Antibonding States Drive Anharmonicity and Low Thermal Conductivity in Edge-Sharing Metal Chalcogenides.

Stereochemically active lone pairs (SCALPs) and orbital hybridization in edge-sharing polyhedra play a crucial role in suppressing lattice thermal conductivity (κL) in thermoelectric materials. Strong mixing between pnictogen s- and chalcogen p-orbitals generates active lone pairs, which enhance lattice anharmonicity and lead to ultralow κL. In this study, we leverage machine learning interatomic potential to systematically probe bonding-driven mechanisms and their influence on thermal transport in noncentrosymmetric pnictogen chalcogens. We show that the combined effects of SCALPs, Pn-Pn bonding, and edge-sharing polyhedra enable the formation of antibonding states near the valence band maxima, intensifying phonon scattering. To quantify the underlying anharmonicity, we introduce a set of bonding descriptors─lone pair angle, lone pair distance, ionicity, and hybridization─that capture the influence of local structural motifs and antibonding features near the valence band edge. This bonding-centric framework not only elucidates the origins of ultralow κL but also offers a rational design strategy for accelerating the discovery of high-performance thermoelectric materials.