Crystal Phase and Electrical Conductivity in the Pyrochlore-Type Composition Systems, Ln2Ce2O7 (Ln=La, Nd, Sm, Eu, Gd, Y and Yb)

Abstract

The relationship between electrical conductivity and crystal structure in Ln2Ce2O7 (Ln=La, Nd, Sm, Eu, Gd, Y and Yb) systems was investigated. The crystal phases were assigned to a fluorite (F)-type phase accompanying the rare earth C-type superstructure, when the ionic radius ratio, r(Ln3+)/r(Ce4+) were larger than 1.17. On the other hand, La2Ce2O7 was the only F-type phase without the superstructure. The lattice constant increased linearly with increasing the ionic radius of Ln3+ independent of whether or not the superstructure was present. Oxygen partial pressure dependence of the electrical conductivity suggested that the charge carrier was oxide-ion, except for the Nd system. The electrical conductivity at 800°C in air for the Ln2Ce2O7 systems also increased with increasing the ionic radius ratio. The activation energy for the conduction decreased remarkably in the fluorite-type phase region. The compositional change of the oxide-ion conductivity was understood by the introduction of an unit cell free volume. This result was also supported by Raman spectra.