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Anion Disorder in Lanthanoid Zirconates Gd<sub>2–<i>x</i></sub>Tb<sub><i>x</i></sub>Zr<sub>2</sub>O<sub>7</sub>
The pyrochlore–defect fluorite
order–disorder transition has been studied for a series of
oxides of the type Gd<sub>2–<i>x</i></sub>Tb<sub><i>x</i></sub>Zr<sub>2</sub>O<sub>7</sub> 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
Ln<sub>2</sub>Zr<sub>2</sub>O<sub>7</sub> is rather gradual. Such
disorder was also evident in X-ray absorption measurements at the
Zr L<sub>3</sub>-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<sub>2–<i>x</i></sub>Tb<sub><i>x</i></sub>Zr<sub>2</sub>O<sub>7</sub> series