Structural Disorder in Doped Zirconias, Part I: The Zr_0.8Sc_0.2−xY_xO_1.9 (0.0 ≤ x ≤ 0.2) System

The influence of local ordering of the anion vacancies and cation−anion vacancy interactions on the ionic conductivity of the anion-deficient fluorite Zr 0.8 Sc 0.2− x Y x O 1.9 (0.0 ≤ x ≤ 0.2) system have been investigated using impedance spectroscopy, molecular dynamics (MD) simulations, and reverse Monte Carlo (RMC) analysis of neutron powder diffraction data. At 1000 K, the ionic conductivity decreases by a factor of ∼2 as x increases from 0.0 to 0.2, while the oxygen anion partial radial distribution function, g OO ( r ), remains similar across the entire solid solution, even though the cation−oxygen interactions change with increasing Y 2 O 3 content. These experimental data are used to validate the MD simulations, which probe the details of the vacancy−vacancy interactions within the x = 0.0 and x = 0.2 end members. Both possess similar vacancy−vacancy ordering that favors the formation of pairs along ⟨111⟩ directions. Significantly, an increased proportion of the oxygen vacancies are associated with the Zr 4+ cations in Zr 0.8 Y 0.2 O 1.9, while in Zr 0.8 Sc 0.2 O 1.9 they show no significant preference for being nearest neighbor to a Sc 3+ or a Zr 4+ cation. Thus, it is concluded that the lower ionic conductivity at x = 0.2 is predominantly a consequence of the larger size of the Y 3+ cation, which induces strain in the lattice and hinders diffusion of the O 2−, rather than changes in the local ordering of the anion vacancies.