General

Indium is a chemical element with the symbol In and the atomic number 49. In the periodic table of the elements it is in the 5th period and is the fourth element of the 3rd main group (group 13 according to the new count) or boron group. Indium is a rare, silvery white and soft heavy metal. Its abundance in the earth's crust is comparable to that of silver. Indium is not essential for the human body, nor are any toxic effects known. Most of the metal is now processed into indium tin oxide, which is used as a transparent conductor for flat screens and touch screens. Since the turn of the millennium, the associated increased demand has led to a significant rise in indium prices and to discussions about the range of the deposits.

Indium was discovered in 1863 by the German chemists Ferdinand Reich and Theodor Richter at the Bergakademie Freiberg. They examined a sphalerite sample found in the area for thallium. Instead of the expected thallium lines, they found a previously unknown indigo blue spectral line in the absorption spectrum and thus a previously unknown element. The new element was later named after this. A short time later they were initially able to produce indium chloride and oxide, and the metal by reducing indium oxide with hydrogen. A large amount of indium was first shown at the World Exhibition in Paris in 1867. 

After its first use in 1933 as an alloy component in dental gold, the extensive use of indium began with the Second World War. The United States used it as a coating in highly stressed aircraft bearings. After the Second World War, indium was mainly used in the electronics industry, as a soldering material and in low-melting alloys. Use in control rods of nuclear reactors also became important with the increasing use of nuclear energy. This led to the first sharp rise in the price of indium by 1980. However, after the Three Mile Island nuclear accident, both demand and price fell significantly.

From 1987 two new indium compounds, the semiconductor indium phosphide and the transparent indium tin oxide, which is conductive and transparent in thin layers, were developed. Indium tin oxide in particular became technically interesting with the development of liquid crystal screens. Due to the high demand, most of the indium has been processed into indium tin oxide since 1992.

occurrence 

Indium is a rare element, its share in the continental crust is only 0,05 ppm. It is thus of a similar frequency as silver and mercury. In its solid state, indium has only been found in one single find in eastern Siberia. Only a few indium minerals are known. These are mainly sulphidic minerals such as Indit FeIn₂S₄ and Roquésit CuInS₂. However, these are rare and do not play a role in the extraction of indium. The largest deposits of indium are in zinc ores, especially sphalerite. Theoretical reserves are estimated at 16.000 tons, of which around 11.000 tons are economically recoverable. The largest deposits are in Canada, China and Peru. Ores containing indium are also found in Australia, Bolivia, Brazil, Japan, Russia, South Africa, the USA, Afghanistan and some European countries. In Germany there are deposits in the Ore Mountains (Freiberg, Marienberg, Geyer) and on the Rammelsberg in the Harz Mountains.

Extraction and presentation 

Indium is obtained almost exclusively as a by-product in the production of zinc or lead. Economic extraction is possible if indium accumulates at certain points in the production process. These include flue dusts that are generated during the roasting of zinc sulfide and residues that are left behind during electrolysis during the wet process of zinc production. These are reacted with sulfuric acid or hydrochloric acid and thus brought into solution. Since the concentration of indium in the acid is too low, it must be enriched. This is done, for example, by extraction with tributyl phosphate or precipitation as indium phosphate.

The actual indium production takes place electrolytically. A solution of indium (III) chloride in hydrochloric acid is used for this. This is converted into elemental indium with the help of mercury electrodes. During the electrolysis, care must be taken that the solution no longer contains any thallium, as the standard potentials of the two elements are very similar.

The crude product can be further purified by means of suitable processes such as zone melting processes or repeated electrolysis of indium (I) chloride salt melts and thus over 99,99% pure indium can be obtained.

Production 

The primary production (mine production) of indium was between 2006 in year 500 and 580 tons. Due to the low natural reserves of 11.000 tons and high demand, indium is one of the scarcest raw materials on earth. In 2008, in particular for China, the information on natural indium reserves grew from 280 to 8.000 tons, which extended the static range from 6 to 19 years. Secondary production, i.e. recycling, exceeds primary production and was 2008 tons in 800.

