Indium

Indium,  ₄₉In

General properties
Name, symbol	indium, In
Appearance	silvery lustrous gray
Pronunciation	/ˈɪndiəm/ IN-dee-əm
Indium in the periodic table
Ga ↑ In ↓ Tl cadmium ← indium → tin
Atomic number (Z)	49
Group, block	group 13, p-block
Period	period 5
Element category	post-transition metal
Standard atomic weight (±) (Ar)	114.818(1)[1]
Electron configuration	[Kr] 4d^10 5s^2 5p^1
per shell	2, 8, 18, 18, 3
Physical properties
Phase	solid
Melting point	429.7485 K ​(156.5985 °C, ​313.8773 °F)
Boiling point	2345 K ​(2072 °C, ​3762 °F)
Density near r.t.	7.31 g/cm^3
when liquid, at m.p.	7.02 g/cm^3
Triple point	429.7445 K, ​~1 kPa[2]
Heat of fusion	3.281 kJ/mol
Heat of vaporization	231.8 kJ/mol
Molar heat capacity	26.74 J/(mol·K)
vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 1196 1325 1485 1690 1962 2340
Atomic properties
Oxidation states	3, 2, 1, −1, −2, −5[3] ​(an amphoteric oxide)
Electronegativity	Pauling scale: 1.78
Ionization energies	1st: 558.3 kJ/mol 2nd: 1820.7 kJ/mol 3rd: 2704 kJ/mol
Atomic radius	empirical: 167 pm
Covalent radius	142±5 pm
Van der Waals radius	193 pm
Miscellanea
Crystal structure	​tetragonal
Speed of sound thin rod	1215 m/s (at 20 °C)
Thermal expansion	32.1 µm/(m·K) (at 25 °C)
Thermal conductivity	81.8 W/(m·K)
Electrical resistivity	83.7 nΩ·m (at 20 °C)
Magnetic ordering	diamagnetic[4]
Young's modulus	11 GPa
Mohs hardness	1.2
Brinell hardness	8.8–10.0 MPa
CAS Number	7440-74-6
History
Discovery	Ferdinand Reich and Hieronymous Theodor Richter (1863)
First isolation	Hieronymous Theodor Richter (1867)
Most stable isotopes of indium
iso NA half-life DM DE (MeV) DP ^113In 4.29% – (SF) <24.281 ^115In 95.71% 4.41×10^14 y β^− 0.495 ^115Sn
Decay modes in parentheses are predicted, but have not yet been observed

Indium is a chemical element with symbol In and atomic number 49. It is a post-transition metallic element that is rare in Earth's crust. The metal is very soft, malleable and easily fusible, with a melting point higher than sodium, but lower than lithium or tin. Chemically, indium is similar to gallium and thallium, and it is largely intermediate between the two in terms of its properties.^[5] It has no obvious role in biological processes and common compounds are not toxic. It is most notably used in low melting point metal alloys such as solders, in soft metal high vacuum seals, and in the production of transparent conductive coatings of indium tin oxide (ITO) on glass.

Properties

Indium wetting the glass surface of a test tube

Indium is a very soft, silvery-white, highly ductile, relatively rare post-transition metal with a bright luster.^[6] It is so soft (Mohs hardness 1.2) that the metal can be cut with a knife, as can sodium. It also leaves a visible line on paper.^[7] Like tin, when it is bent indium emits a high-pitched "cry".^[6] Like gallium, indium is able to wet glass. Like both, indium has a low melting point, 156.60 °C (313.88 °F); higher than its lighter homologue, gallium, but lower than its heavier homologue, thallium, and lower than tin. Only mercury, gallium, and most of the alkali metals have lower melting points.^[8] Its boiling point is 2072 °C (3762 °F), higher than that of thallium, but lower than that of gallium, showing opposition to the melting points' trend. Indium thus has a very large liquid range of around 2000 °C. The density of indium, 7.31 g·cm⁻³, is also higher than that of gallium, but lower than that of thallium. Below its critical temperature of 3.41 K, indium becomes a superconductor. At standard temperature and pressure, indium crystallizes in the tetragonal crystal system in the space group I4/mmm (lattice parameters: a = 325 pm, c = 495 pm).^[8]

