Fcc breathing instability in BaBiO_3 from first principles

TL;DR

This study uses first-principles density-functional calculations to analyze the structural breathing instability in cubic BaBiO_3, explaining an IR spectral feature through a simple structural distortion.

Contribution

It demonstrates that a breathing distortion alone can account for the IR spectral peak, challenging the need for charge disproportionation explanations.

Findings

Breathing distortion explains the IR spectral peak.

Oscillator strength of the mode matches experimental data.

Structural instability involves oxygen-octahedra breathing and rotation.

Abstract

We present first-principles density-functional calculations using the local density approximation to investigate the structural instability of cubic perovskite BaBiO_3. This material might exhibit charge disproportionation and some evidence thereof has been linked to the appearance of an additional, fourth peak in the experimental IR spectrum. However, our results suggest that the origin of this additional peak can be understood within the picture of a simple structural instability. While the true instability consists of an oxygen-octahedra breathing distortion and a small octahedra rotation, we find that the breathing alone in a fcc-type cell doubling is sufficient to explain the fourth peak in the IR spectrum. Our results show that the oscillator strength of this particular mode is of the same order of magnitude as the other three modes, in agreement with experiment.