Structural features of Sm- and Gd-doped ceria studied by synchrotron X-ray diffraction and -raman spectroscopy

A structural study of Sm- and Gd-doped ceria was performed with the aim to clarify some unexplained structural features. (Ce<inf>1-x</inf>RE<inf>x</inf>)O<inf>2-x/2</inf> 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 CeO<inf>2</inf> structure at low RE content, and a hybrid one at higher RE content, intermediate between the CeO<inf>2</inf> and the RE<inf>2</inf>O<inf>3</inf> 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.