Hybridization effects and bond disproportionation in the bismuth perovskites

We propose a microscopic description of the bond-disproportionated insulating state in the bismuth perovskites XBiO3 (X = Ba,Sr) that recognizes the bismuth-oxygen hybridization as a dominant energy scale. It is demonstrated by using electronic structure methods that the breathing distortion is accompanied by spatial condensation of hole pairs into local, molecularlike orbitals of the A(1g) symmetry composed of O-2p(sigma) and Bi-6s atomic orbitals of collapsed BiO6 octahedra. The primary importance of oxygen p states is thus revealed, in contrast to a popular picture of a purely ionic Bi3+/Bi5+ charge disproportionation. Octahedra tilting is shown to enhance the breathing instability by means of a nonuniform band narrowing. We argue that the formation of localized states upon breathing distortion is, to a large extent, a property of the oxygen sublattice, and we expect similar hybridization effects in other perovskites involving formally high oxidation state cations.