POINT-DEFECT ANALYSIS AND MICROSTRUCTURAL EFFECTS IN PURE AND DONOR-DOPED CERIA

Ceria(CeO2-x) can be made electronically conductive by doping with pentavalent or hexavalent cations. A technique has been developed, within the framework of standard Kroger-Vink defect analysis, by which the concentrations of three coexisting major defect species (pentavalent impurity, free electrons, and one of several possible point defects) can be modeled in ceria or other MO2-type oxides. This analysis provides diagnostic information regarding the dominant defect behavior that cannot be obtained from the standard Brouwer approximation. Comparisons of microstructures and electrical conductivity were made between Nb-doped ceria synthesized by a conventional mixed-oxide route and by hydroxide coprecipitation. The mixed oxide materials contained silicate intergranular phases that acted as sinks for the dopant. Coprecipitated materials were virtually free of intergranular contamination and exhibited electrical conductivities approximately 25% higher than the mixed oxide materials. Finally, experimental results on the electrical conductivity of Nb-, Ta-, and W-doped ceria indicate that all are electronic conductors, with the W-doped materials showing the highest overall conductivity in air in the temperature range 600-900-degrees-C.