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 using electronic structure methods that the breathing distortion is accompanied by spatial condensation of hole pairs into local, molecular-like orbitals of the A1g symmetry composed of O-2pσ and Bi-6s atomic orbitals of collapsed BiO6 octahedra. 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 non-uniform band-narrowing. We argue that formation of localized states upon breathing distortion is, to a large extent, a property of the oxygen sublattice and expect similar hybridization effects in other perovskites involving formally high oxidation state cations.
@article{arxiv.1410.0945,
title = {Hybridization effects and bond-disproportionation in the bismuth perovskites},
author = {Kateryna Foyevtsova and Arash Khazraie and Ilya Elfimov and George A. Sawatzky},
journal= {arXiv preprint arXiv:1410.0945},
year = {2015}
}