Grain–bulk versus grain–boundary sensitivities to redox reaction in yttria-doped ceria ceramics viewed from impedance spectroscopy

Yttria-doped ceria ceramics were prepared and reduced in an oxygen-deficient (argon) ambient. Electrical characterization through impedance spectroscopy revealed ionic-type conduction processes in as-sintered samples, with grain–bulk and grain–boundary activation energies (H) of about 1.00 eV and 1.05 eV, respectively. Electrical results from the reduced materials showed a predominant electronic-type, relatively high conductivity for the grains (H=0.52 eV), in contrast to a still ionic-like, relatively poor conductivity for the grain boundaries (H=0.95 eV). With the support of the results processed after re-oxidizing the materials in air combined with information from literature, this apparently contradictory behavior is discussed in terms of electron trapping at (Ce3+: $V_{\text{O}^..}$ )-type defect complexes. The overall work strongly supports the idea that surfaces (e.g., grain boundaries) in polycrystalline ceria are indeed much more sensitive to redox interactions than lattice.