Abstract
It has been known that the ceria-zirconia solid solutions (Ce2Zr2O7+x, 0 ≤ x ≤ 1) have an excellent ability for oxygen absorption/release. The local charge states of cations in the CeO2-ZrO2 solid solutions have been studied by means of electron energy-loss spectroscopy (EELS) attached to a transmission electron microscope. The relative intensity of cerium-M4,5 white-line peaks from Ce2Zr2O7, Ce2Zr2O7.5 and Ce2Zr2O8 showed a systematic change, which corresponded to the valence change of cerium; Ce3+ in Ce2Zr2O7, Ce4+ in Ce2Zr2O8. By contrast, the valence of the zirconium ions, which are mainly at the nearest neighbor sites of absorbed or emitted oxygen atoms, remains to be quaternary irrespective of oxygen contents. It was found that the energy-loss near edge structure (ELNES) of the cerium-M4,5 edge in Ce2Zr2O7.5 was well reproduced by the sum of the ELNES profiles of Ce2Zr2O7 and Ce2Zr2O8 with an equal weight, the phenomenon of which is known as the ‘valence fluctuation,’ or ‘mixed valence state,’ often observed in rare earth metal compounds.
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