Characterization of Ordered Phases in Ceria-Zirconia Solid Solution by Electron Energy-Loss Spectroscopy

Abstract

The ceria-zirconia solid solution (Ce₂Zr₂O_7+x, 0≤x≤1) has an excellent ability for oxygen absorption/release. Ce₂Zr₂O₇ with pyrochlore structure is fully oxidized to Ce₂Zr₂O₈, in which the absorbed oxygen occupies particular atomic sites, depending on the degree of oxidation. The local atomic arrangement of CeO₂-ZrO₂ solid solution has been studied in order to analyze the valence change of cations by means of electron energy-loss spectroscopy (EELS) in a transmission electron microscope. EEL spectra from Ce₂Zr₂O₇, Ce₂Zr₂O_7.5 and Ce₂Zr₂O₈ showed a systematic change in the relative intensity ratio of cerium-M_4,5 white-line peaks, which corresponds to the valence change of cerium. The valence number of cerium ions in the CeO₂-ZrO₂ solid solution changes from 3 to 4 according to its oxygen content; Ce³⁺ in Ce₂Zr₂O₇, Ce⁴⁺ in Ce₂Zr₂O₈. By contrast, the valence of the zirconium ion remains quaternary on either case. The energy-loss near edge structure (ELNES) of cerium-M_4,5 edge in Ce₂Zr₂O_7.5 is well reproduced by the equiweight sum of the ELNES profiles of Ce₂Zr₂O₇ and Ce₂Zr₂O₈, the phenomenon of which is known as the ‘valence fluctuation’, often observed in rare earth compounds.