Novel valence state of cerium in Ce2Zr2O7.5 elucidated by electron energy-loss spectroscopy under electron channeling conditions

Rare earth compounds are known to often exhibit peculiar valence states. In the present study, we show, by electron energy loss spectroscopy (EELS) measurements under electron channeling conditions, the existence of a novel valence state for the cerium atoms in Ce2Zr2O7.5, where the valence state is split into two states, with the ions exhibiting a trivalent character (Ce3+) when facing oxygen planes with oxygen vacancies, and a tetravalent character (Ce4+) when facing oxygen planes with no oxygen vacancies. This result should prompt reconsideration of the conventional concept of the valence fluctuation in rare earth compounds.