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It was found that oxide-ion conductivity of Nd-doped CeO2 (Ce1-xNdxO2-x/2) increased with an increase in Nd content x up to x = 0.2, showing a maximum value at x = 0.2, and then decreased with x. In order to clarify the origin of the decrease in the conductivity of the heavily Nd-doped CeO2, Rietveld analyses of the X-ray diffraction and neutron diffraction data were carried out, assuming two phases with oxygen deficient fluorite-type and rare-earth C-type structures (0.45 ≤ x ≤ 0.7). The oxide-ion conductivity of the heavily Nd-doped CeO2 is discussed in relation to the formation of the rare-earth C-type structure, which derives from the oxygen ordering of the defect fluorite-type structure.

Crystal structures of the Ln 2 Zr 2 O 7 (Ln = Nd and La) were refined by the Rietveld analysis of powder neutron diffraction (ND) data at room temperature. La 2 Zr 2 O 7 had a pyrochlore (P)-type structure of almost completely ordered for oxygen vacancy. On the other hand, Nd 2 Zr 2 O 7 had the P-type structure for the oxygen vacancy distributed between the O2 (8b) and O3 (48f ) sites. The Rietveld refinement results of powder X-ray diffraction (XRD) data of Ln 2 Zr 2 O 7 (Ln = Eu, Nd and La) indicated that the Ln 3+ and Zr 4+ ions exist in ordered states in 16c and 16d sites, respectively. The refined lattice parameters (a), occupancies (g) for oxygen sites (O1, O2 and O3) and x-positional parameters for O3 (48f ) site of La 2 Zr 2 O 7 and Nd 2 Zr 2 O 7 by using powder XRD data were in almost agreement with those by using powder ND data. The refined ND and XRD data of Ln 2 Zr 2 O 7 (Ln = Eu, Nd and La) system revealed that the O3 (48f ) site occupancy (g 48f ) decreases with increasing O2 (8b) site occupancy, while O1 (8a) site occupancy keeping 1.0. The change in oxide-ion conductivity (·) in Ln 2 Zr 2 O 7 (Ln = Eu, Nd and La) system can be related to a change in the product of site occupancy (g 48f ) and the oxygen vacancy rate (1-g 48f ) in the O3 (48f ) site, g 48f (1-g 48f ).

The pyrochlore-type Ln2Zr2O7 (Ln = La, Nd, Eu) systems were refined by Rietveld analyses of XRD data combined with the MEM-based pattern fitting at RT, 873, 1073, and 1273 K. All the samples kept a single cubic phase of the pyrochlore-type structure in the temperature range up to 1273 K. The oxide-ion conductivities of the present systems were discussed on a basis of ordering of oxygen vacancy in the pyrochlore-type structure. Theoxide-ion conduction of Eu2Zr2O7 was ascribed to diffusion of oxygen along the 〈100〉 direction, while the hole conduction of La2Zr2O7 occurred along the 〈110〉 direction, according to the charge density map estimated by the MEM analysis.

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