a family of 14 chemical elements, with atomic numbers from 58 to 71, that follow lanthanum in the sixth period of the Mendeleev periodic system (see Table 1). The lanthanides and the elements scandium, yttrium, and lanthanum, which are similar to them, make up the rare earths. This name stems from the fact that the elements are rarely encountered and form refractory oxides that are insoluble in water and were known in the ancient terminology as earths. The rare earths are found in a side subgroup of Group III of the periodic system.

The lanthanides are very close to one another in their chemical properties as a result of the electron shell structure of their atoms. As the charge of the nucleus increases, the structure of the two outer electron shells does not change, because electrons are added to the third from the outer shell—the deep-lying 4f level. The maximum possible number of electrons in an/level is 14, which determines the number of elements in the lanthanide family. The lanthanides are divided into two subgroups—the cerium subgroup, including cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), and europium (Eu); and the yttrium subgroup, including gadolinium

(Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). This division results from the periodicity of the change in a number of properties within the lanthanide family; the names of the subgroups are of historical origin.

History. The mineral ytterbite (later renamed gadolinite) was found in 1788 in the Swedish village of Ytterby. In 1794, Johan Gadolin discovered the new yttrium earth in ytterbite. In 1803, J. J. Berzelius and W. Hisinger (1766–1852) and, independently, M. Klaproth (1743–1817) discovered cerium earth (named for the asteroid Ceres) in “hard rock from Bastnäs.” At first both of these earths were thought to be oxides of the previously unknown metals yttrium and cerium. In 1843 the Swedish chemist C. G. Mosander (1797–1858) separated yttrium earth into yttrium, erbium, and terbium earths (all three names are derived from Ytterby). In 1878, J. Marignac separated ytterbium from erbium earth, and in 1879 the Swedish chemist P. T. Cleve separated holmium earth (from Holmia, the Latin name for Stockholm) and thulium earth (from Thule, the ancient Greek name for lands in the far north) from it. In 1886, P. E. Lecoq de Boisbaudran separated holmium earth into holmium and dysprosium earths (from the Greek dysprositos, “difficult of access”). In 1907 the French chemist G. Urbain (1872–1938) found lutetium in yttrium earth (from Lutetia, the Latin name for Paris).

The same process of discovery was repeated for cerium earth. In 1839–41, Mosander separated cerium into lanthanum earth (from the Greek lanthano, “I hide”), didymium earth (from the Greek didymos, “twin”), and cerium earth. In 1879, while studying didymium earth obtained from the mineral samarskite, found in the Urals (so named in 1847 by Heinrich Rose [1795–1864] for the director of the Russian Corps of Mining Engineers, V. E. Samarskii-Bykhovets [1803–70], who supplied Rose with a considerable quantity of the mineral), Lecoq de Boisbaudran isolated samarium earth. In 1886 he succeeded in isolating gadolinium earth (named for Gadolin) from didymium earth, which proved to be identical with the earth discovered in samarskite by Marignac in 1880. In 1885 the Austrian chemist C. Auer von Welsbach (1858–1929) separated didymium earth into praseodymium earth (from the Greek prasios, “light green”) and neodymium earth (from the Greek neos, “new”). In 1901 the French chemist E. Dermarçay (1852–1904) separated samarium earth into samarium and europium.

Thus, all the lanthanides had been discovered by the beginning of the 20th century, except for the radioactive element with atomic number 61, which is not found in nature. It was obtained only in 1947 by the American physicists J. Marinsky, L. Glendinen, and C. Coryell from uranium fission fragments in a nuclear reactor and was named promethium (from the name Prometheus).

Although the lanthanides had been discovered by the beginning of the 20th century, many of them had neither been isolated in sufficiently pure form nor carefully studied. The efficient methods of isolation developed in the last 20 years make possible the production of both the lanthanide compounds and the lanthanide metals themselves in highly pure form.

