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. (Ce1–xREx)­O2–x/2 samples (RE ≡ Sm, Gd; x = 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.