velocities and shear modulus, and the Grüneisen parameter (γ) calculated by sound velocities can reach at 1.67, 1.85 and 1.94 in PbBi₂S₄, Pb₃Bi₂S₆ and Pb₆Bi₂S₉, respectively. Finally, the intrinsic low lattice thermal conductivities in Pb_mBi₂S_3+m (m = 1-10) contribute to promising thermoelectric performance, and the maximum ZT values of 0.47, 0.38 and 0.45 can be achieved in undoped PbBi₂S₄, Pb₃Bi₂S₆ and Pb₆Bi₂S₉, respectively.

Key words: Crystal structure evolution, Lattice thermal conductivity, Phonon dispersion, Lattice anharmonicity, Lone pair electrons