Indium metal does not react with water, but it is oxidized by stronger oxidizing agents, such as halogens to give indium(III) compounds. It does not form a boride, silicide or carbide, and the hydride InH₃ has only a transitory existence, even at low temperatures, before reverting to the elements.^[9]

Isotopes

Indium has 39 known isotopes, ranging in mass from 97 to 135. Only two isotopes occur naturally in primordial nuclides: indium-113, the only stable isotope, and indium-115, which has a half-life of 4.41×10¹⁴ years, four orders of magnitude longer than the age of the universe and nearly 50,000 times longer than that of natural thorium.^[10] Indium-115 makes up 95.7% of all indium.

The most stable artificial indium isotope is indium-111, which has half-life of approximately 2.8 days. All other isotopes have half-lives shorter than 5 hours. Indium also has 47 meta states; indium-114m1 is the most stable, being more stable than the ground state of any indium isotope other than the primordial ones.

Chemical compounds

Indium is a post-transition metal and chemically, is the intermediate element between its group 13 neighbors gallium and thallium.

Indium has 49 electrons, having an electronic configuration of [Kr]4d¹⁰5s²5p¹. In its compounds, indium most commonly loses its three outermost electrons, becoming indium(III) ions, In³⁺, but in some cases the pair of 5s-electrons do not ionize, resulting in indium(I), In⁺. The stabilization of the monovalent state is attributed to the inert pair effect, the stabilization of 5s-orbital due to relativistic effects, which are greater for heavier elements. Thallium (indium's heavier homolog) shows an even stronger effect, making oxidation to thallium(I) more likely than to thallium(III), making +1 the more likely oxidation state,^[11] whereas gallium (indium's lighter homolog) commonly shows only the +3 oxidation state. Thus, although thallium(III) is a moderately strong oxidizing agent, indium(III) is not, and indium(I) compounds are often powerful reducing agents.^[9]

A number of standard electrode potentials, depending on the reaction under study,^[12] are reported for indium:

−0.40	In^2+ + e^−	↔ In^+
−0.49	In^3+ + e^−	↔ In^2+
−0.443	In^3+ + 2 e^−	↔ In^+
−0.3382	In^3+ + 3 e^−	↔ In
−0.14	In^+ + e^−	↔ In

Indium(III) compounds

Indium(III) oxide forms when the element is burned in air or when the hydroxide or nitrate is heated. In₂O₃ adopts a structure like alumina. It is amphoteric, i.e., it can react with both acids and bases. Its reaction with water results in insoluble indium(III) hydroxide, which is also amphoteric, reacting with alkalis to give indates(III) and with acids to give indium(III) salts:

In(OH)₃ + 3 HCl → InCl₃ + 3 H₂O

Indium forms the expected trihalides. Chlorination, bromination, and iodination of In gives, respectively, colorless InCl₃ and InBr₃, and yellow InI₃. The compounds are Lewis acids, somewhat akin to the better known aluminium trihalides. Again like the related aluminium compound, InF₃ is polymeric.

See also Indium halides

Indium(I) compounds

Indium(I) compounds are not common. The chloride, bromide, and iodide are deeply colored, unlike the parent trihalides from which they are prepared. Other In(I) compounds include a sulfide. Indium(I) sulfide is the product of reaction between indium and sulfur or indium and hydrogen sulfide, and can be received at 700—1000 °C. Indium(I) oxide black powder is received at 850 °C during reaction between indium and carbon dioxide or during decomposition of indium(III) oxide at 1200 °C. Cyclopentadienylindium(I), which was the first organoindium(I) compound reported,^[13] is polymer consisting of zigzag chains of alternating indium atoms and cyclopentadienyl complexes.

