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Indium
Indium
Indium
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Indium, 49In
Indium
Pronunciation/ˈɪndiəm/ ⓘ ​(IN-dee-əm)
Appearancesilvery lustrous gray
Standard atomic weight Ar°(In)
Indium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Ga
↑
In
↓
Tl
cadmium ← indium → tin
Atomic number (Z)49
Groupgroup 13 (boron group)
Periodperiod 5
Block  p-block
Electron configuration[Kr] 4d10 5s2 5p1
Electrons per shell2, 8, 18, 18, 3
Physical properties
Phase at STPsolid
Melting point429.7485 K ​(156.5985 °C, ​313.8773 °F)
Boiling point2345 K ​(2072 °C, ​3762 °F)
Density (at 20° C)7.290 g/cm3[3]
when liquid (at m.p.)7.02 g/cm3
Triple point429.7445 K, ​~1 kPa[4]
Heat of fusion3.281 kJ/mol
Heat of vaporization231.8 kJ/mol
Molar heat capacity26.74 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 statescommon: +3
−5,[5] −2,[6] −1,[7] 0,[8] +1,[9] +2[9]
ElectronegativityPauling scale: 1.78
Ionization energies
  • 1st: 558.3 kJ/mol
  • 2nd: 1820.7 kJ/mol
  • 3rd: 2704 kJ/mol
Atomic radiusempirical: 167 pm
Covalent radius142±5 pm
Van der Waals radius193 pm
Color lines in a spectral range
Spectral lines of indium
Other properties
Natural occurrenceprimordial
Crystal structure ​body-centered tetragonal (tI2)
Lattice constants
Body-centered-tetragonal crystal structure for indium
a = 325.16 pm
c = 494.71 pm (at 20 °C)[3]
Thermal expansion32.2×10−6/K (at 20 °C)[a]
Thermal conductivity81.8 W/(m⋅K)
Electrical resistivity83.7 nΩ⋅m (at 20 °C)
Magnetic orderingdiamagnetic[10]
Molar magnetic susceptibility−64.0×10−6 cm3/mol (298 K)[11]
Young's modulus11 GPa
Speed of sound thin rod1215 m/s (at 20 °C)
Mohs hardness1.2
Brinell hardness8.8–10.0 MPa
CAS Number7440-74-6
History
Namingfor the indigo blue line in its spectrum
DiscoveryFerdinand Reich and Hieronymous Theodor Richter (1863)
First isolationHieronymous Theodor Richter (1864)
Isotopes of indium
Main isotopes[12] Decay
Isotope abun­dance half-life (t1/2) mode pro­duct
111In synth 2.8048 d ε 111Cd
113In 4.28% stable
115In 95.7% 4.41×1014 y β− 115Sn
 Category: Indium
| references

Indium is a chemical element; it has symbol In and atomic number 49. It is a silvery-white post-transition metal and one of the softest elements. Chemically, indium is similar to gallium and thallium, and its properties are largely intermediate between the two. It was discovered in 1863 by Ferdinand Reich and Hieronymous Theodor Richter by spectroscopic methods and named for the indigo blue line in its spectrum.[13]

Indium is used primarily in the production of flat-panel displays as indium tin oxide (ITO), a transparent and conductive coating applied to glass.[14] It is also used in the semiconductor industry, in low-melting-point metal alloys such as solders and soft-metal high-vacuum seals.[15] It is used in the manufacture of blue and white LED circuits, mainly to produce Indium gallium nitride p-type semiconductor substrates.[16] It is produced exclusively as a by-product during the processing of the ores of other metals, chiefly from sphalerite and other zinc sulfide ores.[17]

Indium has no biological role and its compounds are toxic when inhaled or injected into the bloodstream, although they are poorly absorbed following ingestion.[18][19]

Etymology

[edit]

The name comes from the Latin word indicum meaning violet or indigo.[20] The word indicum means "Indian", as the naturally based dye indigo was originally exported to Europe from India.

