Charge order of bismuth ions and nature of chemical bonds in double perovskite-type oxide BaBiO3 visualized by synchrotron radiation X-ray diffraction

Abstract

Synchrotron radiation X-ray powder diffraction has visualized the charge order of Bi3+ and Bi5+ ions and the nature of chemical bonds in BaBiO3 in electron charge density distribution maps. The Bi3+–O bond is considerably more ionic than the Bi5+–O bond in all four phases. BaBiO3 can be regarded as an ionic crystal in the prototype structure comprising isolated Ba2+ and Bi3+ ions as well as [Bi5+O6]7− octahedra in the Fmm symmetry; a three-dimensional covalent bonding network is formed in the R and I2/m symmetries, and a two-dimensional layered structure comprising Bi3+O4 planar squares and Bi5+O6 octahedra is established in the P21/n symmetry at low temperatures. Our study provides a comprehensive understanding of the structural characteristics, electron charge density distribution, and chemical bonds of BaBiO3, which may be useful for developing BaBiO3-based functional materials such as superconductors.