Abstract
The structures of three defect pyrochlore oxides were refined using powder neutron or synchrotron X-ray diffraction data: Bi 1.89GaSbO 6.84 ( Fd3̄ m; a = 10.3807(4) Å); Pb 2(Ga 0.5Sb 1.5)O 6.5 and Pb 2(Ni 0.333Sb 1.667)O 6.5 ( F4̄3 m; a = 10.4433(2) and 10.5052(2) Å, respectively). Based on these and previously reported structures, some generalizations on the structure of defect pyrochlores are made. It was found that the contraction of the lattice in metallic Bi 2B 2O 7−y-type oxides, relative to that of the isostructural insulating oxides, is not a result of B–B bonding. Rather, there is a contraction of the BiO 8 polyhedron in the metallic oxides, due to the decreased stereochemical activity of Bi 6s electrons, as a consequence of their mixing with B d orbitals. In A 2B 2O 6.5 compounds, where A is an easily polarizable cation, oxygen-vacancy ordering accompanied by A-cation displacement can occur. The extent of the displacement of the A-type cation is shown to be strongly dependent on the cell parameter.
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