Properties

[edit]

Physical

[edit]
Indium wetting the glass surface of a test tube

Indium is a shiny silvery-white, highly ductile post-transition metal with a bright luster.[21] It is so soft (Mohs hardness 1.2) that it can be cut with a knife and leaves a visible line like a pencil when rubbed on paper.[22] It is a member of group 13 on the periodic table and its properties are mostly intermediate between its vertical neighbors gallium and thallium. As with tin, a high-pitched cry is heard when indium is bent – a crackling sound due to crystal twinning.[21] 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.[23] The boiling point is 2072 °C (3762 °F), higher than that of thallium, but lower than gallium, conversely to the general trend of melting points, but similarly to the trends down the other post-transition metal groups because of the weakness of the metallic bonding with few electrons delocalized.[24]

The density of indium, 7.31 g/cm3, is also greater than gallium, but lower than thallium. Below the critical temperature, 3.41 K, indium becomes a superconductor. Indium crystallizes in the body-centered tetragonal crystal system in the space group I4/mmm (lattice parameters: a = 325 pm, c = 495 pm):[23] this is a slightly distorted face-centered cubic structure, where each indium atom has four neighbours at 324 pm distance and eight neighbours slightly further (336 pm).[25] Indium has greater solubility in liquid mercury than any other metal (more than 50 mass percent of indium at 0 °C).[26] Indium displays a ductile viscoplastic response, found to be size-independent in tension and compression. However it does have a size effect in bending and indentation, associated to a length-scale of order 50–100 μm,[27] significantly large when compared with other metals.

Isotopes

[edit]
Main article: Isotopes of indium

Indium has 39 known isotopes, ranging in mass number from 97 to 135. Only two isotopes occur naturally as 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 greater than the age of the Universe and nearly 30,000 times greater than half-life of thorium-232.[28] The half-life of 115In is very long because the beta decay to 115Sn is spin-forbidden.[29] Indium-115 makes up 95.7% of all indium. Indium is one of three known elements (the others being tellurium and rhenium) of which the stable isotope is less abundant in nature than the long-lived primordial radioisotopes.[30]

The stablest artificial isotope is indium-111, with a half-life of approximately 2.8 days. All other isotopes have half-lives shorter than 5 hours. Indium also has 47 meta states, among which indium-114m1 (half-life about 49.51 days) is the most stable, more stable than the ground state of any indium isotope other than the primordial. All decay by isomeric transition. The indium isotopes lighter than 113In predominantly decay through electron capture or positron emission to form cadmium isotopes, while the indium isotopes heavier than 113In predominantly decay through beta-minus decay to form tin isotopes.[28]

Chemistry

[edit]

Indium has 49 electrons, with an electronic configuration of [Kr]4d105s25p1. In compounds, indium most commonly donates the three outermost electrons to become indium(III), In3+. In some cases, the pair of 5s-electrons are not donated, resulting in indium(I), In+. The stabilization of the monovalent state is attributed to the inert pair effect, in which relativistic effects lowers the energy of the 5s-orbital, observed in heavier elements. Thallium (indium's heavier homolog) shows an even stronger effect, manifested by the pervasiveness of thallium(I) vs thallium(III),[31] Gallium (indium's lighter homolog) is only rarely observed in the +1 oxidation state. Thus, although thallium(III) is a moderately strong oxidizing agent, indium(III) is not, and many indium(I) compounds are powerful reducing agents.[32] While the energy required to include the s-electrons in chemical bonding is lowest for indium among the group 13 metals, bond energies decrease down the group so that by indium, the energy released in forming two additional bonds and attaining the +3 state is not always enough to outweigh the energy needed to involve the 5s-electrons.[33] Indium(I) oxide and hydroxide are more basic and indium(III) oxide and hydroxide are more acidic.[33]

A number of standard electrode potentials, depending on the reaction under study,[34] are reported for indium, reflecting the decreased stability of the +3 oxidation state:[25]

