Rare-earth Element

rare-earth element, any member of the group of chemical elements consisting of three elements in Group 3 (scandium [Sc], yttrium [Y], and lanthanum [La]) and the first extended row of elements below the main body of the periodic table (cerium [Ce] through lutetium [Lu]). The elements cerium through lutetium are called the lanthanides, but many scientists also, though incorrectly, call those elements rare earths.

The rare earths are generally trivalent elements, but a few have other valences. Cerium, praseodymium, and terbium can be tetravalent; samarium, europium and ytterbium, on the other hand, can be divalent. Many introductory science books view the rare earths as being so chemically similar to one another that collectively they can be considered as one element. To a certain degree that is correct—about 25 percent of their uses are based on this close similarity—but the other 75 percent of rare-earth usage is based on the unique properties of the individual elements. Furthermore, a close examination of these elements reveals vast differences in their behaviours and properties; e.g., the melting point of lanthanum, the prototype element of the lanthanide series (918 °C, or 1,684 °F), is much lower than the melting point of lutetium, the last element in the series (1,663 °C, or 3,025 °F). This difference is much larger than that found in many groups of the periodic table; e.g., the melting points of copper, silver, and gold vary by only about 100 °C (180 °F).

The name rare earths itself is a misnomer. At the time of their discovery in the 18th century, they were found to be a component of complex oxides, which were called “earths” at that time. Furthermore, these minerals seemed to be scarce, and thus these newly discovered elements were named “rare earths.” Actually, these elements are quite abundant and exist in many workable deposits throughout the world. The 16 naturally occurring rare earths fall into the 50th percentile of elemental abundances. By the early 21st century, China had become the world’s largest producer of rare-earth elements. Australia, Brazil, India, Kazakhstan, Malaysia, Russia, South Africa, and the United States also extract and refine significant quantities of these materials.

Many people do not realize the enormous impact the rare-earth elements have on their daily lives, but it is almost impossible to avoid a piece of modern technology that does not contain any. Even a product as simple as a lighter flint contains rare-earth elements. Their pervasiveness is exemplified by the modern automobile, one of the biggest consumers of rare-earth products. Dozens of electric motors in a typical automobile, as well as the speakers of its sound system, use neodymium-iron-boron permanent magnets. Electrical sensors employ yttria-stabilized zirconia to measure and control the oxygen content of the fuel. The three-way catalytic converter relies on cerium oxides to reduce nitrogen oxides to nitrogen gas and oxidize carbon monoxide to carbon dioxide and unburned hydrocarbons to carbon dioxide and water in the exhaust products. Phosphors in optical displays contain yttrium, europium, and terbium oxides. The windshield, mirrors, and lenses are polished using cerium oxides. Even the gasoline or diesel fuel that propels the vehicle was refined using rare-earth cracking catalysts containing lanthanum, cerium, or mixed-rare-earth oxides. Hybrid automobiles are powered by a nickel–lanthanum metal hydride rechargeable battery and an electrical traction motor, with permanent magnets containing rare-earth elements. In addition, modern media and communication devices—cell phones, televisions, and computers—all employ rare earths as magnets for speakers and hard drives and phosphors for optical displays. The amounts of rare earths used are quite small (0.1–5 percent by weight, except for permanent magnets, which contain about 25 percent neodymium), but they are critical, and any of those devices would not work as well, or would be significantly heavier, if it were not for the rare earths.