Ionic conductivity of nanocrystalline heavily acceptor doped ceria: The role of the local atomic order

Acceptor doped ceria (CeO2) is a material that over the years has attracted much interest for applications in solid oxide fuel cells and oxygen membranes. This study aims at systematically investigating how the local atomic order and the dopant distribution affect the ionic transport of nanocrystalline CeO2 ceramics doped with samarium, erbium and ytterbium, with concentrations ranging between 10 and 30 at.%. For this purpose, a.c. electrochemical impedance spectroscopy together with Raman and fluorescence spectroscopy as well as XRD and EXAFS analysis have been carried out. In order to have detailed information on the bulk properties of these compositions also microcrystalline samples have been considered. The main findings can be summarized as follows. (i) The nanocrystalline samples exhibit systematically lower bulk conductivities than the microcrystalline counterparts and this can be correlated with a different distribution of the dopants and possible dopant-oxygen vacancies associations. (ii) The grain boundaries of the nanocrystalline samples are proportionally less blocking than those of the microcrystalline ceramics. (iii) Finally, EXAFS data suggest that with increasing dopant content cerium (iv) cations tend to be slightly reduced.