Structure Low Dimensionality and Lone-Pair Stereochemical Activity: the Key to Low Thermal Conductivity in the...

Recently, metal sulfides have begun to receive attention as potential cost-effective materials for thermoelectric applications beyond optoelectronic and photovoltaic devices. Herein, based on a comparative analysis of the structural and transport properties of 2D PbSnS₂ and 1D PbSnS₃, we demonstrate that the intrinsic effects that govern the low lattice thermal conductivity (κ_L) of these sulfides originate from the combination of the low dimensionality of their crystal structures with the stereochemical activity of the lone-pair electrons of cations. The presence of weak bonds in these materials, responsible for phonon scattering, results in inherently low κ_L of 1.0 W/m K in 1D PbSnS₃ and 0.6 W/m K in 2D PbSnS₂ at room temperature. However, the nature of the thermal transport is quite distinct. 1D PbSnS₃ exhibits a higher thermal conductivity with a crystalline-like peak at low temperatures, while 2D PbSnS₂ demonstrates glassy thermal conductivity in the entire temperature range investigated. First-principles density functional theory calculations reveal that the presence of antibonding states below the Fermi level, especially in PbSnS₂, contributes to the very low κ_L. In addition, the calculated phonon dispersions exhibit very soft acoustic phonon branches that give rise to soft lattices and very low speeds of sounds.