Fast-Ion Conducting Y2(ZryTi1-y)2O7 Pyrochlores: Neutron Rietveld Analysis of Disorder Induced by Zr Substitution

Abstract

The structure of Y2(ZryTi1-y)2O7 solid solutions progressively changes, with increasing y, from an ordered pyrochlore structure A2B2O7□, space group Fd3m, to a deflect-fluorite structure (A, B)4 (O0.875□0.125)8, space group Fm3m, at y = 0.90. The anion array consists of three independent sites O(1), O(2), and O(3), occupying positions 48f, 8a and 8b, respectively, of which 8b is unoccupied in a fully ordered pyrochlore. Rietveld powder-profile analysis of data collected with 1.5453-Å thermal neutrons was used to determine the structural state of four samples with increasing Zr content (y = 0.30, 0.45, 0.60, and 0.90). Refinements that employed only the pyrochlore superstructure intensity data provided weighted profile residuals that ranged 8.06 to 8.67% compared with expected values of 7.13 to 7.87% derived from counting statistics. The onset of disorder at y = 0.30 is marked by filling of the vacant 8b site with oxygen ions displaced from the nearest-neighbor anion shell—i.e., O(1) in 48f. Only for y > 0.45 does O(2) participate in the disorder. Mixing of the occupancy of cation sites A and B begins only with under occupancy of the O(2) site. The eight-coordinated A site, position 16c, is occupied solely by Y for y ≤ 0.45 and is predominantly Y at y = 0.60. The substituted Zr4+ thus replaces Ti4+ in the six-coordinated B site for most of the solid-solution series, Complete mixing of all three cation species occurs abruptly over the narrow compositional range 0.60 < y ≤ 0.90. The disordered structural states cannot be described by a single-order parameter. The O(1) ion is displaced 0.46 Å toward the vacant O(3) site in the ordered pyrochlore at y = 0. The displacement relaxes to the ideal position of the fluorite structure with a quadratic dependence on y. Increase of the anisotropic temperature-factor coefficients with y indicates general softening of the structure with increased disorder. The array of O(1) ions forms a continuous path for migration of anions through the structure. The magnitude and composition dependence of the reported change of ionic conductivity with y may be satisfactorily explained by the variation of the product of charge-carrier concentration and vacancy concentration N(N - 1), where N is taken as the site occupancy of O(1).