Other oxidation states

Less frequently, indium forms compounds in oxidation state +2 and even fractional oxidation states. Usually such materials feature In–In bonding, most notably in halides, In₂X₄ and [In₂X₆]²⁻.^[14] Several other compounds are known to combine indium(I) and indium(III), such as In^I₆(In^IIICl₆)Cl₃,^[15] In^I₅(In^IIIBr₄)₂(In^IIIBr₆),^[16] In^IIn^IIIBr₄.^[14] In organic synthesis it is used for indium-mediated allylation.

Organoindium compounds

Organoindium compounds feature In-C bonds. Most are In(III) derivatives, but cyclopentadienylindium(I) is an exception. Perhaps best known is trimethylindium, In(CH₃)₃, which is used to prepare certain semiconducting materials.

Biological and medical functions

Indium has no metabolic role in any organism. In a similar way to aluminium salts, indium(III) ions can be toxic to the kidney when given by injection, but oral indium compounds do not have the chronic toxicity of salts of heavy metals, probably due to poor absorption in basic conditions. Radioactive indium-111 (in very small amounts on a chemical basis) is used in nuclear medicine tests, as a radiotracer to follow the movement of labeled proteins and white blood cells in the body.

Creation

The s-process acting in the range from silver to antimony.

Indium is created via the long-lasting, (up to thousands of years), s-process in low-to-medium mass stars (which range in mass between 0.6 and 10 solar masses). When a silver-109 atom (the isotope of which approximately half of all silver in existence is composed), catches a neutron, it undergoes a beta decay to become cadmium-110. Capturing further neutrons, it becomes cadmium-115, which decays to indium-115 via another beta decay. This explains why the radioactive isotope predominates in abundance compared to the stable one.^[17]

Occurrence

In Earth's crust, indium occurs only in the form of its compounds, except occasionally as rare grains of free metal of no commercial importance. Indium is 68th most abundant element in Earth's crust at approximately 160 ppb, making indium approximately as abundant as cadmium.^[18] Fewer than 10 indium minerals are known, such as dzhalindite (In(OH)₃) and indite (FeIn₂S₄),^[19] but none of these occurs in significant deposits.

Based on content of indium in zinc ore stocks, there is a worldwide reserve of approximately 6,000 tonnes of economically viable indium.^[20] However, the Indium Corporation, the largest processor of indium, claims that, on the basis of increasing recovery yields during extraction, recovery from a wider range of base metals (including tin, copper and other polymetallic deposits) and new mining investments, the long-term supply of indium is sustainable, reliable, and sufficient to meet increasing future demands.^[21] This conclusion may be reasonable considering that silver, which is one-third as abundant as indium in Earth's crust,^[22] is currently mined at approximately 18,300 tonnes per year,^[23] which is 40 times greater than current indium mining rates.

History

In 1863, the German chemists Ferdinand Reich and Hieronymous Theodor Richter were testing ores from the mines around Freiberg, Saxony. They dissolved the minerals pyrite, arsenopyrite, galena and sphalerite in hydrochloric acid and distilled raw zinc chloride. As it was known that ores from that region sometimes contain thallium they searched for the green emission lines with spectroscopy. The green lines were absent but a blue line was present in the spectrum. As no element was known with a bright blue emission they concluded that a new element was present in the minerals. They named the element with the blue spectral line indium, from the indigo color seen in its spectrum.^[24][25] That line was the first indication of an unknown element in zinc ores, and when the free metal was isolated in the following year it was named indium after the colour of the light that had provided a clue to its presence. Zinc ores remain the primary source of indium.

