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+) M5+O6 ordered perovskites have been determined from neutron and X-ray powder diffraction data. Ba2YNbO6 adopts the aristotype high-symmetry cubic structure (space group Fm (3) over barm, 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 Sr2CrTaO6 and Sr2GaTaO6, 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 GdFeO3 occurs in Sr2ScNbO6, Ca2AlNbO6 and Ca2CrTaO6, which lowers the symmetry to monoclinic (space group P2(1)/ n, Z = 2, Glazer tilt system = a(-)a(-)c(+)). The Sr2MTaO6 (M = Cr, Ga, Sc) compounds have unit- cell dimensions that are highly pseudocubic. Ca2AlNbO6 and Ca2CrTaO6 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, unitcell 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.