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In, a chemical element in Group III of the Mendeleev periodic system. Atomic number, 49: atomic mass, 114.82. A lustrous soft white metal, indium comprises a mixture of two isotopes: “3In (4.33 percent) and115In (95.67 percent). The latter isotope exhibits very low β-radioactivity (half-life T 1/2 = 6 X 1014 years).
In 1863, during a spectroscopic analysis of zinc blende, the German scientists F. Reich and T. Richter discovered new spectral lines belonging to an unknown element. The new element was named indium, owing to the bright blue (indigo) color of the lines.
Distribution in nature. Indium is a typical trace element; its average content in the lithosphere comprises 1.4 X 105− percent by weight. Slight accumulation of indium in granite and other acidic rocks occurs during magmatic processes. The main processes of indium concentration in the earth’s crust are connected with hot aqueous solutions, which form hydrothermal deposits in which indium is associated with Zn, Sn, Cd, and Pb. Sphalerites, chalcopyrites, and cassiterites are enriched by indium by an average factor of 100 (approximate content, 1.4 X 103− percent). There are three known indium minerals: native indium, roxite (CuInS2), and indite (In2S,4), all extremely rare. Indium accumulation in sphalerites (up to 0.1 percent, sometimes 1 percent) is of practical importance.
Deposits in the ore belt of the Pacific Ocean are characterized by indium concentration.
Physical and chemical properties. Indium has a tetragonal face-centered crystal lattice with the parameters a = 4.583 Å and c = 4.936 Å. Its atomic radius is 1.66 Å; ionic radii, In3+= 0.92 Å and In+ = 1.30 Å; and density, 7.362 g/cm3. Indium is easily fusible, with a melting point of 156.2°C. Its boiling point is 2075°C. The temperature coefficient of linear expansion is 33 X 10−6 (20°C); specific heat at 0°-150°C is 234.461 joules/ (kg-°K), or 0.056 cal/ (g-°C); specific electrical resistivity at 0°C is 8.2 X 10−8 ohm-m, or 8.2 X 10−6 ohm-cm; modulus of elasticity is 11 henrys/m2, or 1100 kgf/mm2; and Brinell hardness is 9 meganewtons/m2, or 0.9 kgf/mm2.
According to the electron configuration of the atom, 4d105s25p1 indium has a valence of 1, 2, and 3 (primarily) in compounds. In the air, indium remains stable in a solid compact state but oxidizes at high temperatures; above 800°C it burns with a violet-blue flame, yielding the oxide In203—yellow crystals that are highly soluble in acids. Indium readily combines with halogens upon heating, thus forming the soluble halides InCl3, In Br3, and Inl3. The chloride InCl2 is obtained by heating indium in a flow of HCI, while InCl is formed by passing InCl2 vapor over heated indium. The sulfides In2S3 and InS are formed by combining indium with sulfur, which yield the compounds InS-In2S3 and 3InS-In2S3. In water, indium undergoes gradual corrosion at the surface in the presence of oxidizing agents as follows: 4In + 302 + 6H20 = 4In(OH)3. Indium is soluble in acids; its normal electrode potential is equal to— 0.34 V, and it is practically insoluble in alkalis. Indium salts hydro-lyze readily, yielding either basic salts or the hydroxide In(OH)3. The latter is highly soluble in acids and poorly soluble in alkaline solutions (with the formation of salts—indates): In(OH) 3+ 3KOH = K3[In(OH), 6]. Indium compounds with lower degrees of oxidation are fairly unstable; the halides InHal and black oxide ln2O are very strong reducing agents.
Preparation and use. Indium is prepared from the byproducts and intermediate products of industrial zinc, lead, and tin. These raw materials contain from thousandths to tenths of a percent of indium. The process of indium extraction consists of three basic stages: obtaining the enriched product, that is, indium concentrate; processing the concentrate into ferrous metal; and refining. In most cases, the starting raw material is treated with sulfuric acid, converting indium into a solution from which the concentrate is extracted through hydrolytic precipitation. Unrefined indium is extracted primarily by cementation with zinc or aluminum. Refining is carried out using chemical, electrochemical, distillation, and crystallo-physical methods.
Indium and its compounds (for example, nitride InN, phosphide InP, antimonide InSb) are widely used in the production of semiconductors. Indium is used for various anticorrosive coatings (including bearing coatings); these coatings have high reflecting power, which is utilized in the manufacture of mirrors and reflectors. Of commercial importance are certain indium alloys, including fusible alloys, solders for cementing together glass and metal, and others.
REFERENCESKhimiia i tekhnologiia redkikh i rasseiannykh elementov, vols. 1-2. Edited by K. A. Bofshakov. Moscow, 1965-69. (Vol. 1, pp. 88-99; vol. 2, pp. 178-207.)
Zelikman, A. N., O. E. Krein, and G. V. Samsonov. Metallurgiia redkikh metallov. Moscow, 1964. Pages 424—45.
Osnovy metallurgii, vol. 4. Edited by N. S. Greiver, N. P. Sazhin, and I. A. Strigin. Moscow, 1967. Pages 552-61.
N. A. GUROVICH