The room-temperature crystal structures of six A2M3+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 Fm3̄m, 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 = a0a0c-). 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 Sr2MTaO6 (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 A2MTaO6 and A2MNbO6 perovskites shows that in general the octahedral tilt system strongly correlates with the tolerance factor.
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