It was found that the structure of Y2(ZrxTi1-x)2O7 solid solutions progressively changed, with increasing x-value, from an ordered pyrochlore structure, A2B2O7, with a space group of Fd3m, to a defect fluorite structure (A,B)4(O0.8750.125)8, with a space group of Fm3m, at x = 0.90. The anion array consisted of 3 independent sites, O(1), O(2) and O(3), which occupied the positions, 48f, 8a and 8b, respectively. Of these, 8b was unoccupied in a fully-ordered pyrochlore. A Rietveld powder analysis of data which had been collected using 0.15453nm thermal neutrons was used to determine the structural state of 4 samples with an increasing Zr content (x = 0.30, 0.45, 0.60, 0.90). Refinements which used only pyrochlore superstructure intensity data provided weighted profile residuals that ranged from 8.06 to 8.67%, as compared with the expected values of 7.13 to 7.87% which were deduced from counting statistics. An onset of disorder at x = 0.30 was marked by the filling of the vacant 8b site by O ions which were displaced from the nearest-neighbor anion shell. That is, O(1) in 48f. Only when x was greater than 0.45 did O(2) participate in the disorder. Mixing of the occupancy of the A and B cation sites began only with under-occupancy of the O(2) site. The 8-coordinated A site (16c) was occupied only by Y when x was less than 0.45, and was predominantly occupied by Y when x was equal to 0.60. The substituted Zr4+ therefore replaced Ti4+ in the 6-coordinated B site over most of the solid-solution range. Complete mixing of all 3 cation species occurred for x-values of between 0.60 and 0.90. The disordered structural states could not be described by a single-order parameter. The O(1) ion was displaced by 0.046nm towards the vacant O(3) site in the ordered pyrochlore at x = 0. The displacement relaxed to the ideal position for the fluorite structure, with a quadratic dependence upon x. An increase in the anisotropic temperature-factor coefficients, as a function of x, reflected a general softening of the structure with increasing disorder. The array of O(1) ions constituted a continuous path for the migration of anions through the structure. The magnitude and composition dependence of the reported change in ionic conductivity with x-value could be explained by the variation in the product of charge-carrier concentration and vacancy concentration, N(N-1), where N was the site occupancy of O(1).

C.Heremans, B.J.Wuensch, J.K.Stalick, E.Prince: Journal of Solid State Chemistry, 1995, 117[1], 108-21