Refinery production of indium by countries

The indium production in China has only recently increased. In 1994 the amount produced was 10 tons. Since then, China's share of world production has increased to 60% in 2005. Production in other countries such as Japan, Canada or France could only be increased to a small extent or decreased due to the depletion of deposits. In 2006, for example, the Japanese Toyoha mine was closed, thereby significantly reducing production there.

Temporal development of indium production

Since the demand for indium has grown faster than production, the price of indium rose sharply from $ 97 in 2002 to $ 827 per kilogram in 2005. Indium is recycled primarily by recycling residues from sputtering. The only country where indium is currently recovered in large quantities is Japan.

If demand continues to rise and the price goes up, recycling of materials with only a small amount of indium will become profitable. Likewise, it makes economic sense to exploit ores with lower indium contents. This will probably make it possible to delay the drying up of resources.

Indium can be replaced by other substances in most applications, but this often worsens the properties of the product or the profitability of production. For example, indium phosphide can be replaced by gallium arsenide, and some substitutes for indium tin oxide, albeit of poorer quality, are possible.

Features

Physical Properties 

Crystallographic data^[22]

crystal system	tetragonal
space group
lattice parameters (Unit cell)	a = (b) = 325 pm c = 495 pm
Number (Z) of formula units	Z = 2

Unit cell of indium with coordination environment of the central indium atom

Coordination polyhedron of an indium atom made up of 4 + 8 = 12 neighboring atoms in the shape of a distorted cuboctahedron

Indium is a silvery-white metal with a low melting point of 156,5985 ° C. Among the metals, only mercury, gallium and most alkali metals have a lower melting point. The metal is liquid over a very large range of almost 2000 K. Liquid indium permanently leaves a thin film on glass (wetting). The similar gallium has the same property.

The metal has a high ductility and very low hardness (Mohs hardness: 1,2). It is therefore possible to cut indium like sodium with a knife. At the same time, it leaves a visible line on paper. Indium is superconducting below a transition temperature of 3,41 Kelvin. A peculiarity of indium, which it has in common with tin, is the characteristic noises that can be heard when indium is bent (“pewter screams”).

Only one crystalline modification is known of indium under normal conditions, that in the tetragonal crystal system in the space group  and thus in a tetragonal body-centered grid with the grid parameters a = 325 pm and c = 495 pm and two formula units crystallize in the unit cell.

An indium atom is surrounded by twelve other atoms in the crystal structure, four of which come from the neighboring unit cells and are closer together (325 pm; red bonds) than the eight atoms located on the corners of the unit cell (337 pm; green bonds). The coordination number 4 + 8 = 12 results in a distorted cuboctahedron as the coordination polyhedron. The crystal structure can therefore be described as a tetragonally distorted, cubic closest packing of spheres.

A further modification was discovered in high pressure experiments, which is stable above 45 GPa and in the orthorhombic crystal system in the space group Fmmm crystallized.

Chemical properties 

The chemical properties of indium are similar to those of its group neighbors gallium and thallium. Like the other two elements, indium is a base element that can react with many non-metals at high temperatures. In the air it is stable at room temperature because, as with aluminum, a dense oxide layer forms that protects the material from further oxidation through passivation. The reaction to indium (III) oxide only takes place at high temperatures.

While indium is attacked by mineral acids such as nitric acid or sulfuric acid, it is not soluble in hot water, bases and most organic acids. Salt water does not attack indium either. Indium is the most soluble metal in mercury at room temperature.

Isotope 

There are 38 different isotopes and another 45 core isomers of indium ⁹⁷In to ¹³⁵In known. Only two isotopes occur in nature, ¹¹³In (64 neutrons) with 4,29% and ¹¹⁵In (66 neutrons) with 95,71% share in the natural isotope distribution. The common isotope ¹¹⁵In is weakly radioactive, it is a beta emitter with a half-life of 4,41 · 10¹⁴ Years. Both natural isotopes can be detected with the help of NMR spectroscopy. The most stable artificial isotopes ¹¹¹In and ^114mIn have half lives of several days, ^113mIn only about one and a half hours. ¹¹¹In and ^113mIn are used in nuclear medicine.

Usage 

Metal 

Indium wire is used in indium seals.

Indium is versatile, but its use is limited by its rarity and high price. Most of the indium produced is not used as metal, but is processed into a series of compounds. For the production of indium tin oxide alone, 2000% of total indium production was used in 65. Other compounds such as indium phosphide and indium arsenide are also obtained from the indium produced. More details on the use of indium compounds can be found in section Links.

