Effect of Cation Radius Ratio and Unit Cell Free Volume on Oxide-Ion Conductivity in Oxide Systems with Pyrochlore-Type Composition

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  • NISHINO Hanako
    Department of Applied Chemistry, Faculty of Engineering, Kanagawa University
  • YAMAMURA Hiroshi
    Department of Applied Chemistry, Faculty of Engineering, Kanagawa University
  • ARAI Takahiko
    Department of Applied Chemistry, Faculty of Engineering, Kanagawa University
  • KAKINUMA Katsuyoshi
    Department of Applied Chemistry, Faculty of Engineering, Kanagawa University
  • NOMURA Katsuhiro
    National Institute of Advanced Industrial Science and Technology-Kansai

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  • パイロクロア型組成酸化物系の酸化物イオン伝導度に及ぼす陽イオン半径比と単位格子自由体積の効果

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The oxide-ion conductivity of two solid-solution systems of a pyrochlore-type composition, (Y1-xLax)2(Ce1-xZrx)2O7 (YLa) and (Nd1-xYbx)2(Ce1-xZrx)2O7 (NdYb), was investigated. The crystal phase of the YLa system with a variable cation radius ratio, r(A3+)/r(B4+), but a constant lattice parameter, changed from the rare-earth-C (C)-type phase to the pyrochlore (P)-type phase through the fluorite (F)-type phase with an increase in composition (x), depending on the cation radius ratio (i.e., a measure of ordering degree of oxygen vacancies). On the other hand, the NdYb system, with a variable lattice constant, but a constant cation radius ratio, showed the F-type phase in the entire composition range, except for Nd2Ce2O7, which was assigned to the C-type phase. The local ionic arrangement speculated from the analysis of the Raman spectra of the two systems agreed with the results of X-ray diffraction (XRD) analysis. It was found by conductivity measurements that, when the lattice constant was constant in the whole system, oxide-ion conductivity decreased with increasing ordering of oxygen vacancies and when the ordering of oxygen vacancies was constant, the conductivity increased with increasing unit cell free volume.<br>

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