Anion Disorder in Lanthanoid Zirconates Gd_2–xTb_xZr₂O₇

The pyrochlore–defect fluorite order–disorder transition has been studied for a series of oxides of the type Gd_2–xTb_xZr₂O₇ by a combination of diffraction and spectroscopy techniques. Synchrotron X-ray diffraction data suggest an abrupt transition from the coexistence of pyrochlore and defect fluorite phases to a single defect fluorite phase with increasing Tb content. However neutron diffraction data, obtained at λ ≈ 0.497 Å for all Gd-containing samples to minimize absorption, not only provide evidence for independent ordering of the anion and cation sublattices but also suggest that the disorder transition across the pyrochlore–defect fluorite boundary of Ln₂Zr₂O₇ is rather gradual. Such disorder was also evident in X-ray absorption measurements at the Zr L₃-edge, which showed a gradual increase in the effective coordination number of the Zr from near 6-coordinate in the pyrochlore rich samples to near 7-coordinate in the Tb rich defect fluorites. These results indicate the presence of ordered domains throughout the defect fluorite region, and demonstrate the gradual nature of the order–disorder transition across the Gd_2–xTb_xZr₂O₇ series.