The Role of Dopant Ionic Radius in O^2- — Conducting Solid Electrolytes

Abstract

The oxides of the fluorite structure (ZrO₂, CeO₂, ThO₂, HfO₂) have proved to be attractive systems for the study of mass transport. The focus has been primarily on transport on the oxygen-ion sublattice, for the cations are known to migrate much more slowly and, in fact, the entire cation sublattice is generally frozen-in below ∿ 1000 °C.^l Migration on the oxygen-ion sublattice can be enhanced by the introduction of oxygen vacancies, V ^¨_o , through doping with lower valent cations. The study of such ionic transport is stimulated, in part, by the actual and potential applications of such oxides as oxygen sensors and oxygen-ion conductors in high-temperature fuel cells.² Stabilized zirconia has received the most attention, but that material requires ∿ 8 mole %; of dopant to stabilize the fluorite structure. Ceria and thoria, on the other hand, are more versatile for fundamental studies since they possess the fluorite structure in the pure state; thus it is possible to study the entire range, from very low to high dopant concentrations.