Cerium(IV) oxide

Cerium(IV) oxide, ceric oxide, ceria, or sometimes simply cerium oxide or cerium dioxide, is a pale yellow-white powder, CeO2. It is used in ceramics, to polish glass, and to sensitize photosensitive glass. It is also used in lapidary as "jeweller's rouge"; it is also known as "optician's rouge". Ceria is used in the walls of self-cleaning ovens as a hydrocarbon catalyst during the high-temperature cleaning process. It has high absorption of ultraviolet radiation while it is transparent for visible light, so it is a prospective replacement of zirconia and titania in sunscreens, as it has lower photocatalytic activity. However its thermal catalytic properties have to be decreased by coating the particles by amorphous silica or boron nitride.

It is slightly hygroscopic and will also absorb a small amount of carbon dioxide from the atmosphere.

Note that cerium also forms cerium(III) oxide, Ce2O3.

As a fuel cell electrolyte
In the doped form, ceria has seen interest as a material for solid oxide fuel cells or SOFCs because of its relatively high oxygen ion conductivity (i.e. oxygen atoms readily move through it) at intermediate temperatures (500-800 °C). Undoped and doped ceria also exhibit high electronic conductivity at low partial pressures of oxygen due to the formation of small polarons. However, doped ceria has an extended electrolytic region (area of predominant ionic conductivity), over that of ceria, that allows its use as an electrolyte in SOFCs. Substituting a fraction of the ceria with gadolinium or samarium will introduce oxygen vacancies in the crystal without adding electronic charge carriers. This increases the ionic conductivity and results in a better electrolyte.

Under reducing conditions, those experienced on the anode side of the fuel cell, a large amount of oxygen vacancies within the ceria electrolyte can be formed. This results in the normally pale yellow ceria to turn black or grey as the result of color center formation. Some of the cerium(IV) oxide is also reduced to cerium(III) oxide under these conditions which consequently increases the electronic conductivity of the material. Finally, ceria undergoes what is described as a chemical expansion under reducing conditions as a result of reduction of the cerium cation from a 4+ to a 3+ state in order to charge compensate for oxygen vacancy formation.

As a catalyst
Ceria has been used in catalytic converters in automotive applications. Since ceria can become non-stoichioimetric in oxygen content (i.e. it can give up oxygen without decomposing) depending on its ambient partial pressure of oxygen, it can release or take in oxygen in the exhaust stream of a combustion engine. In association with other catalysts, ceria can effectively reduce NOx emissions as well as convert harmful carbon monoxide to the less harmful carbon dioxide.

Defects
In the fluorite structure, ceria 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.