Observations on pyrochlore oxide structures

The structures of three defect pyrochlore oxides were refined using powder neutron or synchrotron X-ray diffraction data: Bi1.89GaSbO6.84 (Fd (3) over bar m; a = 10.3807(4) Angstrom); Pb-2(Ga0.5Sb1.5)O-6.5 and Pb-2(Ni0.333Sb1 .667)O-6.5 (F (4) over bar 3m; a = 10.4433(2) and 10.5052(2) Angstrom, resp ectively). Based on these and previously reported structures, some generali zations on the structure of defect pyrochlores are made. It was found that the contraction of the lattice in metallic Bi2B2O7-y-type oxides, relative to that of the isostructural insulating oxides, is not a result of B-B bond ing. Rather, there is a contraction of the BiO8 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(2)B(2)O(6.5) compoun ds, where A is an easily polarizable cation, oxygen-vacancy ordering accomp anied 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. (C) 1999 Elsevier Science Ltd.