Structural properties of Sm-doped ceria electrolytes at the fuel cell operating temperatures

A high temperature structural study (673-1073 K) was performed by means of synchrotron x-ray diffraction and ri-Rarnan spectroscopy on several compositions belonging to the Ce1-xSmxO2-x/2 system with the aim to investigate the crystallographic features of Sm-doped ceria electrolytes at the fuel cell operating temperatures; ionic conductivity of samples with x ranging between 0.1 and 0.4 was measured too in order to correlate the main structural features with transport properties. A slight shift toward lower x values of the fluorite-based/hybrid region boundary is observed with increasing temperature; moreover, the coefficient of thermal expansion reveals a strong slope change close to x = 0.3, which represents the crossover composition between the two atomic arrangements. The presence of C-structured RE2O3 nanodomains within the fluorite structure starting from x similar to 0.2 is revealed by mu-Raman spectroscopy and confirmed by the behaviour of total conductivity. The ordering effect exerted at each temperature by the Sm-vacancies aggregates on the fluorite structure within the hybrid region influences the behaviour of both the intensity and the full width at half maximum of the Raman signal typical of the CeO2 structure. The obtained results are discussed in comparison to the ones deriving from the Gd-doped ceria system.