Richter went on to isolate the metal in 1864.^[26] At the World Fair 1867 an ingot of 0.5 kg (1.1 lb) was presented.^[27]

In 1924, indium was found to have a valued ability to stabilize non-ferrous metals, which was the first significant use for the element.^[28] It took until 1936 for the U.S. Bureau of Mines to list indium as a commodity, and even in the early 1950s only very limited applications for indium were known, the most important of which was making light-emitting diodes and coating bearings for aircraft engines during World War II. The start of production of indium-containing semiconductors started in 1952. The development and widespread use of indium-containing nuclear control rods increased demand during the 1970s, and the use of indium tin oxide in liquid crystal displays increased and became the major application by 1992.^{[29][30][31][32][33]}

Currently the demand for indium is driven by the manufacture of transparent electrodes from indium tin oxide (ITO). The electrodes are used in liquid crystal displays and touchscreens. The metal also is used in a wide range of alloys;^[34] one of its first large-volume applications was in high-performance bearing alloys (for aircraft) in WWII. It is also used for making particularly low melting point alloys, and is a component in some solders. One of its unusual attributes is that, like gallium, molten indium wets glass, so that it can be used as a solder in glass seals. It also is used in a wide range of electric and electronic roles, and has been used in superconducting alloys.

Production

World production trend^[35]

The lack of indium mineral deposits and the fact that indium is enriched in sulfidic lead, tin, copper, iron and predominately in zinc deposits, makes zinc production the main source for indium. The indium is leached from slag and dust of zinc production. Further purification is done by electrolysis. The exact process varies with the exact composition of the slag and dust.^[6][27]

Indium is produced mainly from residues generated during zinc ore processing but is also found in iron, lead, and copper ores.^[6] China is a leading producer of indium (390 tonnes in 2012), followed by Canada, Japan and South Korea with 70 tonnes each.^[36] The Teck Cominco refinery in Trail, British Columbia, is a large single-source indium producer, with an output of 32.5 tonnes in 2005, 41.8 tonnes in 2004 and 36.1 tonnes in 2003. South American Silver Corporation's Malku Khota property in Bolivia is a large resource of indium with an indicated resource of 1,481 tonnes and inferred resource of 935 tonnes.^[37] Adex Mining Inc.’s Mount Pleasant Mine in New Brunswick, Canada, holds some of the world’s total known indium resources.^[38]

The amount of indium consumed is largely a function of worldwide LCD production. Worldwide production in 2007 was 475 tonnes per year from mining and a further 650 tonnes per year from recycling.^[21] Demand has risen rapidly in recent years with the popularity of LCD computer monitors and television sets, which now account for 50% of indium consumption.^[39] Increased manufacturing efficiency and recycling (especially in Japan) maintain a balance between demand and supply. According to the UNEP, indium's end-of-life recycling rate is less than 1%.^[40] Demand increased as the metal is used in LCDs and televisions, and supply decreased when a number of Chinese mining concerns stopped extracting indium from their zinc tailings. In 2002, the price was US$94 per kilogram. The recent changes in demand and supply have resulted in high and fluctuating prices of indium, which from 2006 to 2009 ranged from US$382/kg to US$918/kg.

It has been estimated that there are fewer than 14 years left of indium supplies, based on current rates of extraction, demonstrating the need for additional recycling.^[41]

Applications

A magnified image of an LCD screen showing RGB pixels. Individual transistors are seen as white dots in the bottom part.

The first large-scale application for indium was as a coating for bearings in high-performance aircraft engines during World War II. Afterward, production gradually increased as new uses were found in fusible alloys, solders, and electronics. In the 1950s, tiny beads of it were used for the emitters and collectors of PNP alloy junction transistors. In the middle and late 1980s, the development of indium phosphide semiconductors and indium tin oxide thin films for liquid crystal displays (LCD) aroused much interest. By 1992, the thin-film application had become the largest end use.^[42][43]

Electronics

• Indium oxide (In₂O₃) and indium tin oxide (ITO) are used as a transparent conductive coating applied to glass substrates in the making of electroluminescent panels.^[44]
• Some indium compounds such as indium antimonide, indium phosphide,^[45] and indium nitride^[46] are semiconductors with useful properties.
• Indium is used in photovoltaics for the synthesis of the semiconductor copper indium gallium selenide (CIGS), which is used for the manufacture of CIGS solar cells, a type of second generation thin film solar cell.^[47]
• Used in light-emitting diodes (LEDs) and laser diodes based on compound semiconductors such as InGaN, InGaP that are fabricated by Metalorganic Vapor Phase Epitaxy (MOVPE) technology.^[48]
• Ultrapure metalorganics of indium include high purity trimethylindium (TMI), which is used as a precursor in III–V compound semiconductors, whereas it is also used as the semiconductor dopant in II–VI compound semiconductors.^[49]
• One of many substitutes for mercury in alkaline batteries to prevent the zinc from corroding and releasing hydrogen gas.^[50]

