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(Ti1–xZrx)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 ≤ x ≤ 1.0. The average crystal structure shows a continuous transformation from the long-range ordered pyrochlore structure (Fd3̅m, a = 10.0967(1) Å, Z = 8, for x = 0.00, Y2Ti2O7) to a disordered fluorite structure (Fm3̅m, a = 5.2042(1) Å, Z = 1, for x = 1.00, Zr2Y2O7) in agreement with previous reports. However, on increasing x 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 x ≈ 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 O2– 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.