Occurrence in nature. The total content of lanthanum and the lanthanides in the earth’s crust is 1.78 × 10⁻² percent by mass; lanthanides with even atomic numbers are more abundant than their neighbors with odd numbers. Lanthanides are characteristic elements of the earth’s crust; they are rare in rock of the mantle and in rock meteorites. In magmatic processes, lanthanides accumulate in granitoids and particularly in alkali rocks. There are 33 known cerium minerals and nine lanthanum minerals. The remaining lanthanides are found as isomorphic impurities in the crystal lattices of other minerals, mainly the rare earths. In many minerals, lanthanides isomorphically replace calcium, uranium, and thorium. In the biosphere lanthanides are relatively immobile, which accounts for their accumulation in placers. The content of lanthanides in natural waters and in organisms is negligible. The aqueous and biogenic migration of lanthanides has not been studied in detail. Deposits of lanthanide phosphates, fluocarbonates, and fluorides are known, but complex deposits associated with alkali magmatic rock (such as the nepheline syenites from the Kola Peninsula) and carbonatites, as well as deposits of sedimentary phosphorites, the crust of weathering of alkali rocks, seashore and alluvial deposits of xenotime and monazite, have greater industrial importance.

Physical properties. Lanthanides are metals with a silver-white color (some, such as praseodymium and neodymium, are slightly yellowish). Most of the lanthanides have a closely packed hexagonal crystal structure. Exceptions are γ-cerium and α-ytterbium, which have a cubic face-centered structure; samarium, which has a rhombohedral structure; and europium, which has a cubic body-centered structure. The fact that the number of electrons in the two outer shells usually does not change in passing from cerium to lutetium, whereas the positive charge of the nucleus increases steadily, leads to a stronger attraction of the electrons to the nucleus and results in so-called lanthanide contraction. Thus, the radii of neutral lanthanide atoms and ions of identical valence decrease somewhat with increasing atomic number. The melting points of elements of the cerium subgroup are significantly lower than those of the yttrium subgroup.

Lanthanides of high purity are plastic and easily deformable (for example, by forging and rolling). The mechanical properties depend strongly on the content of impurities, particularly oxygen, sulfur, nitrogen, and carbon. Values of the ultimate strength and elastic modulus of metals of the yttrium subgroup (except ytterbium) are greater than for the cerium subgroup. All lanthanides except lanthanum and lutetium are highly paramagnetic at greater than room temperature as a result of uncompensated spin and orbital magnetic moments in their 4f subshells. In the low-temperature region, most of the lanthanides of the cerium subgroup (neodymium, praseodymium, and samarium) are found in an antiferromagnetic state, whereas the lanthanides of the yttrium subgroup (terbium, dysprosium, holmium, erbium, and thulium) are in a ferromagnetic state at very low temperatures and undergo transition to the helicoid antiferromagnetic state at higher temperatures. At temperatures below 293°K (that is, up to the Curie temperature), gadolinium is in a ferromagnetic state.

Terbium, dysprosium, holmium, erbium, and thulium have high magnetic susceptibility and extremely high values of the energies of magnetic anisotropy and magnetostriction, which makes possible the production from them of magnetic materials (alloys, ferrites, and chalcogens) with unique properties.

Alpha-lanthanum becomes a superconductor at 4.9°K; β-lanthanum, at 5.85°K. Superconductivity of other lanthanides has not been observed.

Chemical properties. Lanthanides are characterized by high chemical activity. Upon heating they react with hydrogen, carbon, nitrogen, phosphorus, hydrocarbons, and carbon monoxide and dioxide. Lanthanides decompose water and dissolve in hydrochloric, sulfuric, and nitric acids. Above 180°–200°C, lanthanides oxidize rapidly in air. A valence of 3 is characteristic of all lanthanides; some of them also have valences of 4 or 2.

The oxides of lanthanides and lanthanum are refractory. Hydroxides of the type R(OH)₃ are basic and are insoluble in alkalies. The chlorides, sulfates, and nitrates of trivalent lanthanides are soluble in water and crystallize mostly in the form of crystal hydrates of various compositions. Their fluorides, oxalates, phosphates, carbonates, and ferrocyanides are only slightly soluble in water and dilute mineral acids. The triply charged cations of cerium, gadolinium, terbium, ytterbium, and lutetium are colorless; those of promethium, europium, and erbium, pink; those of samarium, dysprosium, and holmium, yellow; those of praseodymium and thulium, green; and those of neodymium, violet-red.

Most simple lanthanide salts tend to form double salts with the salts of alkali metals, ammonia, and magnesium. Lanthanides form complexes with many organic compounds. The more important such complexes are those with citric acid and a series of aminopolyacetic acids, such as nitrilotriacetic and ethylenediaminetetraacetic acids and other “complexons”. Such compounds are used in processes for the isolation of lanthanides.

