Abstract
The stability of the cubic phase (c′) of Ce1−xZrxO2−y nanoparticles was studied by x-ray absorption near edge spectroscopy (XANES), time-resolved high temperature x-ray diffraction (XRD), and room temperature XRD. A clear relationship between the phase stability of the c′ phase and the Ce3+ concentration was found in zirconia-doped ceria, prepared in an oxidizing environment. The percentage of the 3+ oxidation state of cerium was measured from the relative Ce3+ peak intensity at the CeLIII edge in XANES. A concentration of 11% of the larger Ce3+ ions, among all cerium ions, helps to release the local stress induced by the smaller Zr4+ ions and stabilizes the c′ phase even under high zirconia concentrations of 40%–60%. A smaller particle size enhances this effect. XANES results at the ZrLIII edge supported the cubic phase stabilization. When the homogenization anneal was performed under a reducing environment instead of in air, the solubility limit of the cubic phase Ce1−xZrxO2−y was extended to above 90% zirconia, while the Ce3+ concentration increased, reaching 94% of all [Ce] in Ce0.1Zr0.9O2−y, which confirms the crucial role of Ce3+ ions in stabilizing the cubic phase as well as the importance of the gaseous environment during the homogenization anneal.
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