The oxide-ion conductivity of two solid-solution systems of a pyrochlore-type composition, (Y
1-xLa
x)
2(Ce
1-xZr
x)
2O
7 (YLa) and (Nd
1-xYb
x)
2(Ce
1-xZr
x)
2O
7 (NdYb), was investigated. The crystal phase of the YLa system with a variable cation radius ratio,
r(A
3+)/
r(B
4+), 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 Nd
2Ce
2O
7, 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.
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