Materials with low thermal conductivity effectively reduce heat dissipation, finding significant applications in thermoelectric devices and thermal barrier coatings. We report the low lattice thermal conductivity and high thermoelectric figure-of-merit ($\mathit{ZT}$) of a new ${\mathrm{PbBi}}_2{\mathrm{Te}}_4$ phase. Based on the dual-phonon transport theory, its lattice thermal conductivity is $\sim 0.3\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ (Criterion I) to $\sim 0.4\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ (Criterion III) at 600 K. Such low lattice thermal conductivity originates from the $s^2$ lone pair electrons residing on both Pb and Bi sites. The presence of double lone pair electrons induces rattling-like vibrations, which intensify phonon scattering and suppress thermal transport. The coexistence of a multiple-valley conduction band and a flat valence band benefits the Seebeck coefficient. The $n$- and $p$-type ${\mathrm{PbBi}}_2{\mathrm{Te}}_4$ compounds exhibit optimal $\mathit{ZT}$ values exceeding or approaching 1.0 at room temperature, rising to $\sim$ 2.01 (Criterion III) to $\sim$ 2.13 (Criterion I) and $\sim$ 1.87 (Criterion III) to $\sim$ 2.01 (Criterion I), respectively, in the intermediate-temperature range. The $n$-type ${\mathrm{PbBi}}_2{\mathrm{Te}}_4$ displays a more pronounced thermoelectric advantage than the $p$-type counterparts. Our findings provide hints for designing new thermoelectric materials with intrinsically low thermal conductivity.

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• Thermal conductivity
• Thermoelectrics
• Density functional theory
• Thermal techniques