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.
Ling
Chris D
CD
School of Chemistry, The University of Sydney, Sydney 2006, Australia. c.ling@chem.usyd.edu.au
Avdeev
Maxim
M
Aivazian
Karina
K