Metallic workpieces can be protected by galvanically deposited indium coatings. Materials made of steel, lead or cadmium, for example, coated in this way are more resistant to corrosion from organic acids or salt solutions and, above all, to abrasion. In the past, indium protective layers were often used for sliding bearings in automobiles or aircraft. However, since the sharp rise in the price of indium, this is no longer economical. Surfaces coated with indium have a high and uniform degree of reflection across all colors and can therefore be used as a mirror.

The melting point of indium is relatively low and can be determined very precisely. For this reason, it is one of the fixed points when setting up the temperature scale. This property is also used for calibration in Dynamic Differential Calorimetry (DSC).

Because of the large capture cross-section for both slow and fast neutrons, indium is a suitable material for control rods in nuclear reactors. Indium foils can also be used as neutron detectors. Indium is gas-tight and easy to deform even at low temperatures and is therefore used in so-called indium seals in cryostats.

Indium also plays a role as a solder for many materials due to some special properties. It only deforms slightly when it cools down. This is especially important when soldering semiconductors for transistors. The fact that indium is also able to solder non-metallic materials such as glass and ceramics also plays a role.

With “indium pills”, germanium platelets were alloyed on both sides in order to produce the first transistors.

alloys 

Indium can be alloyed with many metals. Many of these alloys, especially with the metals bismuth, tin, cadmium and lead, have a low melting point of 50 to 100 ° C. This results in possible applications in sprinkler systems, thermostats and fuses, for example. Since lead, which can also be used, is poisonous, indium serves as a harmless substitute. The purpose of these alloys is that they melt when the ambient temperature is too high, caused by fire or high currents. The melting then interrupts the circuit or triggers the sprinkler system. Indium-gallium alloys often have even lower melting points and are contained in high-temperature thermometers. A special gallium-indium-tin alloy is Galinstan. This is liquid at room temperature and serves as a harmless substitute for mercury or sodium-potassium alloys.

There are several other alloys containing indium that are used in different areas. Indium is used with copper, manganese and magnesium as an alloy component in magnetic materials. Occasionally indium (maximum 5%) with silver, tin, copper, mercury and zinc is used as an admixture in amalgam fillings. The storage layer of a CD-RW contains indium, among other things.

Proof

A possible chemical proof is the precipitation of indium ions with the help of 8-hydroxyquinoline from acetic acid solution. Normally, indium is not detected chemically, but using suitable spectroscopic methods. Indium can easily be detected using the characteristic spectral lines at 451,14 nm and 410,18 nm. Since these are in the blue spectral range, the typical blue flame color results. X-ray fluorescence analysis and mass spectrometry are available as examination methods for a more precise quantitative determination.

Toxicity and safety 

While indium metal is not known to have any toxic effects, it has been shown that indium ions have embryonic and teratogenic effects in animal experiments with rats and rabbits. With a single dose of 0,4 mg * kg^-1 InCl₃ Malformations such as cleft palates and oligodactyly have been observed in pregnant rats. These phenomena were found more frequently when the indium was applied on the 10th day of pregnancy. In contrast, no malformations were observed in mice. Indium nitrate has been found to be toxic to aquatic organisms (aquatic toxicity).

Compact indium metal is not flammable. In the finely divided state as powder or dust, on the other hand, like many metals, it is highly flammable and combustible. Burning indium must not be extinguished with water because of the risk of explosion from the hydrogen produced, but must be extinguished with metal fire extinguishers (class D).

Connections 

Indium forms a number of compounds. In them, the metal usually has the oxidation state + III. Level + I is rarer and more unstable. The oxidation state + II does not exist, compounds in which formally divalent indium occurs are in reality mixed compounds of monovalent and trivalent indium.

indium 

Indium (III) oxide is a yellow, stable salt. Pure indium (III) oxide is rarely used; in technology, most of it is processed into indium tin oxide. It is indium (III) oxide doped with a small amount of tin (IV) oxide. This turns the connection into a transparent and conductive oxide (TCO material). This combination of properties, which only a few other materials have, requires a wide range of applications. Indium tin oxide is used in particular as a conductor in liquid crystal screens (LCD), organic light-emitting diodes (OLED), touch screens and solar cells. In other applications such as heated car windows and solar cells, the expensive indium tin oxide could be replaced by cheaper aluminum-doped zinc oxide.

