Investigation of thermoelectric properties in layered PbSnS<sub>2</sub> from first-principles calculations

<p indent="0mm">Thermoelectric materials are solid-state materials that can convert thermal energy into electrical energy and vice versa. In recent years, there has been significant progress in the development of advanced thermoelectric materials with improved efficiency and cost-effectiveness. To date, high figure-of-merit was obtained in n-type and p-type SnSe, providing a solid foundation for the construction of thermoelectric devices. The low cost and environmentally friendly SnS, an analogue of SnSe, has also been greatly developed and a high average <italic>ZT</italic> of 1.25 was reported in p-type SnS_0.91Se_0.09 crystals. However, the thermoelectric performance of n-type SnS is far lagging due to the great difficulties and challenges on achieving effective electron doping to optimize the poor n-type electrical transports. A novel sulfide compound, PbSnS₂, is isostructural to SnS and exhibits promising n-type thermoelectric performance due to its intrinsically low thermal conductivity. The PbSnS_1.96Cl_0.04 crystal exhibits a <italic>ZT</italic>_max of ~1.20 at <sc>773 K</sc> and a <italic>ZT</italic>_ave of ~0.79 within the temperature range of <sc>300–823 K,</sc> specifically along out-of-plane directions. In contrast, along the in-plane direction, a record-high <italic>ZT</italic> value of ~1.20 was achieved at <sc>473 K.</sc> However, this performance is still unsatisfactory for the realization of low-cost and high-efficiency thermoelectric devices with p-type SnS. In this work, we investigate the thermoelectric properties of PbSnS₂ compounds based on first-principles calculations and Boltzmann transport theory. The results show that the intrinsically low thermal conductivity of PbSnS₂ originates from the strong lattice anharmonicity since the multiple chemical bonding between Pb/Sn and S atoms, and the lone-pair electrons of Pb/Sn. The sharp band edge at the bottom of the conduction band is conducive to achieving high mobility, and the flatten valence band helps to obtain high Seebeck coefficients, leading to strong anisotropic transport properties. The doping can effectively improve the electrical transport through increasing the carrier concentration, while resulting in higher electronic thermal conductivity. Moreover, the contribution of total thermal conductivity from electronic parts is more dominant than that from lattice parts, especially in the high carrier concentration region. Finally, a peak <italic>ZT</italic> (~2.81) is obtained in n<italic>-</italic>type PbSnS₂ along the out-of-plane direction at <sc>900 K.</sc> While for p<italic>-</italic>type doping, a peak <italic>ZT</italic> (~1.05 @ <sc>900 K)</sc> can be found along the in-plane direction. This work elucidates the physical origin of the intrinsically low lattice thermal conductivity, predicts the anisotropic thermoelectric properties with acoustic deformation potential scattering, ionized impurity scattering, and polar optical phonon scattering considered, and proposes the strategy for the further optimization of thermoelectric performance through prompting the carrier concentration. The evidence of experimental results is still needed.