The ceria-zirconia solid solution (Ce
2Zr
2O
7+x, 0≤
x≤1) has an excellent ability for oxygen absorption/release. Ce
2Zr
2O
7 with pyrochlore structure is fully oxidized to Ce
2Zr
2O
8, in which the absorbed oxygen occupies particular atomic sites, depending on the degree of oxidation. The local atomic arrangement of CeO
2-ZrO
2 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 Ce
2Zr
2O
7, Ce
2Zr
2O
7.5 and Ce
2Zr
2O
8 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 CeO
2-ZrO
2 solid solution changes from 3 to 4 according to its oxygen content; Ce
3+ in Ce
2Zr
2O
7, Ce
4+ in Ce
2Zr
2O
8. 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 Ce
2Zr
2O
7.5 is well reproduced by the equiweight sum of the ELNES profiles of Ce
2Zr
2O
7 and Ce
2Zr
2O
8, the phenomenon of which is known as the ‘valence fluctuation’, often observed in rare earth compounds.
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