パイロクロア型組成系Ln_2Ce_2O_7(Ln=La, Nd, Sm, Eu, Gd, Y and Yb)の結晶相と電気伝導度

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The relationship between electrical conductivity and crystal structure in Ln^2Ce^2O_7 (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 (Ln^<3+>) /r (Ce^<4+>) were larger than 1.17. On the other hand, La_2Ce_2O_7 was the only F-type phase without the superstructure. The lattice constant increased linearly with increasing the ionic radius of Ln^<3+> 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℃ in air for the Ln_2Ce_2O_7 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.
社団法人日本セラミックス協会の論文
2003-12-01