Acta Cryst. (2007). B63, 584-588 [ doi:10.1107/S0108768107019441 ]
Abstract: The structure of Ba3BaSb2O9 is reported for the first time as a high-temperature phase with an ideal hexagonal BaTiO3-type structure (space group P63/mmc) and Rietveld-refined against high-temperature synchrotron X-ray powder diffraction data. The structure is remarkable for the extreme size difference between the pairs of face-sharing Sb5+O6 octahedra (with mean Sb-O bonds of 1.99 Å) and the single corner-sharing Ba2+O6 octahedra (with mean Ba-O bonds of 2.46 Å), which is greater than for any other reported 6H perovskite. This is consistent with the very different ionic radii of Sb5+ (0.60 Å) and Ba2+ (1.35 Å), and accounts for the instability of this phase at room temperature. The known symmetry-lowering modes of closely related 6H perovskites such as Ba3SrNb2O9 and Ba3SrTa2O9 are considered, but found not to account for the behaviour of Ba3BaSb2O9 on cooling from high temperatures.
Keywords: hexagonal perovskite; synchrotron X-ray diffraction; Rietveld refinement; high-temperature phase.
Rietveld powder data file (CIF format) (156.8 kbytes)
Rietveld powder data file (CIF format) (147.9 kbytes)
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