Oxygen holes and hybridization in the bismuthates

Motivated by the recently renewed interest in the superconducting bismuth perovskites, we investigate the electronic structure of the parent compounds ABiO(3) (A = Sr, Ba) using ab initio methods and tight-binding (TB) modeling. We use the density functional theory (DFT) in the local density approximation (LDA) to understand the role of various interactions in shaping the ABiO(3) band structure near the Fermi level. It is established that interatomic hybridization involving Bi-6s and O-2p orbitals plays the most important role. Based on our DFT calculations, we derive aminimalTBmodel and demonstrate that it can describe the properties of the band structure as a function of lattice distortions, such as the opening of a charge gap with the onset of the breathing distortion and the associated condensation of holes onto a(1g) -symmetric molecular orbitals formed by the O-2ps orbitals on collapsed octahedra. We also derive a single band model involving the hopping of an extended molecular orbital involving both Bi-6s and a linear combination of six O-2p(sigma) orbitals which provides a very good description of the dispersion and band gaps of the low energy scale bands straddling the chemical potential.