The present paper reviews the atomic-scale characterization of ion diffusion in ceria-based catalysts and fluorite-type ionic conductors and discusses the correlation between their bulk ion diffusivity and structural properties. Bulk oxide-ion diffusion is an important step of oxygen storage and release in ceria-based catalysts, and the oxide-ion diffusivity is essential for high OSC and efficiency of the SOFCs and the solar-energy-to-fuel conversion. The c/ a F ratio, and the oxygen displacement are smaller in the nanocrystalline Ce 0.5Zr 0.5O 2 than in bulk Ce 0.5Zr 0.5O 2. The tetragonal symmetry of compositionally homogeneous nano-sized Ce 0.5Zr 0.5O 2 in air is retained up to 1176 K. The c–t″ phase boundary at room temperature is located at around x = 0.85–0.9 in both bulk and nanocrystalline Ce xZr 1– xO 2. The c–t″ transition is continuous and might be of higher order. The c–t″ phase transition of Ce xZr 1– xO 2 is induced by the oxygen displacement from the regular fluorite position 1/4,1/4,1/4 along the c-axis. The composition (CeO 2 content x)-induced t′–t″ transition in Ce xZr 1– xO 2 is discrete and of first order. Formation of the t′- and t″-Ce xZr 1– xO 2 is depicted in the “metastable” phase diagram consisting of allotropic phase boundaries in the CeO 2–ZrO 2 system and is explained using the schematic Gibbs energy–composition ( G– x) diagram. The axial ratio, c/ a F where the subscript F represents the pseudo-fluorite lattice, of the t′-form is larger than unity, while the c/ a F ratio of the t″-form equals unity. Here, the t′- and t″-forms are unstable compared to the two-phase mixture of stable ZrO 2-rich tetragonal and CeO 2-rich cubic (or t″) phases, but are stable in the partitionless, compositionally homogeneous phases. “Metastable,” compositionally ( x) homogeneous t′- and t″-ceria–zirconia Ce xZr 1– xO 2 solid solutions (0.2 < x < 0.9) are key materials to obtain a high OSC, leading to high catalytic activity. The present paper is a critical review on the atomic-scale characterization of oxide-ion diffusion pathway, the existing phases, the phase transformations, “metastable” and stable phase diagrams, and oxygen storage capacity (OSC) of ceria-based materials. Ceria-based materials are utilized as automotive exhaust catalysts for the removal of noxious compounds, as catalysts for reforming ethanol and methane to produce hydrogen in fuel cells, as materials for solar-energy-to-fuel conversion, and as cathode, anode, and electrolyte materials in solid oxide fuel cells (SOFCs).
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