Localized versus Delocalized Bonding in the K5Bi4 Metallic Salt

The electronic structure of one member of the A5Pn4 family of metallic salts, K5Bi4, has been studied by means of first-principles density functional calculations. It is shown that the alkali and pnictide orbitals mix considerably providing for a large covalent bonding contribution to the stability of the compound. It is suggested that solids of the A5Pn4 family of compounds (A = K, Rb, Cs; Pn = As, Sb, Bi) should have better conductivity in the direction perpendicular to the plane of the tetrameric pnictide units and that the low-temperature transition in the antimonides is an orderdisorder transition associated with one of the alkali positions.