Bismuth valence order-disorder study in BaBiO(3) by powder neutron diffraction.

Two structural arrangements have been shown to exist for the perovskite-like compound BaBi(0.5)(3+)Bi(0.5)(5+)O(3) by powder neutron diffraction data. The first is characterized by a partial (75%) order of the Bi(3+) and Bi(5+) cations on the two crystallographically independent sites, while in the second arrangement the two cations are almost 100% disordered. The structure of eleven differently-prepared BaBiO(3) samples showed that the deciding factor for obtaining one or the other arrangement is the temperature at which the sample is prepared or subsequently heat-treated. On the contrary, the cooling rate and the atmosphere (air or oxygen) do not seem to be important parameters.