Abstract
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 3̄ 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 2 CrTaO 6 and Sr 2 GaTaO 6 , which lowers the symmetry to tetragonal (space group = I 4/ 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 P 2 1 / n , Z = 2, Glazer tilt system = a - a - c + ). The Sr 2 M TaO 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 M TaO 6 and A 2 M NbO 6 perovskites shows that in general the octahedral tilt system strongly correlates with the tolerance factor.
| Original language | English |
|---|---|
| Pages (from-to) | 384-396 |
| Number of pages | 13 |
| Journal | Acta Crystallographica Section B: Structural Science |
| Volume | 62 |
| Issue number | 3 |
| DOIs | |
| State | Published - Jun 2006 |
| Externally published | Yes |
ASJC Scopus Subject Areas
- General Biochemistry,Genetics and Molecular Biology
Keywords
- pseudosymmetry; perovskites; elpasolites; octahedral tilting; cation ordering; powder diffraction
Disciplines
- Chemistry