In2+ + e− ⇌ In+ E0 = −0.40 V
In3+ + e− ⇌ In2+ E0 = −0.49 V
In3+ + 2 e− ⇌ In+ E0 = −0.443 V
In3+ + 3 e− ⇌ In E0 = −0.3382 V
In+ + e− ⇌ In E0 = −0.14 V

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. Indium is rather basic in aqueous solution, showing only slight amphoteric characteristics, and unlike its lighter homologs aluminium and gallium, it is insoluble in aqueous alkaline solutions.[35]

Indium(III) compounds

[edit]
See also: Indium chalcogenides and Category:Indium compounds
InCl3 (structure pictured) is a common compound of indium.

Hydrides and halides

[edit]

The hydride InH3 has at best a transitory existence in ethereal solutions at low temperatures. It polymerizes in the absence of bases.[32] Lewis bases stabilize a rich collection of indium hydrides of the formula LInH3 (L = tertiary phosphine and N-Heterocyclic carbenes).[36]

Chlorination, bromination, and iodination of In produce colorless InCl3, 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.[37]

Indium halides dissolves in water to give aquo complexes such as [Ir(H2O)6]3+ and [IrCl2(H2O)4]+. Similar complexes can be prepared from nitrates and acetates. Overall, the pattern is similar to that for aluminium(III).[36]

Chalcogenides and pnictides

[edit]

Indium derivatives of chalcogenides (O, S, Se, Te) are well developed. Indium(III) oxide, In2O3, forms when indium metal is burned in air or when the hydroxide or nitrate is heated.[38] The analogous sesqui-chalcogenides with sulfur, selenium, and tellurium are also known.[39]

The chemistry of indium pnictides (N, P, As, Sb) is also well known, motivated by their relevance to semiconductor technology. Direct reaction of indium metal with the pnictogens For applications in microelectronics, the P, As, and Sb derivatives are made by reactions of trimethylindium:

In(CH3)3 + H3E → InE + 3 CH4 (E = P, As, Sb)

Many of these derivatives are prone to hydrolysis.[40]

Indium(I) compounds

[edit]

Indium(I) compounds are not common. The chloride, bromide, and iodide are deeply colored, unlike the parent trihalides from which they are prepared. The fluoride is known only as an unstable gas.[41] Indium(I) oxide black powder is produced when indium(III) oxide decomposes upon heating to 700 °C.[38]

Compounds in other oxidation states

[edit]

Less frequently, indium forms compounds in oxidation state +2 and even fractional oxidation states. Usually such materials feature In–In bonding, most notably in the halides In2X4 and [In2X6]2−,[42] and various subchalcogenides such as In4Se3.[43] Several other compounds are known to combine indium(I) and indium(III), such as InI6(InIIICl6)Cl3,[44] InI5(InIIIBr4)2(InIIIBr6),[45] and InIInIIIBr4.[42]

Organoindium compounds

[edit]

Organoindium compounds feature In–C bonds. Most are In(III) derivatives, but cyclopentadienylindium(I) is an exception. It was the first known organoindium(I) compound,[46] and is polymeric, consisting of zigzag chains of alternating indium atoms and cyclopentadienyl complexes.[47] Perhaps the best-known organoindium compound is trimethylindium, In(CH3)3, used to prepare certain semiconducting materials.[48][49]

History

[edit]

In 1863, German chemists Ferdinand Reich and Hieronymus 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. Reich, who was color-blind, employed Richter as an assistant for detecting the colored spectral lines. Knowing that ores from that region sometimes contain thallium, they searched for the green thallium emission spectrum lines. Instead, they found a bright blue line. Because that blue line did not match any known element, they hypothesized a new element was present in the minerals. They named the element indium, from the indigo color seen in its spectrum, after the Latin indicum, meaning 'of India'.[50][51][52][53]

Richter went on to isolate the metal in 1864.[54] An ingot of 0.5 kg (1.1 lb) was presented at the World Fair 1867.[55] Reich and Richter later fell out when the latter claimed to be the sole discoverer.[53]

