Abstract
The ceria-zirconia solid solution (Ce2Zr2O7+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, Ce2Zr2O7.5 and Ce2Zr2O8 showed a systematic change in the relative intensity ratio of cerium-M4,5 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-M4,5 edge in Ce2Zr2O7.5 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.
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