Crystal Growth and Structural Investigations of the Oxygen Ion Conductor BaBi3O5.5

Abstract

Large single crystals of the oxygen ion conductor BaBi3O5.5 have been obtained, and its structure was investigated by single-crystal X-ray diffraction (XRD) and neutron diffraction (ND), electron diffraction, and high-resolution electron microscopy (HREM). The basic three-dimensional structure was refined, using single crystal ND data, in space group Im3m to a weighted R value of 5.3% for 17 unique reflections, with Ba and Bi atoms statistically distributed on the 2(a) sites (0, 0, 0) and the O atoms on the 6(b) sites (12, 0, 0) with a refined occupancy of 36(1)%. The refined thermal-displacement parameters for the O atoms are highly anisotropic, with U11=U22=0.27(1) and U33=0.082(7) Å2. In addition, the structure 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:Im3m:Im3m, with a=4.3798(2) Å and modulation vectors q1=[α, 0, 0]*, q2=[0, α, 0]*, and q3=[0, 0, α]* with α=0.3835(1). The diffraction data and the HREM images indicate that the modulation is predominantly occuptional. The satellite reflections have comparable intensities in powder XRD and ND patterns, indicating furthermore that the modulation originates from ordering of both O atoms/vacancies and metal atoms.