Occurrence

[edit]
yellow squares with red and blue arrows
The s-process acting in the range from silver to antimony

Indium is created by the long-lasting (up to thousands of years) s-process (slow neutron capture) in low-to-medium-mass stars (range in mass between 0.6 and 10 solar masses). When a silver-109 atom captures a neutron, it transmutes into silver-110, which then undergoes beta decay to become cadmium-110. Capturing further neutrons, it becomes cadmium-115, which decays to indium-115 by another beta decay. This explains why the radioactive isotope is more abundant than the stable one.[56] The stable indium isotope, indium-113, is one of the p-nuclei, the origin of which is not fully understood; although indium-113 is known to be made directly in the s- and r-processes (rapid neutron capture), and also as the daughter of very long-lived cadmium-113, which has a half-life of about eight quadrillion years, this cannot account for all indium-113.[57][58]

Indium is the 68th most abundant element in Earth's crust at approximately 50 ppb. This is similar to the crustal abundance of silver, bismuth and mercury. It very rarely forms its own minerals, or occurs in elemental form. Fewer than 10 indium minerals such as roquesite (CuInS2) are known, and none occur at sufficient concentrations for economic extraction.[59] Instead, indium is usually a trace constituent of more common ore minerals, such as sphalerite and chalcopyrite.[60][61] From these, it can be extracted as a by-product during smelting.[17] While the enrichment of indium in these deposits is high relative to its crustal abundance, it is insufficient, at current prices, to support extraction of indium as the main product.[59]

Different estimates exist of the amounts of indium contained within the ores of other metals.[62][63] However, these amounts are not extractable without mining of the host materials (see Production and availability). Thus, the availability of indium is fundamentally determined by the rate at which these ores are extracted, and not their absolute amount. This is an aspect that is often forgotten in the current debate, e.g. by the Graedel group at Yale in their criticality assessments,[64] explaining the paradoxically low depletion times some studies cite.[65][17]

Production and availability

[edit]
World production trend[66]

Indium is produced exclusively as a by-product during the processing of the ores of other metals. Its main source material are sulfidic zinc ores, where it is mostly hosted by sphalerite.[17] Minor amounts are also extracted from sulfidic copper ores. During the roast-leach-electrowinning process of zinc smelting, indium accumulates in the iron-rich residues. From these, it can be extracted in different ways. It may also be recovered directly from the process solutions. Further purification is done by electrolysis.[67] The exact process varies with the mode of operation of the smelter.[21][17]

Its by-product status means that indium production is constrained by the amount of sulfidic zinc (and copper) ores extracted each year. Therefore, its availability needs to be discussed in terms of supply potential. The supply potential of a by-product is defined as that amount which is economically extractable from its host materials per year under current market conditions (i.e. technology and price).[68] Reserves and resources are not relevant for by-products, since they cannot be extracted independently from the main-products.[17] Recent estimates put the supply potential of indium at a minimum of 1,300 t/yr from sulfidic zinc ores and 20 t/yr from sulfidic copper ores.[17] These figures are significantly greater than current production (655 t in 2016).[69] Thus, major future increases in the by-product production of indium will be possible without significant increases in production costs or price. The average indium price in 2016 was US$240/kg, down from US$705/kg in 2014.[70]

China is a leading producer of indium (290 tonnes in 2016), followed by South Korea (195 t), Japan (70 t) and Canada (65 t).[69] The Teck Resources 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.