Metal and alloys

Ductile indium wire

• Very small amounts used in aluminium alloy sacrificial anodes (for salt water applications) to prevent passivation of the aluminium.
• To bond gold electrical test leads to superconductors, indium is used as a conducting adhesive and applied under a microscope with precision tweezers.^[51]
• In the form of a wire it is used as a vacuum seal and a thermal conductor in cryogenics and ultra-high-vacuum applications, for example, in manufacturing gaskets that deform to fill gaps.^[52]
• Used as a calibration material for differential scanning calorimetry.^[53]
• It is an ingredient in the gallium–indium–tin alloy galinstan, which is liquid at room temperature but not toxic like mercury.^[54]
• Indium soldering is the preferred method to create a stress-free electrically and thermally conducting bond between a ceramic sputter target and a target holder.^[55]

Other uses

• Indium tin oxide is used as a light filter in low-pressure sodium vapor lamps. The infrared radiation is reflected back into the lamp, which increases the temperature within the tube and therefore improves the performance of the lamp.^[43]
• Indium is a component of some dental amalgam alloys, in order to decrease the surface tension of the mercury and allow for less mercury and easier amalgamation.^[56]
• Indium's melting point of 429.7485 K (156.5985 °C) is a defining fixed point on the international temperature scale ITS-90.^[57]
• Indium's high neutron capture cross section for thermal neutrons makes it suitable for use in control rods for nuclear reactors, typically in an alloy containing 80% silver, 15% indium, and 5% cadmium.^[58]
• In nuclear engineering, the (n,n') reactions of ¹¹³In and ¹¹⁵In are used to determine magnitudes of neutron fluxes.^[59]
• Indium is also used as a thermal interface material by personal computer enthusiasts in the form of pre-shaped foil sheets fitted between the heat-transfer surface of a microprocessor and its heat sink. The application of heat partially melts the foil and allows the indium metal to fill in any microscopic gaps and pits between the two surfaces, removing any insulating air pockets that would otherwise compromise heat transfer efficiency.^[60]
• ¹¹¹In emits gamma radiation and is used in indium leukocyte imaging, or indium scintigraphy, a technique of medical imaging that is particularly helpful in differentiating conditions such as osteomyelitis from decubitus ulcers for assessment of route and duration of antibiotic therapy.^[61] Indium leukocyte scintigraphy has many applications, including early phase drug development, and the monitoring of activity of white blood cells. For the test, blood is taken from the patient, white cells removed, labeled with the radioactive ¹¹¹In, then re-injected back into the patient. Gamma imaging will then reveal any areas of on-going white-cell localization such as new and developing areas of infection.

Health issues

The health effects of exposure to Indium have been little studied. The EU does not consider it a chemical of "high concern". Indium tin oxide and indium phosphide have been shown to cause harm to the pulmonary and immune systems, predominantly through ionic indium.^[62] Mild eye irritation may result from exposure to its dust or vapor. Lab studies in animals have shown injection of indium salts may cause liver and kidney damage. Because of its rarity, little is known about its ecological fate, and bioaccumulation has not been ruled out.

Occupational health

People can be exposed to indium in the workplace by breathing it in, swallowing it, skin contact, and eye contact. The National Institute for Occupational Safety and Health has set a recommended exposure limit (REL) of 0.1 mg/m³ over an 8-hour workday.^[63]

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External links

• Indium at The Periodic Table of Videos (University of Nottingham)
• Reducing Agents > Indium low valent
• NIOSH Pocket Guide to Chemical Hazards (Centers for Disease Control and Prevention)