Production. The main sources of rare earth elements of the cerium subgroup are the minerals monazite (phosphates of rare earths and thorium), bastnaesite (rare-earth fluocarbonates), and loparite (a complex titanoniobate of sodium, calcium, and rare earth). The main sources of rare earths of the yttrium subgroup are euxenite, fergusonite, xenotime (yttroparisite), and gadolinite.

To extract rare earths, monazite and bastnaesite concentrates are decomposed by concentrated sulfuric acid upon heating to 200°C, with subsequent leaching of the material with water. Thorium is initially isolated from sulfuric acid solutions, and then the rare earths are precipitated as oxalates, double sulfates, and other compounds. For decomposition of the monazite concentrates, treatment with alkali solutions is also used, with dissolution of the resulting mixture of hydroxides in hydrochloric or nitric acid. Bastnaesite concentrates are calcined at 400°–800°C to produce partial or complete decomposition of the mineral, accompanied by the evolution of CO_2. The product of the calcination is treated with nitric acid. The rare earths are precipitated as fluorides or double sulfates or are removed by extraction with tributyl phosphate. Complex raw material of the loparite type is chlorinated in the presence of carbon at 700°–800°C. The volatile chlorides of titanium, niobium, and tantalum are removed with the gases. A melt composed of chlorides of rare earths remains in the furnace. The chlorides are dissolved in water, and then the oxalates of rare earths are isolated. The chlorination method is also recommended for the treatment of euxenite.

The methods for separating lanthanides are based on the small differences in the properties of their compounds. Fractional crystallization of their salts, such as double nitrates, and fractional precipitation of their hydroxides, sulfates, and oxalates were previously used for this purpose. At present the main methods are based on extraction, in which the difference in the distribution coefficients between aqueous and organic solvents is used. These methods, together with ion-exchange chromatography, make possible the production of all the lanthanides with a high degree of purity. In addition, the ability of some lanthanides to oxidize to the tetravalent state (for cerium) or to reduce to the divalent state (for samarium, europium, and ytterbium) is also used in separation schemes.

Metallothermy or electrolysis is used to obtain lanthanide metals. The metallothermic method is based on the reduction of anhydrous chlorides or fluorides by pure calcium. The process is carried out in a steel bomb lined with calcium oxide or in

tantalum crucibles in a pure argon atmosphere. This method may be used to produce all the lanthanides except samarium, europium, and ytterbium. The latter may be reduced from their oxides by lanthanum, with subsequent distillation of the metals.

All lanthanides may be obtained by electrolysis of their compounds in salt melts. Metals of the cerium subgroup are separated by the electrolysis of their anhydrous chlorides in KC1 + CaCh₂ or KC1 + NaCl melts. In the case of metals of the yttrium subgroup, which are more refractory, the electrolysis is carried out with a liquid cadmium or zinc cathode, which is distilled out under vacuum. Metals produced electrolytically are less pure than those produced by metallothermic methods.

Use. Lanthanides are used in the form of metals, alloys, and chemical compounds in various areas of technology. Lanthanide additives (mainly cerium or its alloy with lanthanum) improve the structure, mechanical properties, and corrosion and heat resistance of steel, cast iron, and magnesium and aluminum alloys. The addition of oxides of various lanthanides imparts special physical properties and coloring to glass. Cerium dioxide, CeO₂, is used for polishing optical glass. Lanthanide oxides are used for coloring china and lacquered and enamel ware. Cerium or an alloy of a lanthanide of the cerium subgroup (misch metal) is added to nonexpanding gas absorbers (getters) in electrovacuum instruments. The borides of some lanthanides are used to make the cathodes of large electronic devices.

Rare-earth ferrite garnets and orthoferrites are used in super-high-frequency electronics and computer technology. Rare-earth alloys of the SmCo₅ type are used in radio electronics and microelectronics for making permanent magnets of record power. Lanthanides are added to crystals for lasers (additives of lanthanide compounds in crystals of CaF₂ and other salts).

In atomic technology, lanthanides with a large thermal neutron capture cross section, such as gadolinium, samarium, and europium, are used for protection against radiation and to control the operation of reactors.

In the chemical industry and light industry, lanthanide compounds are used in the production of paints and varnishes, luminescent preparations (phosphors), catalysts, and photographic reagents.

Some radioactive lanthanide isotopes have found important uses: ¹⁴⁷Pm is used for making microscopic batteries, and ¹⁷⁰Tm is used in portable medical X-ray devices. In agriculture, lanthanide compounds are used as insecticides and trace fertilizers. This list does not exhaust the various uses of the lanthanides.