Pyrochlore to Fluorite
Transition: The Y<sub>2</sub>(Ti<sub>1–<i>x</i></sub>Zr<sub><i>x</i></sub>)<sub>2</sub>O<sub>7</sub> (0.0 ≤ <i>x</i> ≤ 1.0) System
- Publication date
- Publisher
Abstract
The structural properties of the system Y<sub>2</sub>(Ti<sub>1–<i>x</i></sub>Zr<sub><i>x</i></sub>)<sub>2</sub>O<sub>7</sub> have been investigated using the
neutron powder diffraction
technique, including a detailed analysis of the “total scattering”
using reverse Monte Carlo modeling to probe the short-range ion–ion
correlations over sample range 0.0 ≤ <i>x</i> ≤
1.0. The average crystal structure shows a continuous transformation
from the long-range ordered pyrochlore structure (<i>Fd</i>3̅<i>m</i>, <i>a</i> = 10.0967(1) Å, <i>Z</i> = 8, for <i>x</i> = 0.00, Y<sub>2</sub>Ti<sub>2</sub>O<sub>7</sub>) to a disordered fluorite structure (<i>Fm</i>3̅<i>m</i>, <i>a</i> = 5.2042(1)
Å, <i>Z</i> = 1, for <i>x</i> = 1.00, Zr<sub>2</sub>Y<sub>2</sub>O<sub>7</sub>) in agreement with previous reports.
However, on increasing <i>x</i> the disordering of both
the cation and the anion sublattices occurs in stages, with the Zr<sup>4+</sup> initially only substituting onto the Ti<sup>4+</sup> site
and adopting a cubic, rather than octahedral, local anion environment.
At concentrations in excess of <i>x</i> ≈ 0.4 there
is a gradual disordering of the Y<sup>3+</sup>, Ti<sup>4+</sup>, and
Zr<sup>4+</sup> species over all the cation sites, coupled with a
redistribution of the O<sup>2–</sup> which initially only involves
those anions on the O1 sites. The relationship between the composition
dependences of the structure properties and the ionic conductivity
is discussed