Hybridization effects and bond-disproportionation in the bismuth perovskites

TL;DR

This paper presents a microscopic model highlighting the dominant role of bismuth-oxygen hybridization in the bond-disproportionated insulating state of bismuth perovskites, emphasizing oxygen's importance over ionic charge models.

Contribution

It introduces a new understanding of the bond-disproportionation mechanism emphasizing oxygen hybridization and local molecular orbitals, contrasting with traditional ionic charge-disproportionation views.

Findings

Breathing distortion involves hole pairs in O-2p and Bi-6s orbitals.

Octahedral tilting enhances breathing instability.

Localized states mainly form on the oxygen sublattice.

Abstract

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.