Relationship between Oxide-Ion Conductivity and Oxygen Vacancy in Ln_2Ce_2O_7 (Ln = La, Nd, Sm, Eu and Gd) Having a Pyrochlore Composition

摘要

Crystal structures of the Ln_2Ce_2O_7 (Ln = La, Nd, Sm, Eu and Gd) with a pyrochlore-type composition were refined by the Rietveld analysis of X-ray diffraction (XRD) data. Only La_2Ce_2O_7 showed a single phase of a fluorite-type structure and the other compounds showed a rare earth C-type structure. The Rietveld analysis of neutron powder diffraction data for La_2Ce_2O_7 and Nd_2Ce_2O_7 supported the analysis of XRD data. The oxide-ion conductivity of Ln_2Ce_2O_7 increased with increasing occupancy ratio of oxygen 16c site in the C-type structure, suggesting the oxide-ion conductivity is strongly dependent on ordering degree of oxygen vacancy.