The primary consumption of indium worldwide is LCD production. Demand rose rapidly from the late 1990s to 2010 with the popularity of LCD computer monitors and television sets, which now account for 50% of indium consumption.[71] 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%.[72]

Applications

[edit]

Industrial uses

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

In 1924, indium was found to have a valued property of stabilizing non-ferrous metals, and that became the first significant use for the element.[73] The first large-scale application for indium was coating bearings in high-performance aircraft engines during World War II, to protect against damage and corrosion; this is no longer a major use of the element.[67] New uses were found in fusible alloys, solders, and electronics. In the 1950s, tiny beads of indium 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.[74][75]

Indium(III) oxide and indium tin oxide (ITO) are used as a transparent conductive coating on glass substrates in electroluminescent panels.[76] 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 improves the performance of the lamp.[75]

Indium has many semiconductor-related applications. Some indium compounds, such as indium antimonide and indium phosphide,[77] are semiconductors with useful properties: one precursor is usually trimethylindium (TMI), which is also used as the semiconductor dopant in II–VI compound semiconductors.[49] InAs and InSb are used for low-temperature transistors and InP for high-temperature transistors.[67] The compound semiconductors InGaN and InGaP are used in light-emitting diodes (LEDs) and laser diodes.[78] Indium is used in photovoltaics as the semiconductor copper indium gallium selenide (CIGS), also called CIGS solar cells, a type of second-generation thin-film solar cell.[79] Indium is used in PNP bipolar junction transistors with germanium: when soldered at low temperature, indium does not stress the germanium.[67]

Ductile indium wire

Indium wire is used as a vacuum seal and a thermal conductor in cryogenics and ultra-high-vacuum applications, in such manufacturing applications as gaskets that deform to fill gaps.[80] Owing to its great plasticity and adhesion to metals, Indium sheets are sometimes used for cold-soldering in microwave circuits and waveguide joints, where direct soldering is complicated. Indium is an ingredient in the gallium–indium–tin alloy galinstan, which is liquid at room temperature and replaces mercury in some thermometers.[81] Other alloys of indium with bismuth, cadmium, lead, and tin, which have higher but still low melting points (between 50 and 100 °C), are used in fire sprinkler systems and heat regulators.[67]

Indium is one of many substitutes for mercury in alkaline batteries to prevent the zinc from corroding and releasing hydrogen gas.[82] Indium is added to some dental amalgam alloys to decrease the surface tension of the mercury and allow for less mercury and easier amalgamation.[83]

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 of 80% silver, 15% indium, and 5% cadmium.[84] In nuclear engineering, the (n,n') reactions of 113In and 115In are used to determine magnitudes of neutron fluxes.[85]

In 2009, Professor Mas Subramanian and former graduate student Andrew Smith at Oregon State University discovered that indium can be combined with yttrium and manganese to form an intensely blue, non-toxic, inert, fade-resistant pigment, YInMn blue, the first new inorganic blue pigment discovered in 200 years.[86]

Medical applications

[edit]

Radioactive indium-111 (in very small amounts) is used in nuclear medicine tests, as a radiotracer to follow the movement of labeled proteins and white blood cells to diagnose different types of infection.[87][88] Indium compounds are mostly not absorbed upon ingestion and are only moderately absorbed on inhalation; they tend to be stored temporarily in the muscles, skin, and bones before being excreted, and the biological half-life of indium is about two weeks in humans.[89] It is also tagged to growth hormone analogues like octreotide to find growth hormone receptors in neuroendocrine tumors.[90]

Biological role and precautions

[edit]
Indium
Hazards
GHS labelling:
GHS07: Exclamation mark
Warning
H302, H312, H315, H319, H332, H335
P261, P280, P305+P351+P338[91]
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
2
0
0

Indium has no metabolic role in any organism. According to one overview "no evidence of any health hazard from industrial use of indium."[92]

Notes

[edit]

References

[edit]

