Achieving high thermoelectric performance in metal sulfide PbSnS2 through strain-mediated lone-pair expression amplification and multi-band conduction

Abstract

PbSnS₂ is a low-cost and earth-abundant metal sulfide that exhibits promising thermoelectric performance, characterized by low lattice thermal conductivity associated with stereochemically active 5s² (Sn) and 6s² (Pb) lone-pair electrons. In this work, strain-driven modulation of lone-pair activity, phonon transport, lattice thermal conductivity, and thermoelectric performance in layered PbSnS₂ is systematically revealed by first-principles calculations. Biaxial strain ranging from −2% (compressive) to +2% (tensile) progressively weakens interatomic bonding and enhances lone-pair stereochemical activity. This enhancement leads to a simultaneous suppression of particle-like and wave-like heat transport through phonon softening, reduced phonon group velocities, and strengthened three-phonon scattering, resulting in a marked reduction of lattice thermal conductivity from 2.0 to 1.22 W m⁻¹ K⁻¹ at 300 K. At −2% compressive strain, the presence of pronounced multi-band features near the Fermi level gives rise to an unusual decoupling between electrical conductivity and the Seebeck coefficient. Consequently, the power factor reaches 3.23 mW m⁻¹ K⁻² under −2% compressive strain, nearly twice that achieved under +2% tensile strain, and delivers a substantially enhanced thermoelectric figure of merit with a maximum zT of 1.18 at 900 K. These results establish strain engineering as an effective and dynamic strategy for optimizing thermoelectric performance in PbSnS₂ and provide general design guidelines for lone-pair-active metal sulfide thermoelectrics.