Cerium is the most abundant and relatively the cheapest of the lanthanide rare earths, and one of the rare-earth elements actually used in the largest industrial quantities. It has a rare gift shared by few other rare earths—it switches flexibly between the trivalent and tetravalent states—making it a natural redox all-rounder. In automotive three-way catalysts, cerium-based oxide is an indispensable oxygen-storage promoter: as the engine's air-fuel ratio fluctuates, cerium oxide temporarily stores or releases oxygen, keeping platinum, palladium, and rhodium catalysts working in their optimal window, and is a basic ingredient of the modern gasoline three-way catalytic coating. The glass industry is another major outlet: cerium oxide polishing powder is the dominant chemical-mechanical polishing material for optical lenses, LCD glass, and quartz wafers, with polishing efficiency and surface finish far superior to traditional iron oxide powders. At the same time, adding cerium dioxide to glass absorbs ultraviolet light and the green tint of iron, producing UV-blocking sunglass glass and decolorized glass. Flint (cerium-iron alloy) is the material that strikes a spark in lighters and signal pistols; its main constituent is a cerium-iron mixed rare earth, because cerium readily oxidizes and ignites under friction. In metallurgy, cerium is added as a rare-earth additive to steel and cast iron to desulfurize, deoxidize, and spheroidize graphite, producing cleaner steel and more ductile nodular cast iron.
Cerium dioxide (CeO2, also called ceric oxide) is cerium's most important industrial compound, a pale yellow or white powder. Owing to its Ce4+/Ce3+ valence change, it serves both as the oxygen-storage component of automotive exhaust catalysts and as polishing powder for optical glass and LCD substrates, and is also used in UV-shielding glass, ceramic glazes, and electrolyte doping in solid oxide fuel cells. Cerium sesquioxide (Ce2O3) is the lower oxide of cerium, a purple or bluish-purple powder used mainly in the laboratory and in ceramic and glass coloration as a reducing cerium source, and also in certain optical crystals and electronic ceramics. Cerium fluoride (CeF3) is a white crystal used in arc-carbon electrodes (to raise arc brightness and stability), as an electrolyte component in rare-earth electrolysis, and as an additive in certain optical crystals and metal-halide lamps; it is also used as a polishing and lubricant additive.
