Synthesis, structure, and stability of the high-temperature 6H-type perovskite phase Ba3BaSb2O9

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 Aa) and the single corner-sharing Ba2+O6 octahedra (with mean Ba-O bonds of 2.46 Aa), which is greater than for any other reported 6H perovskite. This is consistent with the very different ionic radii of Sb5+ (0.60 Aa) and Ba2+ (1.35 Aa), 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. (orig.)