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Solute segregation at grain boundaries has been correlated with grain‐boundary conductivity in high‐purity 15‐mol%‐CaO‐stabilized ZrO2. STEM measurements of solute coverage show that the segregation of impurity silicon (present at bulk levels <80 ppm) is grain‐size dependent. The boundary coverage of silicon can be systematically varied by varying grain size at concentrations low enough that a discrete siliceous film does not form. The cosegregation of calcium and silicon is observed. The grain‐boundary solute coverage (Tsi+Ca) has been correlated with the specific grain‐boundary conductivity (σspgb) determined using impedance spectroscopy. At monolayer segregation levels, the specific boundary conductivity is less than the bulk conductivity by a factor >103 at 500°C. At the lowest levels of segregation achieved, <0.1 monolayer, σspgb remains ∼102 less, and possibly represents an “intrinsic” limiting value for the grain boundary. Comparison with Y2O3‐doped ZrO2 suggests similar behavior in this system. The control of grain‐boundary segregation through purity, microstructure, and thermal history is discussed from the objective of engineering the grain‐boundary impedance of polycrystalline ionic conductors.

It is shown that unique defect thermodynamics and transport properties result for oxides of a few nanometers crystallite size. Fully-dense CeO2−x polycrystals of ∼10 nm grain size were synthesized, and their electrical properties compared with those of samples coarsened from the same material. The nanocrystals showed reduced grain boundary resistance, 104 higher electronic conductivity, and less than one-half the heat of reduction of its coarse-grained counterpart. These properties are attributed to a dominant role of interfacial defect formation.