Structure determination of A2M3+TaO6 and A2M3+NbO6 ordered perovskites: 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\overline 3m, 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 = I4/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 P2(1)/n, Z = 2, Glazer tilt system = a(-)a(-)c(+)). The Sr(2)MTaO(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)MTaO(6) and A(2)MNbO(6) perovskites shows that in general the octahedral tilt system strongly correlates with the tolerance factor.