Bismuth valence order-disorder study in BaBiO3 by powder neutron diffraction

Abstract

Two structural arrangements have been shown to exist for the Perovskite-like compound BaBi0.53+ Bi0.55+ O3 by powder neutron diffraction data. The first is characterized by a partial (75%) order of the Bi3+ and Bi5+ 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 BaBiO3 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. DTA measurements indicated that BaBiO3 undergoes an additional phase transition at 860°C on heating and 801°C on cooling. This transition probably corresponds to the disproportionation of the Bi into Bi3+ and Bi5+ cations. This conjecture explains why the heat-treatment at 800°C yields compounds containing partial cation ordering. Since the symmetry is cubic above and below the 801°C transition, the 75% ordered BaBiO3 stable between 801°C and 480°C would contain a trigonal dynamical distortion which would be the precursor of the static trigonal distortion which takes place at 480°C. In every compound the partial ordering is always 75%. This could be interpreted as an indication that an additional ordering of Au3Cu type exists on the two Bi sublattices.