A μ-Raman study of complex doped ceria systems at the liquid nitrogen temperature

RE-doped ceria systems (RE= trivalent rare earth) are able to conduct O2- ions between 673 and 973 K, thus being possible electrolytes for Solid Oxide Fuel and Electrolysis Cells working in the intermediate temperature range (IT-SOFCs and IT-SOECs, respectively). In Ce1-xRExO2-x/2 systems, the ionic conductivity is given by the oxide structure, which is strongly influenced by the nature and the amount of the RE employed [1]. In general, the highest ionic conductivity is observed for x≤0.20, in a compositional region where the systems show the fluorite-type structure of pure ceria, a fraction of Ce4+ ions is randomly replaced by RE, and oxygen vacancies formed to maintain the charge neutrality are free to move through the lattice. However, defect aggregates in the structure can hinder the vacancies movement, reducing the ionic conductivity of the oxide: an in-depth structural characterization is therefore essential to evaluate the behavior of these systems in fuel cells. Since ceria doped using different RE in combination generally shows improved ionic conductivity properties, our research group synthesized and characterized [2, 3, 4] the Ce1-x(Nd0.74Tm0.26)xO2-x/2 and the Ce1-x(Nd0.63Dy0.37)xO2-x/2 systems, in which the RE average ionic radii reproduce that of Sm3+, the singly doped system that shows the highest ionic conductivity. Since μ-Raman spectroscopy plays a key role in the detection of defects in doped ceria structure, due to its higher sensitivity as a local technique, the spectra of the samples with 0.1≤x≤0.6 belonging to the cited systems were collected at the liquid nitrogen temperature (80 K), to further increase their resolution. This allowed to detect additional defect-related Raman modes that are not clearly observable at ambient temperature, being partially covered by the main signals: all the results from low temperature Raman measurements will be discussed.