Synthesis, structure, and stability of the high-temperature 6H-type perovskite phase Ba(3)BaSb(2)O(9)

The structure of Ba(3)BaSb(2)O(9) is reported for the first time as a high-temperature phase with an ideal hexagonal BaTiO(3)-type structure (space group P6(3)/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 Sb(5+)O(6) octahedra (with mean Sb-O bonds of 1.99 Angstroms) and the single corner-sharing Ba(2+)O(6) octahedra (with mean Ba-O bonds of 2.46 Angstrom), which is greater than for any other reported 6H perovskite. This is consistent with the very different ionic radii of Sb(5+) (0.60 Angstroms) and Ba(2+) (1.35 Angstroms), and accounts for the instability of this phase at room temperature. The known symmetry-lowering modes of closely related 6H perovskites such as Ba(3)SrNb(2)O(9) and Ba(3)SrTa(2)O(9) are considered, but found not to account for the behaviour of Ba(3)BaSb(2)O(9) on cooling from high temperatures.