Hybridization effects and bond-disproportionation in the bismuth perovskites

We propose a microscopic description of the bond-disproportionated insulating state in the bismuth perovskites $X$BiO$_3$ ($X$=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 $A_{1g}$ symmetry composed of O-$2p_{\sigma}$ and Bi-$6s$ atomic orbitals of collapsed BiO$_6$ octahedra. Primary importance of oxygen $p$-states is thus revealed, in contrast to a popular picture of a purely ionic Bi$^{3+}$/Bi$^{5+}$ 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.