δ ‐Bonding and Spin‐Orbit Coupling Make SrAg 4 Sb 2 a Topological Insulator

Abstract Bonding interactions and spin‐orbit coupling in the topological insulator SrAg 4 Sb 2 are investigated using DFT with orbital projection analysis. Ag−Ag delta bonding is a key ingredient in the topological insulating state because the delta antibonding band forms a band inversion with the 5 s sigma bonding band. Spin‐orbit coupling is required to lift d orbital degeneracies and lower the antibonding band enough to create the band inversion. These bonding effects are enabled by a longer‐than‐covalent Ag−Ag distance in the crystal lattice, which might be a structural characteristic of other transition metal based topological insulators. A simplified model of the topological bands is constructed to capture the essence of the topological insulating state in a way that may be engineered in other materials.