BaBiO3

BaBiO3 is a barium bismuth oxide that crystallizes in a perovskite-derived structure with the formula ABiO3, where A is Ba. It is notable for its mixed-valence Bi3+/Bi5+ state, which leads to a breathing-mode distortion of the BiO6 octahedra and charge-ordering that stabilizes an insulating ground state at room temperature.

In the crystal, Bi sits in octahedral coordination with oxygen, forming a network of BiO6 units connected to Ba in a three-dimensional perovskite framework. The Bi3+/Bi5+ charge disproportionation causes alternating short and long Bi–O bonds, reducing the symmetry relative to an ideal cubic perovskite. The average Bi oxidation state is Bi4+, but the system exhibits long-range charge order associated with the Bi3+/Bi5+ pattern.

Electronic properties of BaBiO3 are dominated by its insulating behavior, with a band gap typically reported in the range of about 1.5 to 2.0 eV. The charge-order distortion localizes carriers and suppresses metallic conductivity. External perturbations such as chemical doping or high pressure can suppress the charge ordering and drive the material toward metallicity.

Doping BaBiO3 with holes, most notably by substituting Ba with K to form Ba1−xKxBiO3, yields superconductivity with critical temperatures up to about 30 K around x ≈ 0.4. Related bismuthate systems, such as BaPb1−xBixO3, also become superconducting, albeit with lower Tc values. The superconducting state is attributed to strong electron–phonon coupling in the Bi–O network and the suppression of charge ordering upon doping.

BaBiO3 and its doped derivatives are studied as prototypical examples of charge ordering, structural distortions, and unconventional superconductivity in oxide materials.