The double activation energy to ionic conduction in doped ceria: a journey through impedance and Raman spectroscopy

Ceria doped by trivalent rare earths (RE) ions is widely studied for its high ionic conductivity in the intermediate temperature range, namely between 773 and 973 K, which makes it a promising electrolyte for solid oxides cells (SOCs). In addition to the well known Gd- and Sm-doped systems, especially interesting are co-doped systems, since co-doping by two or more ions is considered as one of the most effective methods to improve ionic transport thanks to the expected lowering of activation energy to ionic conduction and, in the end, the enhancement of ionic conductivity. In many systems two different values of activation energy to ionic conduction occur within different temperature ranges, with the low/high temperature crossover being located at ?750 K. This evidence has been observed both in singly [1,2] and in co-doped systems [3,4], and it seems to be an intrinsic property of the material. Roughly in correspondence of the threshold temperature, even the Raman shift and the linewidth of the ceria most intense Raman signal present a slope change which can be hardly explained by applying the most common models. On the contrary, all these features find a comprehensive interpretation in the light of the defect chemistry of these oxides, which points at the presence of different defect aggregates having a well-known crystal structure and hindering ionic conductivity. The evaluation of their stability with increasing temperature based on their configurational entropy, suggests a key for the understanding of the observed transport and spectroscopic features. Results obtained on several singly (Gd, Sm-) and doubly (GdSm-, NdTm-, NdDy-) doped systems will be discussed, also in comparison with literature data.