Rare+earth+element

**Rare Earth Elements—Critical Resources for High Technology**
The rare earth elements (REE) form the largest chemically coherent group in the periodic table. Though generally unfamiliar, the REE are essential for many hundreds of applications. The versatility and specificity of the REE has given them a level of technological, environmental, and economic importance considerably greater than might be expected from their relative obscurity. The United States once was largely self-sufficient in these critical materials, but over the past decade has become dependent upon imports (fig. 1). In 1999 and 2000, more than 90% of REE required by U.S. industry came from deposits in China. Although the 15 naturally occurring REE (table 1; fig. 2) are generally similar in their geochemical properties, their individual abundances in the Earth are by no means equal. In the continental crust and its REE ore deposits, concentrations of the most and least abundant REE typically differ by two to five orders of magnitude (fig. 3). As technological applications of REE have multiplied over the past several decades, demand for several of the less abundant (and formerly quite obscure) REE has increased dramatically. The diverse nuclear, metallurgical, chemical, catalytic, electrical, magnetic, and optical properties of the REE have led to an ever increasing variety of applications. These uses range from mundane (lighter flints, glass polishing) to high-tech (phosphors, lasers, magnets, batteries, magnetic refrigeration) to futuristic (high-temperature superconductivity, safe storage and transport of hydrogen for a post-hydrocarbon economy). **Some Applications of the Rare Earth Elements** Many applications of REE are characterized by high specificity and high unit value. For example, color cathode-ray tubes and liquid-crystal displays used in computer monitors and televisions employ europium as the red phosphor; no substitute is known. Owing to relatively low abundance and high demand, Eu is quite valuable—$250 to $1,700/kg (for Eu2O3) over the past decade. Fiber-optic telecommunication cables provide much greater bandwidth than the copper wires and cables they have largely replaced. Fiber-optic cables can transmit signals over long distances because they incorporate periodically spaced lengths of erbium-doped fiber that function as laser amplifiers. Er is used in these laser repeaters, despite its high cost (~$700/kg), because it alone possesses the required optical properties. Specificity is not limited to the more exotic REE, such as Eu or Er. Cerium, the most abundant and least expensive REE, has dozens of applications, some highly specific. For example, Ce oxide is uniquely suited as a polishing agent for glass. The polishing action of CeO2 depends on both its physical and chemical properties, including the two accessible oxidation states of cerium, Ce,3+ and Ce4+, in aqueous solution. Virtually all polished glass products, from ordinary mirrors and eyeglasses to precision lenses, are finished with CeO2. Permanent magnet technology has been revolutionized by alloys containing Nd, Sm, Gd, Dy, or Pr. Small, lightweight, high-strength REE magnets have allowed miniaturization of numerous electrical and electronic components used in appliances, audio and video equipment, computers, automobiles, communications systems, and military gear. Many recent technological innovations already taken for granted (for example, miniaturized multi-gigabyte portable disk drives and DVD drives) would not be possible without REE magnets. Environmental applications of REE have increased markedly over the past three decades. This trend will undoubtedly continue, given growing concerns about global warming and energy efficiency. Several REE are essential constituents of both petroleum fluid cracking catalysts and automotive pollution-control catalytic converters. Use of REE magnets reduces the weight of automobiles. Widespread adoption of new energy-efficient fluorescent lamps (using Y, La, Ce, Eu, Gd, and Tb) for institutional lighting could potentially achieve reductions in U.S. carbon dioxide emissions equivalent to removing one-third of the automobiles currently on the road. Large-scale application of magnetic-refrigeration technology (described below) also could significantly reduce energy consumption and CO2 emissions.
 * High-technology and environmental applications of the rare earth elements (REE) have grown dramatically in diversity and importance over the past four decades. As many of these applications are highly specific, in that substitutes for the REE are inferior or unknown, the REE have acquired a level of technological significance much greater than expected from their relative obscurity. Although actually more abundant than many familiar industrial metals, the REE have much less tendency to become concentrated in exploitable ore deposits. Consequently, most of the world’s supply comes from only a few sources. The United States once was largely self-sufficient in REE, but in the past decade has become dependent upon imports from China. ||
 * [[image:http://chem103csu.wikispaces.com/site/embedthumbnail/placeholder?w=442&h=268 width="442" height="268" caption="graph showing global rare earth element production"]] ||
 * **Figure 1.** Global rare earth element production (1 kt=106 kg) from 1950 through 2000, in four categories: United States, almost entirely from Mountain Pass, California; China, from several deposits; all other countries combined, largely from monazite-bearing placers; and global total. Four periods of production are evident: the monazite-placer era, starting in the late 1800s and ending abruptly in 1964; the Mountain Pass era, starting in 1965 and ending about 1984; a transitional period from about 1984 to 1991; and the Chinese era, beginning about 1991. ||
 * Table 1. Names and symbols of the REE ||
 * La || lanthanum ||  || Tb || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">terbium ||
 * <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">Ce || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">cerium ||  || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">Dy || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">dysprosium ||
 * <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">Pr || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">praseodymium ||  || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">Ho || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">holmium ||
 * <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">Nd || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">neodymium ||  || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">Er || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">erbium ||
 * <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">Pm || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">promethium ||  || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">Tm || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">thulium ||
 * <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">Sm || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">samarium ||  || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">Yb || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">ytterbium ||
 * <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">Eu || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">europium ||  || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">Lu || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">lutetium ||
 * <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">Gd || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">gadolinium ||  || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">Y || <span style="font-family: Verdana,Arial,Helvetica,sans-serif;">yttrium ||