Cerium dioxide ($\mathrm{Ce}{\mathrm{O}}_2$)

Cerium dioxide is $\mathrm{Ce}{\mathrm{O}}_2$, often called ceria, with a fluorite-related crystal structure. It can accommodate oxygen vacancies together with changes between cerium(IV) and cerium(III).

That coupled defect and redox chemistry supports oxygen storage and release in catalytic materials. A perfectly fixed integer formula is therefore an idealization when discussing oxygen-deficient ceria under changing conditions.