Stability and Structural Evolution of CeIV1–xLnIIIxO2–x/2 Solid Solutions: A Coupled μ-Raman/XRD Approach

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Abstract
Several CeO2-based mixed oxides with general composition Ce1–xLnxO2–x/2 (for 0 ≤ x ≤ 1 and Ln = La, Nd, Sm, Eu, Gd, Dy, Er, or Yb) were prepared using an initial oxalic precipitation leading to a homogeneous distribution of cations in the oxides. After characterization of the Ce/Nd oxalate precursors and then thermal conversion to oxides at T = 1000 °C, investigation of the crystalline structure of these oxides was carried out by XRD and μ-Raman spectroscopy. Typical fluorite Fm3̅m structure was obtained for relatively low LnIII contents, while a cubic Ia3̅ superstructure was evidenced above x ≈ 0.4. Moreover, since Nd2O3 does not crystallize with the Ia3̅-type structure, two-phase systems composed with additional hexagonal Nd2O3 were obtained for xNd ≥ 0.73 in the Ce1–xNdxO2–x/2 series. The effect of heat treatment temperature on these limits was explored through μ-Raman spectroscopy, which allowed determining the presence of small amounts of the different crystal structures observed. In addition, the variation of the Ce1–xLnxO2–x/2 unit cell parameter was found to follow a quadratic relation as a result of the combination between increasing cationic radius, modifications of cation coordination, and decreasing O–O repulsion caused by oxygen vacancies.

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