to the formation of small polarons. Since the oxygen atoms in a ceria crystal occur in planes, diffusion of these anions is facile. The diffusion rate increases as the defect concentration increases. The presence of oxygen vacancies at terminating ceria planes governs the energetics of ceria interactions with adsorbate molecules, and its wettability. Controlling such surface interactions is key to harnessing ceria in catalytic applications. Natural occurrence Cerium(IV) oxide occurs naturally as the mineral cerianite-(Ce). It is a rare example of tetravalent cerium mineral, the other examples being stetindite-(Ce) and dyrnaesite-(La). The "-(Ce)" suffix is known as Levinson modifier and is used to show which element dominates in a particular site in the structure. It is often found in names of minerals bearing rare earth elements (REEs). Occurrence of cerianite-(Ce) is related to some examples of cerium anomaly, where Ce - which is oxidized easily - is separated from other REEs that remain trivalent and thus fit to structures of other minerals than cerianite-(Ce). Applications Cerium has two main applications, which are listed below. The principal industrial application of ceria is for polishing, especially chemical-mechanical planarization (CMP). For this purpose, it has displaced many other oxides that were previously used, such as iron oxide and zirconia. For hobbyists, it is also known as "opticians' rouge". In its other main application, CeO2 is used to decolorize glass. It function by converting green-tinted ferrous impurities to nearly colorless ferric oxides. Other niche and emerging applications Catalysis CeO2 has attracted much attention in the area of heterogeneous catalysis. It catalyses the water gas shift reaction. It oxidizes carbon monoxide. Its reduced derivative Ce2O3 reduces water, with release of hydrogen. . The interconvertibility of CeOx materials is the basis of the use of ceria for an oxidation catalyst. One small but illustrative use is its use in the walls of self-cleaning ovens as a hydrocarbon oxidation catalyst during the high-temperature cleaning process. Another small scale but famous example is its role in oxidation of natural gas in gas mantles. Building on its distinct surface interactions, ceria finds further use as a sensor in catalytic converters in automotive applications, controlling the air-exhaust ratio to reduce NOx and carbon monoxide emissions. Energy & fuels Due to the significant ionic and electronic conduction of cerium oxide, it is well suited to be used as a mixed conductor. As such, cerium oxide is a material of interest for