Sources

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Indium

View on Grokipedia
from Grokipedia
Indium is a with the symbol In and 49, classified as a silvery-white that exhibits properties intermediate between typical metals and metalloids. It is one of the softest non-radioactive metals, with a brilliant luster, high , and a distinctive high-pitched cry when bent, and it wets glass surfaces similar to mercury. Chemically, indium is amphoteric, dissolving in both acids to form indium salts and in concentrated alkalies to produce indates, and it has a low of 156.6 °C (313.9 °F) and a of 2080 °C (3776 °F), with a of 7.31 g/cm³ at . Discovered in 1863 by German chemists Ferdinand Reich and Hieronymus Theodor Richter at the School of Mines while spectroscopically analyzing zinc ore samples, indium was named for the prominent indigo-blue line observed in its atomic . The element's is 114.818, and it has 39 known isotopes, of which two are stable: indium-113 (4.3% natural abundance) and indium-115 (95.7% natural abundance). Although rare in the at an abundance of approximately 0.25 parts per million—more common than silver or mercury—it does not occur as a free metal and is primarily extracted as a by-product from the processing of , lead, and ores, particularly . Global production of refined indium reached about 1,080 metric tons in 2024, with accounting for 70% of output, and the consuming an estimated value of $85 million worth as of 2024, driven by demand in ; U.S. prices rose 42% to $340 per kg amid growing needs for and AI technologies. Indium's key applications leverage its transparency, conductivity, and low-melt characteristics, notably in (ITO) coatings for displays (LCDs), touchscreens, and solar panels, which consume around 90% of supply; it is also used in solders, semiconductors, low-melting-point alloys, and emerging technologies. Due to its scarcity and critical role in high-tech industries, indium is classified as a , with from becoming increasingly important to meet growing demand.

Characteristics

Physical properties

Indium is a silvery-white, lustrous that appears soft and malleable, easily cut with a due to its low hardness. The of indium is 7.31 g/cm³ at 20°C, and unlike but like most metals, it exhibits contraction upon solidification, resulting in a volume decrease of approximately 2.5%. Indium has a low for a metal at 156.60 °C and a high of 2072 °C, contributing to its wide liquidus range. In its solid form, indium adopts a body-centered tetragonal with I4/mmm and lattice parameters a = 3.252 and c = 4.946 . Key thermal properties include a of 0.233 J/g·K and a conductivity of 81.8 W/m·K at . Electrically, indium is a good conductor with an electrical conductivity of 11.6 × 10⁶ S/m at 20°C, corresponding to a resistivity of 8.6 × 10⁻⁸ Ω·m. Indium is diamagnetic, exhibiting a negative of -64.0 × 10⁻⁶ cm³/mol at 298 K. In binary alloys, indium commonly forms eutectic systems and solid solutions, with phase diagrams showing peritectic and behaviors depending on the alloying element, often resulting in depressed melting temperatures.

Chemical properties

Indium (In) is a in of the periodic table, with 49 and positioned in period 5. Its is [Kr] 4d^{10} 5s^2 5p^1, reflecting the filling of the 5p subshell typical for p-block elements in this group. Indium exhibits an of 1.78 on the Pauling scale, indicating moderate electron-attracting ability compared to other group 13 elements. The first is 558.3 kJ/mol, while the second is significantly higher at 1820.7 kJ/mol, highlighting the energy required to remove successive electrons from the neutral atom and In⁺ ion, respectively. The predominant of indium is +3, consistent with its group valence, though +1 and +2 states are accessible due to the , where the 5s² electron pair becomes increasingly reluctant to participate in bonding down the group. The for the In³⁺/In couple is -0.342 V, signifying that indium is a moderately strong relative to the . Indium displays moderate reactivity, reacting directly with halogens to form trihalides and with oxygen at elevated temperatures to yield indium(III) oxide, though it remains stable in ambient air owing to a protective oxide layer that forms on the surface. It dissolves readily in non-oxidizing acids such as hydrochloric and sulfuric acid, but resists nitric acid due to surface passivation by the oxide film. In coordination chemistry, indium(III) commonly adopts octahedral geometry in six-coordinate complexes or tetrahedral arrangements in four-coordinate species, influenced by ligand field effects and steric factors. Thermodynamically, elemental indium in its has a formation ΔH_f° of 0 kJ/mol and a S° of 57.65 J/mol· at 298 , providing baseline values for assessing reaction spontaneity involving the metal.

