Defects
In the most stable fluorite phase of ceria, it exhibits several defects depending on partial pressure of oxygen. The primary defects of concern are oxygen vacancies and small polarons (electrons localized on cerium cations) because these two are located in the "useful" range of ceria. In the case of oxygen defects, the increased diffusion rate of oxygen in the lattice causes increased catalytic activity as well as an increase in ionic conductivity, making ceria interesting as a fuel cell electrolyte in solid-oxide fuel cells.
Read more about this topic: Cerium(IV) Oxide
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—François, Duc De La Rochefoucauld (1613–1680)
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—Peter Cook (b. 1937)