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

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.