Compound semiconductor 

Many indium compounds are compound semiconductors with characteristic band gaps. This applies in particular to compounds with elements of the 15th and 16th main group, such as phosphorus, arsenic or sulfur. Those with elements of the 15th main group are counted among the III-V compound semiconductors, those with chalcogens among the III-VI compound semiconductors. The number depends on the number of valence electrons in the two connection components. Indium nitride, indium phosphide, indium arsenide and indium antimonide have different applications in different diodes, such as light emitting diodes (LED), photodiodes or laser diodes. The exact application depends on the band gap required. Indium (III) sulfide (In₂S₃) is a III-VI semiconductor with a band gap of 2 eV, which is used in place of cadmium sulfidine solar cells. Some of these compounds - most notably indium phosphide and indium arsenide - play a role in nanotechnology. Indium phosphide nanowires have a strongly anisotropic photoluminescence and can possibly be used in highly sensitive photodetectors or optical switches.

In addition to the simple compound semiconductors, there are also semiconducting compounds that contain more than one metal. An example is indium gallium arsenide (In_xGa_1-xAs) a ternary semiconductor with a reduced band gap compared to gallium arsenide. Copper indium diselenide (CuInSe₂) has a high degree of light absorption and is therefore used in thin-film solar cells (CIGS solar cells).

Further indium compounds 

Indium forms a number of compounds with the halogens fluorine, chlorine, bromine and iodine. They are Lewis acids and form complexes with suitable donors. An important indium halide is indium (III) chloride. This is used, among other things, as a catalyst for the reduction of organic compounds.

There are also organic indium compounds with the general formulas InR₃ and InR. Like many organometallic compounds, they are sensitive to oxygen and water. Organic indium compounds are used as doping reagents in the production of semiconductors.

General
Name, symbol, atomic number	Indium, In, 49
Series	Metals
Group, period, block	13, 5, p
Appearance	silvery shiny gray
CAS number	7440-74-6
Mass fraction of the earth shell	0,1 ppm
Atomic
atomic mass	114,818 u
Atomic radius (calculated)	155 (156) pm
Covalent radius	144 pm
Van der Waals radius	193 pm
electron configuration	[Kr] 4d105s25p1
1. ionization	558,3 kJ / mol
2. ionization	1820,7 kJ / mol
3. ionization	2704 kJ / mol
Physically
Physical state	fest
crystal structure	tetragonal
density	7,31 g / cm3
Mohs hardness	1,2
magnetism	diamagnetic ( = -5,1 10−5)
melting point	429,7485[4] K (156,5985 ° C)
boiling point	2345 K (2072 ° C)
Molar volume	15,76 · 10−6 m3/ mol
Heat of vaporization	231,8 kJ / mol
heat of fusion	3,26 kJ / mol
vapor pressure	1 Pa at 1196 K
speed of sound	1215 m / s at 293,15 K
Specific heat capacity	233 J / (kg · K)
Electric conductivity	12,5 · 106 A / (V · m)
thermal conductivity	81,6 W / (m K)
Chemical
oxidation states	3, 1
normal potential	−0,343 V (In3+ + 3e- → In)
electronegativity	1,78 (Pauling scale)
Isotope
isotope NH t1/2 ZA ZE (MeV) ZP 111In {Syn.} 2,8047 d ε 0,865 111Cd 113In 4,3% Stabil 114In {Syn.} 71,9 s β- 1,989 114Sn ε 1,452 114Cd 115In 95,7 % 4,41 · 1014 a β- 0,495 115Sn
NMR properties
Spin γ in rad * T−1· s−1 Er(1H) fL at W = 4,7 T in MHz 113In 9/2 5,8845 · 107 0,0151 21,87 115In 9/2 5.8972 · 107 0,271 38,86
safety instructions
GHS hazardous substances labeling Danger H and P phrases H: 228-315-319-332-335 EUH: no EUH rates P: 210-​261-​305+351+338 Hazardous Informationpowder Light- flammable (F) R and S phrases R: 11 S: 9-16-29-33

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