Evolution of the local structure at the phase transition in CeO2-Gd2O3solid solutions

Abstract

We investigated the local environment of the Ce and Gd ions in one of the most important oxygen ion conductors, the CeO${}_{2}$-Gd${}_{2}$O${}_{3}$ solid solution, using ${L}_{\mathrm{III}}$ edge extended x-ray absorption fine structure (EXAFS) spectroscopy and x-ray absorption near edge spectroscopy (XANES). The average Gd-O distance, as deduced by EXAFS, decreases gradually with increasing Gd content, whereas a sharp decrease in the average Ce-O distance occurs at 25 mol$%$ Gd, accompanying the appearance of structural features characteristic of double fluorite symmetry. An abrupt change in the local environment of Ce, rather than of Gd, is also supported by the XANES spectra. Efforts at stabilizing the oxygen conducting $Fm$-3$m$ phase in CeO${}_{2}$-Gd${}_{2}$O${}_{3}$ solid solutions should therefore concentrate on tailoring the local environment of the Ce ion. We further suggest that our data may provide an explanation for the recently discovered giant electrostriction effect in Ce${}_{0}$.${}_{8}$Gd${}_{0}$.${}_{2}$O${}_{1.9}$. Since this composition is at the limit of stability of the fluorite phase, local distortions in Ce${}_{0}$.${}_{8}$Gd${}_{0}$.${}_{2}$O${}_{1.9}$ may be modulated by an external electric field more readily than those of the other solid solutions.