Crystal growth and structural investigations of the oxygen ion conductor BaBi3O5.5

Large single crystals of the oxygen ion conductor BaBi3O5.5 have been obtai ned, and its structure was investigated by single-crystal X-ray diffraction (XRD) and neutron diffraction (ND), electron diffraction, and high-resolut ion electron microscopy (HREM), The basic three-dimensional structure was r efined, using single crystal ND data, in space group Im (3) over bar m to a weighted R value of 5.3% for 17 unique reflections, with Ba and Bi atoms s tatistically distributed on the 2(a) sites (0, 0, 0) and the O atoms on the 6(b) sites (1/2, 0, 0) with a refined occupancy of 36(1)%, The refined the rmal-displacement parameters for the O atoms are highly anisotropic, with U -11 = U-22 = 0.27(1) and U-33 = 0.082(7) Angstrom(2). In addition, the stru cture is incommensurately modulated with the systematic absence conditions F < hklmnp > = 0 unless h + k + l = 2j and m + n + p = 2j, implying the six -dimensional superspace group P:Im (3) over bar m:Im (3) over bar m, with 4 .3798(2) Angstrom and modulation vectors q(1) = [alpha, 0, 0]*, q(2) = [ 0, alpha, 0]*, and q(3) = [0, 0, alpha]* with alpha = 0.3835(1), The diffract ion data and the HREM images indicate that the modulation is predominantly occuptional. The satellite reflections have comparable intensities in powde r XRD and ND patterns, indicating furthermore that the modulation originate s from ordering of both O atoms/vacancies and metal atoms. (C) 2000 Academi c Press.