cerium

cerium (Ce), chemical element, the most abundant of the rare-earth metals.

Commercial-grade cerium is iron-gray in colour, silvery when in a pure form, and about as soft and ductile as tin. It oxidizes in air at room temperature to form CeO₂. The metal slowly reacts with water, and it quickly dissolves in diluted acids, except hydrofluoric acid (HF) that leads to the formation of the protective fluoride (CeF₃) layer on the surface of the metal. Cerium turnings (from when the metal is filed, ground, or machined) easily self-ignite in air, burning white-hot. Its pyrophoric nature accounts for one of its important metallurgical applications in lighter flints. The metal should be stored either in vacuum or in an inert atmosphere. The metal is a moderately strong paramagnet both below and above room temperature and becomes antiferromagnetic below 13 K (−260 °C, or −436 °F). It becomes superconducting in the millikelvin range at pressures exceeding 20 kbar.

Cerium as the oxide (ceria) was discovered in 1803 by Swedish chemists Jöns Jacob Berzelius and Wilhelm Hisinger, working together, and independently by German chemist Martin Klaproth. It was named after the asteroid Ceres, which was discovered in 1801. Cerium occurs in bastnasite, monazite, and many other minerals. It also is found among the fission products of uranium, plutonium, and thorium. Cerium is about as abundant as copper and nearly three times as abundant as lead in the igneous rocks of Earth’s crust.

Four isotopes occur in nature: stable cerium-140 (88.45 percent) and radioactive cerium-142 (11.11 percent), cerium-138 (0.25 percent), and cerium-136 (0.19 percent). Excluding nuclear isomers, a total of 38 radioactive isotopes of cerium have been characterized. They range in mass from 119 to 157 with half-lives as short as 1.02 seconds for cerium-151 and as long as 5 × 10¹⁶ years for cerium-142.

The metal is prepared by electrolysis of the anhydrous fused halides or by metallothermic reduction of the halides with alkali or alkaline-earth metals. It exists in four allotropic (structural) forms. The α-phase is face-centred cubic with a = 4.85 Å at 77 K (−196 °C, or −321 °F). The β-phase forms just below room-temperature and is double close-packed hexagonal with a = 3.6810 Å and c = 11.857 Å. The γ-phase is the room temperature form and is face-centred cubic with a = 5.1610 Å at 24 °C (75 °F). The δ-phase is body-centred cubic with a = 4.12 Å at 757 °C (1,395 °F).

Cerium compounds have a number of practical applications. The dioxide is employed in the optics industry for fine polishing of glass, as a decolourizer in glass manufacturing, in petroleum cracking catalysts, and as a three-way automotive emission catalyst that makes use of its dual valence (3+/4+) characteristics. Together with the other rare-earth metals, cerium is a constituent of numerous ferrous alloys to scavenge sulfur and oxygen and to nodulize cast iron. It is also used in nonferrous alloys, most commonly to improve high-temperature oxidation resistance of superalloys. Misch metal (typically 50 percent cerium, 25 percent lanthanum, 18 percent neodymium, 5 percent praseodymium, and 2 percent other rare earths) is primarily used for lighter flints and alloying additions.

Along with praseodymium and terbium, cerium is different from the other rare earths in that it forms compounds in which its oxidation state is +4; it is the only rare earth that exhibits a +4 oxidation state in solution. Salts of the Ce⁴⁺ ion (ceric salts), which are powerful but stable oxidizing agents, are used in analytical chemistry to determine oxidizable substances such as ferrous iron (iron in the +2 oxidation state). Cerium in its +3 oxidation state behaves as a typical rare earth.