Structure Determination of A2M3+TaO6 and A2M3+NbO6 Ordered Perovskites: Octahedral Tilting and Pseudosymmetry: Octahedral tilting and pseudosymmetry

The room-temperature crystal structures of six A ₂ M ³⁺ M ⁵⁺ O ₆ ordered perovskites have been determined from neutron and X-ray powder diffraction data. Ba ₂ YNbO ₆ adopts the aristotype high-symmetry cubic structure (space group Fm 3̄ m , Z = 4). The symmetries of the remaining five compounds were lowered by octahedral tilting distortions. Out-of-phase rotations of the octahedra about the c axis were observed in Sr ₂ CrTaO ₆ and Sr ₂ GaTaO ₆ , which lowers the symmetry to tetragonal (space group = I 4/ m , Z = 2, Glazer tilt system = a ⁰ a ⁰ c ^- ). Octahedral tilting analogous to that seen in GdFeO ₃ occurs in Sr ₂ ScNbO ₆ , Ca ₂ AlNbO ₆ and Ca ₂ CrTaO ₆ , which lowers the symmetry to monoclinic (space group P 2 ₁ / n , Z = 2, Glazer tilt system = a ^- a ^- c ⁺ ). The Sr ₂ M TaO ₆ ( M = Cr, Ga, Sc) compounds have unit-cell dimensions that are highly pseudo-cubic. Ca ₂ AlNbO ₆ and Ca ₂ CrTaO ₆ have unit-cell dimensions that are strongly pseudo-orthorhombic. This high degree of pseudosymmetry complicates the space-group assignment and structure determination. The space-group symmetries, unit-cell dimensions and cation ordering characteristics of an additional 13 compositions, as determined from X-ray powder diffraction data, are also reported. An analysis of the crystal structures of 32 A ₂ M TaO ₆ and A ₂ M NbO ₆ perovskites shows that in general the octahedral tilt system strongly correlates with the tolerance factor.