# Indium

properties
[ Kr ] 4 d 10 5 s 2 5 p 1
49 in
General
Name , symbol , atomic number Indium, In, 49
Element category Metals
Group , period , block 13 , 5 , p
Appearance shiny silver gray
CAS number 7440-74-6
EC number 231-180-0
ECHA InfoCard 100.028.345
Mass fraction of the earth's envelope 0.1 ppm
Atomic
Atomic mass 114.818 (1) &
Atomic radius (calculated) 155 (156) pm
Van der Waals radius 193 pm
Electron configuration [ Kr ] 4 d 10 5 s 2 5 p 1
1. Ionization energy 5.786 355 6 (7) eV 558.3 kJ / mol
2. Ionization energy 18th.87041 (3) eV1 820.72 kJ / mol
3. Ionization energy 28.04415 (12) eV2 705.85 kJ / mol
4. Ionization energy 55.45 (4) eV5 350 kJ / mol
5. Ionization energy 69.3 (1.2) eV6 686 kJ / mol
Physically
Physical state firmly
Crystal structure tetragonal
density 7.31 g / cm 3
Mohs hardness 1.2
magnetism diamagnetic ( Χ m = −5.1 10 −5 )
Melting point 429.7485 K (156.5985 ° C)
boiling point 2273 K (2000 ° C)
Molar volume 15.76 · 10 −6 m 3 · mol −1
Heat of evaporation 225 kJ / mol
Heat of fusion 3.26 kJ mol −1
Vapor pressure 1 Pa at 1196 K
Speed ​​of sound 1215 m s −1 at 293.15 K.
Specific heat capacity 233 J kg −1 K −1
Electric conductivity 12.5 · 10 6 A · V −1 · m −1
Thermal conductivity 81.6 W m −1 K −1
Chemically
Oxidation states 3 , 1
Normal potential −0.343 V (In 3+ + 3e - → In)
Electronegativity 1.78 ( Pauling scale )
Isotopes
isotope NH t 1/2 ZA ZE (M eV ) ZP
111 in {syn.} 2.8047 d ε 0.865 111 Cd
113 in 4.3% Stable
114 in {syn.} 71.9 s β - 1.989 114 Sn
ε 1.452 114 Cd
115 in 95.7  % 4.41 · 10 14 a β - 0.495 115 Sn
For other isotopes see list of isotopes
NMR properties
Spin
quantum
number I
γ in
rad · T −1 · s −1
E r  ( 1 H) f L at
B = 4.7 T
in MHz
113 in 9/2 5.8845 · 10 7 0.0151 21.87
115 in 9/2 5.8972 · 10 7 0.271 38.86
safety instructions
GHS labeling of hazardous substances

powder

danger

H and P phrases H: 228
P: 210-241-280-240-370 + 378
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .

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 , the 13th  IUPAC group , 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 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.

## history

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 after 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 put it as a coating in highly stressed bearings of airplanes one. 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 reactor accident , both demand and price fell significantly.

From 1987 onwards, two new indium compounds, the semiconductor indium phosphide (InP) and the electrically conductive and thin layers of transparent indium tin oxide (ITO) 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 a dignified , i.e. elemental, form, indium has so far (as of 2014) only been discovered very rarely. The type locality is a tantalum ore deposit near Olowjannaja in the Russian region of Transbaikalia . In addition, native indium was found in the Perzhanskoe ore field in the Ukrainian Oblast Zhytomyr and in the Chatkal Mountains in the Uzbek province Tashkent as well as in rock samples from the moon .

Also on minerals containing indium, are only a few known (currently 13 recognized minerals, Booth 2014). These are mainly sulphidic minerals such as Indit finely 2 S 4 , Laforêtit Agins 2 and Roquesit CuInS 2 and the counting to the element minerals, natural alloys Damiaoit PtIn 2 and Yixunit Pt 3 In. 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

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 generated during the roasting of zinc sulfide and residues left over from 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.

${\ displaystyle \ mathrm {In ^ {3 +} + 3 \ e ^ {-} \ {\ xrightarrow {Hg-Elektr.}} \ In}}$

By suitable methods such as zone melting process or multiple electrolysis of indium (I) chloride - molten salt may be further purified and thus more than 99.99% pure indium are obtained the crude product.

## production

The primary production (refinery production) of indium was between 500 and 580 tons in 2006. 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, especially for China, the information on natural indium reserves grew from 280 to 8,000 tons, which extended the range from 6 to 19 years. Secondary production, i.e. recycling, exceeds primary production and was 800 tons in 2008.

