OBSERVATIONS ON PYROCHLORE OXIDE STRUCTURES

摘要

The structures of three defect pyrochlore oxides were refined using powder neutron or synchrotron X-ray diffraction data: Bi_1.89GaSbO_6.84(Fd3m; a = l0.3807(4) A); Pb_2(Ga_0.5Sb_1.5)O_6.5 and Pb_2(Ni_0.333Sb_1.667)O_6.5 (F43m; a = l0.4433(2) and l0.5052(2) A, respectively). Based on these and previously reported structures, some generalizations on the structure of defect pyro- chlores 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 activ- ity 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-va- cancy 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.