Isotopes

Indium has approximately 40 known isotopes, with mass numbers ranging from 97 to 135, including both ground states and metastable isomers. Only two isotopes occur naturally and are considered : indium-113 (¹¹³In) and indium-115 (¹¹⁵In), with natural abundances of 4.29(5)% and 95.71(5)%, respectively. These abundances result in a for indium of 114.818(1) u. Both stable isotopes exhibit nuclear spins of 9/2⁺, reflecting their odd-neutron configuration in the . Although ¹¹⁵In is classified as stable, it undergoes extremely slow to tin-115 with a of 4.41 × 10¹⁴ years. Among the radioactive isotopes, examples include ¹¹¹In, which decays primarily by to cadmium-111 with a of 2.80 days, and ¹¹⁴In, which undergoes to tin-114 with a of 71.9 seconds for the . The long-lived metastable isomer ¹¹⁴ᵐIn has a of 49.51 days and decays via isomeric transition (95.7%) or (4.3%) to the of ¹¹⁴In. Radioactive isotopes of indium are produced artificially, often through . For instance, ¹¹⁴ᵐIn is generated via the reaction ¹¹³In(n,γ)¹¹⁴ᵐIn in nuclear reactors, leveraging the 4.29% natural abundance of ¹¹³In as a target. These radioisotopes find applications in , such as monitoring and as tracers in material science studies, due to their well-characterized decay properties.

History

Etymology

The name "indium" originates from the Latin word indicum, meaning "indigo," referring to the prominent indigo-blue spectral lines observed in its emission spectrum during its identification in 1863. This naming convention was common in 19th-century spectroscopy, where newly discovered elements were often designated based on distinctive colors in their spectral signatures, as seen with elements like rubidium (from Latin rubidus, meaning "red") and caesium (from Latin caesius, meaning "sky blue"). The element was discovered by German chemists Ferdinand Reich and Hieronymus Theodor Richter while analyzing zinc ores, and they proposed the name to reflect these characteristic lines, distinguishing it from unrelated geographic associations like India. The "In" directly derives from "indium," following the standard practice established by in their publication and later formalized in international nomenclature. The term indicum itself traces to the ancient derived from plants, symbolizing the deep blue hue that became emblematic of the element's spectroscopic identity.

Discovery and development

Indium was discovered in by German chemists Ferdinand Reich and Hieronymus Theodor Richter at the Freiberg Mining Academy while spectroscopically examining samples of zinc blende ore from the Himmelfürst mine. Reich, who was color-blind, relied on Richter to interpret the , where they observed prominent indigo-blue emission lines indicating a previously unknown element. This finding occurred amid a surge in spectrochemical discoveries during the mid-19th century, building on techniques pioneered by and , and following closely the 1861 identification of by . Richter subsequently isolated metallic indium in 1864 through electrolytic reduction, producing a small quantity of the soft, silvery-white metal from an of indium . By 1867, they had refined the process to prepare a larger sample, presenting a 0.5 kg at the World Fair, which demonstrated the element's malleability and luster. Early characterization efforts confirmed indium as a distinct element in group 13 of the periodic table, with Richter initially estimating its atomic weight at 75.6 based on assuming a divalent (InCl₂); this was corrected in the 1870s to approximately 113.4 after recognizing its trivalent nature (InCl₃), through analyses by chemists using gravimetric methods on indium halides. Commercial production of indium began in 1934 with the founding of the Indium Corporation of America. In the , it began to be incorporated into low-melting alloys, marking its transition from laboratory curiosity to industrial material. Postwar advancements in the mid-20th century recognized indium's potential in electronic applications, particularly in research.

