Structure Determination of A2M3+TaO6 and A2M3+NbO6 Ordered Perovskites: Octahedral Tilting and Pseudosymmetry
The room-temperature crystal structures of six A{sub 2}M{sup 3+}M{sup 5+}O{sub 6} ordered perovskites have been determined from neutron and X-ray powder diffraction data. Ba{sub 2}YNbO{sub 6} adopts the aristotype high-symmetry cubic structure (space group Fm{bar 3}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{sub 2}CrTaO{sub 6} and Sr{sub 2}GaTaO{sub 6} which lowers the symmetry to tetragonal (space group = I4/m, Z = 2, Glazer tilt system = a{sup 0}a{sup 0}c{sup -}). Octahedral tilting analogous to that seen in GdFeO{sub 3} occurs in Sr{sub 2}ScNbO{sub 6}, Ca{sub 2}AlNbO{sub 6} and Ca{sub 2}CrTaO{sub 6}, which lowers the symmetry to monoclinic (space group P2{sub 1}/n, Z = 2, Glazer tilt system = a{sup -}a{sup -}c{sup +}). The Sr{sub 2}MTaO{sub 6} (M = Cr, Ga, Sc) compounds have unit-cell dimensions that are highly pseudo-cubic. Ca{sub 2}AlNbO{sub 6} and Ca{sub 2}CrTaO{sub 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{sub 2}MTaO{sub 6} and A{sub 2}MNbO{sub 6} perovskites shows that in general the octahedral tilt system strongly correlates with the tolerance factor.
- Research Organization:
- Brookhaven National Lab. (BNL), Upton, NY (United States). National Synchrotron Light Source
- Sponsoring Organization:
- Doe - Office Of Science
- DOE Contract Number:
- DE-AC02-98CH10886
- OSTI ID:
- 914040
- Report Number(s):
- BNL-78608-2007-JA; ACBCAR; TRN: US0801495
- Journal Information:
- Acta Cryst. B, Vol. 62; ISSN 0567-7408
- Country of Publication:
- United States
- Language:
- English
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