from neutron scattering*

BaBiO₃ has attracted considerable research attention, not only as the parent compound of bismuthate superconductors (such as Ba_1-xK_xBiO₃, which exhibit the highest known Tc ​ among non-cuprate superconductors), but also for its unexpected semiconducting behavior.  A simple ionic picture, with Bi in a 4+ oxidation state, would suggest metallicity, yet this is not observed. Instead, Bi is believed to undergo charge disproportionation of the form 2Bi⁴⁺ -> Bi³⁺ + Bi⁵⁺

The local structure studied by pair distribution function (PDF) analysis of neutron diffraction data strongly supports this view, revealing distinct long and short Bi–O bonds consistent with charge disproportionation. Consequently, BaBiO₃ can be described as a double perovskite with two inequivalent Bi sites, each octahedrally coordinated by oxygen, forming an alternating array of large and small BiO₆ octahedra. Furthermore, the charge disproportionation survives with increasing temperature. The local structure analysis also reveals that the Bi³⁺ peak is much broader than the Bi⁵⁺ peak, which cannot be adequately explained by a correlated Debye model. This could hint that the valence state of Bi³⁺ could be fluctuating and has interesting implications about the nature of the charge disproportionation of BaBiO₃ compound.