Acta Crystallographica Section B

Structural Science

Volume 64, Part 2 (April 2008)

research papers

Acta Cryst. (2008). B64, 154-159 [ doi:10.1107/S0108768108005041 ]

Structures of 6H perovskites Ba₃CaSb₂O₉ and Ba₃SrSb₂O₉ determined by synchrotron X-ray diffraction, neutron powder diffraction and ab initio calculations

B. Rowda, M. Avdeev, P. L. Lee, P. F. Henry and C. D. Ling

Abstract: The structures of the 6H perovskites Ba₃B²⁺Sb⁵⁺₂O₉, B = Ca and Sr, have been solved and refined using synchrotron X-ray and neutron powder diffraction data. Ba₃CaSb₂O₉ and Ba₃SrSb₂O₉ have monoclinic C2/c and triclinic $P\bar 1$ space-group symmetries, respectively, while Ba₃MgSb₂O₉ has ideal hexagonal P6₃/mmc space-group symmetry. The symmetry-lowering distortions are a consequence of internal `chemical pressure' owing to the increasing effective ionic radius of the alkaline-earth cation in the perovskite B site from Mg²⁺ (0.72 Å) to Ca²⁺ (1.00 Å) to Sr²⁺ (1.18 Å). Increasing the effective ionic radius further to Ba²⁺ (1.35 Å) leads to decomposition at room temperature. The driving force behind the transition from P6₃/mmc to C2/c is the need to alleviate underbonding of Ba²⁺ cations in the perovskite A site via octahedral rotations, while the transition from C2/c to $P\bar 1$ is driven by the need to regularize the shape of the Sb₂O₉ face-sharing octahedral dimers. Ab initio geometry-optimization calculations were used to find a triclinic starting model for Ba₃SrSb₂O₉.

Keywords: hexagonal perovskite; synchrotron X-ray diffraction; neutron powder diffraction; ab initio geometry optimization.