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Abstract

CeO2 samples doped with 10, 1.0, and 0.1 mol% Y2O3 and undoped
CeO2 samples of high purity were studied by impedance
spectroscopy at temperatures <800degreesC and under various
oxygen partial pressures. According to microstructural
investigations by SEM and analytical STEM (equipped with EDXS),
the grain boundaries were free of any second phase, providing
direct grain-to-grain contacts. An amorphous siliceous phase
was detected at only a few triple junctions, if at all; as a
result, its contribution to the grain-boundary resistance was
negligible. Nevertheless, the specific grain-boundary
conductivities were still 2-7 orders of magnitude lower than
the bulk conductivities, depending on dopant concentration,
temperature, and oxygen partial pressure. The charge carrier
transport across the grain boundaries occurred only through the
grain-to-grain contacts, whose properties were then determined
by the space-charge layer. The space-charge potential in
acceptor-doped CeO2 was positive, causing the simultaneous
depletion of oxygen vacancies and accumulation of electrons in
the space-charge layer. The very low grain-boundary
conductivities can be accounted for by the oxygen-vacancy
depletion; the accumulation of electrons became evident in
weakly doped and undoped CeO2 at high temperatures and under
low oxygen partial pressures.