A TIME-OF-FLIGHT POWDER NEUTRON-DIFFRACTION STUDY OF NONSTOICHIOMETRYIN BARIUM IRIDATE BAIRO3-DELTA

Rietveld refinement of room temperature time-of-flight powder neutron diffraction data for a sample of BaIrO3-delta, prepared by slow coolin g of polycrystalline single-phase material from 1223 K, was carried ou t in the space group C2/m; a = 9.9992(3) Angstrom, b = 5.7490(1) Angst rom, c = 15.1707(2) Angstrom, beta = 103.27(1)degrees. Refinement of o xygen site occupancy factors indicated that the material was almost id eally stoichiometric with a composition BaIrO2.94. Analogous refinemen t of data for a sample produced by rapidly quenching from 1223 K resul ted in similar parameters; space group C2/m, a = 10.0010(4) Angstrom, b = 5.7521(1) Angstrom, c = 15.1747(3) Angstrom, beta = 103.26(1)degre es. Refinement of oxygen site occupancy factors indicated significant numbers of defects on the oxide sublattice corresponding to a composit ion BaIrO2.74. The distribution of these defects and their effect on t he detailed structure of the phases are discussed.