Oxygen Vacancy Ordering and the Conductivity Maximum in Y₂O₃-Doped CeO₂

Abstract

The defect structure and ionic diffusion processes within the anion-deficient, fluorite structured system Ce1–xYxO2–x/2 have been investigated at high temperatures (873 K–1073 K) as a function of dopant concentration, x, using a combination of neutron diffraction studies, impedance spectroscopy measurements, and molecular dynamics (MD) simulations using interionic potentials developed from ab initio calculations. Particular attention is paid to the short-range ion–ion correlations, with no strong evidence that the anion vacancies prefer, at high temperature, to reside in the vicinity of either cationic species. However, the vacancy–vacancy interactions play a more important role, with preferential ordering of vacancy pairs along the ⟨111⟩ directions, driven by their strong repulsion at closer distances, becoming dominant at high values of x. This effect explains the presence of a maximum in the ionic conductivity in the intermediate temperature range as a function of increasing x. The wider implications of these conclusions for understanding the structure–property relationships within anion-deficient fluorite structured oxides are briefly discussed, with reference to complementary studies of yttria and/or scandia doped zirconia published previously.