δ‐bonding and spin‐orbit coupling make SrAg4Sb2 a topological insulator

Bonding interactions and spin‐orbit coupling in the topological insulator SrAg4Sb2 are investigated using DFT with orbital projection analysis. Ag‐Ag delta bonding is a key ingredient in the topological insulating state because the 4dxy + 4dx2 −y2 delta antibonding band forms a band inversion with the 5s 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.