Local structural changes in Ce1-xLnxO2-δ (Ln = La, Gd) solid electrolytes

The local structural changes in La and Gd in doped ceria were investigated using the XAFS　technique. In addition, Ce1-xLnxO2-δ (Ln = La, Gd; LDC) (0 ≤ x ≤ 0.5) was synthesized through a coprecipitation method and the local structures were analyzed based on the XAFS spectra for the first and second neighbor shells of the constituent cations. For these two types of materials, the variation in the coordination number of the Ln and Ce cations showed different trends as a function of the dopant concentration, x. For LDC, a local structural ordering was formed at x ≥ 0.3 similar to a pyrochlore-type structure, in which the oxygen vacancies are preferentially located adjacent to La3+. For Ce1-xGdxO2-δ (GDC), no distinct differences in the coordination number of the cations were observed.
The combined XAFS and XRD results successfully showed that the weighted average distances of the longer Ln-Ln (Ln = La for LDC and Ln = Gd for GDC) and shorter Ce-Ce at each composition, as obtained through XAFS analysis, fit extremely well with the lattice parameter obtained using XRD. A model of the local structural change was derived to explain the variation in the electrical conductivity during the early stage of Ln doping.

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