Pyrochlore to Fluorite Transition: The Y₂(Ti_1–xZr_x)₂O₇ (0.0 ≤ x ≤ 1.0) System

The structural properties of the system Y 2 (Ti 1– x Zr x ) 2 O 7 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 ( Fd 3̅ m, a = 10.0967(1) Å, Z = 8, for x = 0.00, Y 2 Ti 2 O 7 ) to a disordered fluorite structure ( Fm 3̅ m, a = 5.2042(1) Å, Z = 1, for x = 1.00, Zr 2 Y 2 O 7 ) in agreement with previous reports. However, on increasing x the disordering of both the cation and the anion sublattices occurs in stages, with the Zr 4+ initially only substituting onto the Ti 4+ 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 Y 3+, Ti 4+, and Zr 4+ 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.