Pyrochlore to Fluorite Transition: The Y₂(Ti_1–<i>x</i>Zr_<i>x</i>)₂O₇ (0.0 ≤ <i>x</i> ≤ 1.0) System

Abstract

The structural properties of the system Y₂(Ti_1–<i>x</i>Zr_<i>x</i>)₂O₇ 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, Y₂Ti₂O₇) 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, Zr₂Y₂O₇) 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 Zr⁴⁺ initially only substituting onto the Ti⁴⁺ 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 Y³⁺, Ti⁴⁺, and Zr⁴⁺ species over all the cation sites, coupled with a redistribution of the O^2– 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