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

摘要

The oxide-ion conductivity of two solid-solution systems of a pyrochlore-type composition,(Y+_(1-x)Zr_x)_2O_7(YLa)and(Nd_(1-x)Yb_x)_2(Ce_(1-x)Zr_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 rareearth-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 lat tice constant was constant in the whole system,oxide-ion conductivity decreased with increasing ordering of oxy gen vacancies and when the ordering of oxygen vacancies was constant,the conductivity increased with increas ing unit cell free volume.