Structure determination of A 2 M 3+TaO6 and A 2 M 3+NbO6 ordered perovskites: octahedral tilting and pseudosymmetry

The room-temperature crystal structures of six A 2 M 3+ M 5+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\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 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 P21/n, Z = 2, Glazer tilt system = a − a − c +). The Sr2 MTaO6 (M = Cr, Ga, Sc) compounds have unit-cell dimensions that are highly pseudo-cubic. 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, 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 MTaO6 and A 2 MNbO6 perovskites shows that in general the octahedral tilt system strongly correlates with the tolerance factor.