rare-earth metals

rare-earth metals in chemistry, group of metals including those of the lanthanide series and actinide series , usually yttrium , sometimes scandium and thorium , and rarely zirconium . Promethium, which is not found in nature, is not usually considered a rare-earth metal. The metals usually occur together in minerals as their oxides ( rare earths ) and are somewhat difficult to separate because of their chemical similarity. A subgroup of the rare-earth metals, consisting of those with atomic numbers between 57 and 63 and ytterbium, is often called the cerium metals. Misch metal is an alloy of the cerium metals often used in lighter flints, in alloys with other metals (especially magnesium ), and to remove residual gases in the manufacture of vacuum tubes. Individual metals may be isolated as their compounds by ion exchange methods, solvent extraction, or fractional crystallization, and chemically or electrolytically reduced to the pure metal. Uses are discussed in articles on individual elements.

Bibliography: See F. H. Spedding and A. H. Daane, ed., The Rare Earths (1961, repr. 1971); E. C. Subbarao and W. E. Wallace, ed., Science and Technology of Rare Earth Metals (1980).

rare earth (also rare earth el·e·ment or rare earth met·al) • n. Chem. any of a group of chemically similar metallic elements comprising the lanthanide series and (usually) scandium and yttrium. They are not esp. rare, but they tend to occur together in nature and are difficult to separate from one another.

lanthanide series (lanthanide elements, rare-earth metals) Series of 15 rare metallic elements with atomic numbers from 57 to 71. They are, in order of increasing atomic numbers: lanthanum (sometimes not considered a member), cerium, praseodymium, neodymium, promethium (the only radioactive member), samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. Their properties are similar. The shiny metals occur in monazite and other rare minerals and are placed in group III of the periodic table. Each element is analogous to the corresponding radioactive actinide series.