BaBiO_2.5, a new bismuth oxide with a layered structure

A new Bi(III) oxide, BaBiO_2.5, has been prepared by slow-cooling of the perovskite-like BaBiO₃ in a reducing atmosphere. The structure was solved ab initio from a combination of synchrotron X-ray and pulsed neutron powder diffraction data. The phase crystallizes in the monoclinic space group P2₁/c, with a = 7.3412(1) Å, b = 7.5793(1) Å, c = 6.0722(1) Å, and β = 99.187(2)°. A novel layer-type structure is adopted consisting of double BiO_x sheets in the bc plane separated by Ba ions along the a direction. Bi is four-coordinated in a pseudotrigonal bipyramidal environment, with the electron lone-pair occupying one of the equatorial sites, similar to that found in β-Bi₂O₃. There are, thus, significant differences from either the Aurivillius phases or the high-T_c bismuth cuprate superconductors, which may also be regarded as layered bismuth oxides.