Structures of 6H perovskites Ba(3)CaSb(2)O(9) and Ba(3)SrSb(2)O(9) determined by synchrotron X-ray diffraction, neutron powder diffraction and ab initio calculations.

The structures of the 6H perovskites Ba(3)B(2+)Sb(5+)(2)O(9), B = Ca and Sr, have been solved and refined using synchrotron X-ray and neutron powder diffraction data. Ba(3)CaSb(2)O(9) and Ba(3)SrSb(2)O(9) have monoclinic C2/c and triclinic P\bar 1 space-group symmetries, respectively, while Ba(3)MgSb(2)O(9) has ideal hexagonal P6(3)/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 Mg(2+) (0.72 A) to Ca(2+) (1.00 A) to Sr(2+) (1.18 A). Increasing the effective ionic radius further to Ba(2+) (1.35 A) leads to decomposition at room temperature. The driving force behind the transition from P6(3)/mmc to C2/c is the need to alleviate underbonding of Ba(2+) 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 Sb(2)O(9) face-sharing octahedral dimers. Ab initio geometry-optimization calculations were used to find a triclinic starting model for Ba(3)SrSb(2)O(9).