Interplay Between Lone Pair Stereochemical Activity and Structural Anisotropy Drives Ultralow Thermal Conductivity in Layered AGeS3 (A = Pb, Sn) Metal Sulfides

Metal sulfides have recently drawn significant interest from the scientific community due to their nontoxicity and abundance, making them suitable for a wide range of applications, including thermoelectric and optoelectronic technologies. Although numerous ternary metal sulfides have been reported in the literature, their crystal structures and physical properties remain largely unexplored. In the present work, we have synthesized bulk polycrystalline samples of AGeS₃ (A = Pb/Sn) using mechanical alloying followed by spark plasma sintering and studied their crystal structures, microstructures, and thermal and vibrational properties in relation to computational modeling. The low lattice thermal conductivity in this class of compounds is mainly attributed to the weak interlayer bonding (2D character) due to the stereochemical activity of the lone pairs of Sn²⁺ in SnGeS₃ and Pb²⁺ in PbGeS₃. Importantly, we examine the nature of the chemical bonds in AGeS₃ and elucidate the origin of distinct thermal conductivities in these two compounds despite having similar crystal structures. We show that the enhanced stereochemical activity of Sn²⁺ in SnGeS₃, compared to Pb²⁺ in PbGeS₃, leads to a more distinct two-dimensional character. This is demonstrated by stronger intralayer bonding, weaker interlayer interactions, and more prominent interlayer Sn–S antibonding states near the Fermi level. Anisotropic grain growth, observed in our TEM data, further supports this interpretation. Consequently, glass-like lattice thermal conductivity is observed in SnGeS₃ while PbGeS₃ exhibits crystalline-like thermal conductivity. These findings enrich the fundamental knowledge of crystal chemistry and thermal conduction relationships in metal sulfides and encourage further investigations into the design of materials for thermal management applications.