Structure Low Dimensionality and Lone-Pair Stereochemical Activity: the Key to Low Thermal Conductivity in the Pb–Sn–S System
Paribesh Acharyya, Koushik Pal, Bin Zhang, Tristan Barbier, Carmelo Prestipino, Philippe Boullay, B. Raveau, Pierric Lemoine, B. Malaman, Xingchen Shen, Maxime Vaillant, Adèle Renaud, Blas P. Uberuaga, Christophe Candolfi, Xiaoyuan Zhou, Emmanuel Guilmeau
Abstract
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 PbSnS 2 and 1D PbSnS 3, 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 PbSnS 3 and 0.6 W/m K in 2D PbSnS 2 at room temperature. However, the nature of the thermal transport is quite distinct. 1D PbSnS 3 exhibits a higher thermal conductivity with a crystalline-like peak at low temperatures, while 2D PbSnS 2 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 PbSnS 2, 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.