Cerium was discovered in 1803 by both Klaproth and by Berzelius and Hisinger. However, Hillebrand and Hisisnger produced the metal much later in 1875. It is the most abundant of the rare earth metals and is found in a number of minerals, which include allanite (also know as orthite), monazite, bastnasite, cerite and samarskite. Monazite and bastnasite are presently the two most important sources of cerium. Metallic cerium is prepared by metallothermic reduction techniques that produce high-purity cerium. Examples of such processes include the reduction of cerous fluoride with calcium, or the electrolysis of molten cerous chloride or other cerous halides. Cerium is an iron-grey lustrous metal, and is malleable and oxidises very readily at room temperature, especially in moist air. It is the most reactive of the rare earth metals except for europium. Both alkali and acid solutions attack the metals rapidly. The pure metal is likely to ignite if scratched with a knife-like instrument. Cerium has a variable electronic structure, which means only small amounts of energy are required to change the relative occupancy of the electronic levels. This gives rise to dual valency states. An example of this is when cerium is subjected to high pressure or low temperatures a volume change of approximately 10% results. Cerium exhibits complex low temperature behaviour. It is believed that there are four allotropic modifications: · At room temperature and atmospheric pressure is known as g cerium · Below -16°C g cerium changes to b cerium · Below -172°C b cerium begins to change to a cerium · The transformation is complete at -269°C.

Applications

Cerium is a component in misch-metal (German for mixed–metal), which is used in the manufacture of: · Pyrophoric alloys for cigarette lighters. · Making aluminium alloys and in some steels and irons. · In cast iron it opposes graphitisation and produces a malleable iron · In steels it removes sulphides and oxides and completely degasifies. · In stainless steel it is used as a precipitation-hardening agent. · In magnesium alloys for castings it is used from anywhere between 3 to 4% with 0.2 to 0.6% zirconium, both of these refine the grain and give sound casting of complex shapes. It also adds heat resistance to magnesium castings. Other uses for cerium and cerium containing compounds include: · As ceric sulphate it is used extensively as a volumetric oxidising agent in quantitative analysis. · Cerium compounds are used in the manufacture of glass, as a component and a decolouriser. · Cerium oxide is used as a glass-polishing agent. · Cerium in conjunction with other rare earth metals is used in carbon-arc lighting, which is implemented in the motion picture industry. · Cerium is used to store hydrogen as it reacts with it at room temperature to form its hydrides.