Refinery production by country (2008)
country metric tons Share of
world production
People's Republic of China 330 058.1%
Japan 060 010.6%
South Korea 050 008.8%
Belgium 030th 005.3%
Russia 012 002.1%
Peru 006th 001.1%
other 030th 005.3%
Total:  568 100.0%
Development of indium production from 1971 to 2011

The indium production in China has only recently increased. In 1994 the amount produced was still 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 depletion of the deposits. In 2006, for example, the Japanese Toyoha mine was closed, thereby significantly reducing production there.

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

In industrial thin-film technology, the recycling of sputtering targets containing indium is common operational practice due to their lumpy nature and the low dismantling effort with high material value. In contrast to the walls and fixtures of the sputtering chamber and any structuring waste. The substitution of indium in the transparent conductive layer ( TCO ) by means of zinc oxide particularly at copper indium gallium selenide - (CIGS), but also in a-Si - or c-Si - solar cells , like the use of secondary material for use in indium targets of low purity, current topic of publicly funded research in functional and resource-economical materials science.

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 are possible, albeit physically not optimal.

## properties

### Physical Properties

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 156.60 ° C. Of the pure (unalloyed) 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 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 , which is in the tetragonal crystal system in the space group I 4 / mmm (space group no.139) and thus in a tetragonal body-centered lattice with the lattice parameters a  = 325  pm and c  = 495 pm as well 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 on the corners of the unit cell (337 pm; green bonds). As a coordination polyhedron , the coordination number 4 + 8 = 12 results in a distorted cuboctahedron . 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 which crystallizes in the orthorhombic crystal system in the space group  Fmmm (No. 69) .

 Crystal system tetragonal Space group I 4 / mmm (No. 139) Lattice parameter (unit cell ) a  = ( b ) = 325  pm c  = 495 pm Number (Z) of the formula units Z = 2

### 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.

## Isotopes

38 different isotopes and another 45 core isomers from 97 In to 135 In are known of indium . Only two isotopes of these occur in nature, 113 In (64  neutrons ) with 4.29 percent and 115 In (66 neutrons) with 95.71 percent of the natural isotope distribution. The common isotope 115 In is weakly radioactive ; it is a beta emitter with a half-life of 4.41 · 10 14 years. Thus, one kilogram of indium has an activity of 250  Becquerel . Both natural isotopes can be detected with the help of NMR spectroscopy . The most stable artificial isotopes 111 In and 114m In have half-lives of 2.8 and 50 days, respectively , 113m In only 99 minutes. 111 In and 113m In are used in medical diagnostics for imaging procedures ( scintigraphy and SPECT ).

## use

### 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, 65% of total indium production was used in 2000. Other compounds such as indium phosphide and indium arsenide are also obtained from the indium produced. For more information on the use of indium compounds, see the Connections section .

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 slide 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 even reflectance 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. In clinical studies , mercury alloys with copper and 5 or 10% indium as an amalgam filling were tested. 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 by chemical means, but by 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 can be used 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 . A single dose of 0.4 mg kg −1 InCl 3 to pregnant rats resulted in malformations such as cleft palates and oligodactyly . These phenomena were found more frequently when indium trichloride was applied on the 10th day of pregnancy. In contrast, no malformations were observed in mice. Toxicity to aquatic organisms (aquatic toxicity) has been established for indium nitrate.

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).

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 oxides

YInMn blue

Indium (III) oxide is a yellow, stable semiconductor . Pure indium (III) oxide is rarely used; in technology, most of it is further 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, means that it is widely used. 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 (AZO).

YInMn blue is a mixed oxide of yttrium, indium and manganese oxides, which shows a very pure and brilliant blue.

### Compound semiconductors

Many indium compounds are of great importance for semiconductor technology . This applies in particular to compounds with elements of the 5th and 6th main group (15th and 16th IUPAC group), such as phosphorus, arsenic or sulfur. Those with elements of the 5th 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 connecting 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 particular application depends on the band gap required. Indium (III) sulfide (In 2 S 3 ) is a III-VI semiconductor with a band gap of 2  eV that is used instead of cadmium sulfide in 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 x Ga 1 − x As) a ternary semiconductor with a band gap that is smaller than that of gallium arsenide . Copper indium diselenide (CuInSe 2 ) has a high degree of light absorption and is therefore used in thin-film solar cells ( CIGS solar cells ).

### Other indium compounds

Indium forms a series 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 InR 3 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.

## literature

Wiktionary: Indium  - explanations of meanings, word origins, synonyms, translations
Commons : Indium  album with pictures, videos and audio files

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