Structural Features of Sm- and Gd-Doped Ceria Studied by Synchrotron X‑ray Diffraction and μ‑Raman Spectroscopy

A structural study of Sm- and Gd-doped ceria was performed with the aim to clarify some unexplained structural features. (Ce_1–<i>x</i>RE_<i>x</i>)­O_{2–<i>x</i>/2} samples (RE ≡ Sm, Gd; <i>x</i> = 0, 0.1, ..., 1) were prepared by coprecipitation of mixed oxalates and subsequent thermal treatment at 1473, 1173, or 1073 K in air; they were then analyzed at room temperature both by synchrotron X-ray diffraction and μ-Raman spectroscopy. Two structural models were adopted to fit the experimental data, namely, a fluoritic one, resembling the CeO₂ structure at low RE content, and a hybrid one at higher RE content, intermediate between the CeO₂ and the RE₂O₃ structures. Two main transitions were detected along the compositional range: (a) an RE-dependent transition at the boundary between the fluoritic and the hybrid regions, of a chemical nature; (b) an RE-independent transition within the hybrid region at ∼0.5, having a purely geometrical nature. The presence of two finely interlaced F- and C-based structures within the hybrid region was confirmed, and hints of their composition were obtained by μ-Raman spectroscopy. The obtained results indicate a possible explanation for the non-Vegard behavioral trend of the cell parameters