Occurrence and production

Natural occurrence

Indium occurs at low levels in the cosmos, with an estimated abundance of 0.3 by weight, comparable to that of silver at 0.6 ppb. In , indium is present at an average concentration of 0.25 parts per million, ranking it as the 49th most abundant element. It is predominantly dispersed as a in ores, especially ((Zn,Fe)S), where it substitutes for via isomorphous replacement and can reach concentrations up to 1 wt%. Although indium rarely forms independent deposits, it is found in primary minerals such as roquesite (CuInS₂) and dzhalindite (In(OH)₃), with pure indium occurrences being exceptionally scarce. It is chiefly associated with minerals in polymetallic ores of , lead, and , serving as a in these systems. Exploration efforts have recently uncovered new potential sources. In , the Magno in has shown anomalous indium in historical zinc-lead-silver deposits, indicating untapped reserves. In , 2024 discoveries at the Orient in by Iltani Resources have delineated what may be the country's largest silver-indium deposit, with high-grade mineralization that could expand known reserves. Extraterrestrially, indium has been identified in meteorites and lunar samples, with concentrations ranging from 3 to 60 in the latter.

Extraction and refining

Indium is primarily obtained as a of , accounting for approximately 90% of global production. During zinc ore processing, indium concentrates in residues such as slags, dusts, and fumes generated from roasting and smelting ores. The standard industrial process begins with leaching these zinc residues using , which dissolves indium along with other metals into solution. Indium is then selectively precipitated from the as indium hydroxide, In(OH)₃, by adjusting the with a base such as . This precipitate is redissolved in to form indium , In₂(SO₄)₃, solution. The purified indium is subjected to in an acidic , typically using aluminum cathodes, to deposit high-purity indium metal with yields exceeding 99.99%. Alternative methods for indium recovery include solvent extraction using di(2-ethylhexyl) (D2EHPA) as the extractant in an organic phase, which selectively separates indium from impurities in the leach solution, followed by stripping and precipitation. Another approach is cementation, where dust is added to the acidic solution to reduce and precipitate indium metal selectively. Further refining of the electrolytic indium to ultrahigh purity (up to 99.9999%) for applications employs , where a narrow molten zone is passed along the indium to segregate impurities to one end, or , which exploits indium's volatility to separate it from non-volatile impurities. Companies involved in refining high-purity indium for semiconductors include Indium Corporation (USA), Vital Materials, and others such as 5N Plus and Umicore. However, the primary production and much of the refining capacity remain heavily tied to China. Typical recovery rates from residues range from 80% to 95%, depending on the residue type and process efficiency. Recent advances since 2020 include optimized hydrometallurgical techniques, such as oxidative pressure leaching and enhanced solvent extraction systems, enabling efficient recovery from low-grade ores and wastes with indium concentrations below 100 ppm. Global indium production reached 1,020 metric tons in 2023, with an estimated 1,080 metric tons in 2024 driven by increased recovery from zinc processing and recycling efforts. China dominates as the top producer, accounting for about 70% of global output in 2024, followed by South Korea (approximately 20%) and Japan (6%) through refinery operations tied to electronics manufacturing, with much of the refining capacity, particularly for high-purity indium used in semiconductors, heavily tied to China. Recycling supplies a significant portion of indium availability, primarily from indium tin oxide (ITO) scrap generated in display and semiconductor production, with major recovery activities in Japan and South Korea. Quantitative estimates of world reserves are not available, as indium is mainly recovered as a from ores where it occurs at concentrations from less than 1 to 100 ppm. Recent discoveries have bolstered supply potential, including high-grade deposits in Australia's Orient project, identified as the country's largest silver-indium resource, and new exploration sites in such as the Magno project. As of November 2025, indium prices were approximately $350-370 per kilogram, following an increase from the 2024 average of $340 per kilogram, influenced by China's export controls on indium and related products implemented on February 4, 2025, to safeguard and resources. These controls exacerbate risks stemming from heavy dependence on China, which supplies over 70% of global indium and 25% of U.S. imports, prompting initiatives to enhance from ITO scrap and diversify sourcing. Looking ahead, production is forecasted to grow, fueled by rising in and , though potential shortages loom due to the sector's expansion outpacing supply diversification.

Compounds

Indium(III) compounds

Indium(III) compounds represent the most stable and prevalent class of indium derivatives, featuring the metal in its +3 oxidation state, which dominates due to indium's group 13 position and electronic configuration. These compounds exhibit diverse structures, ranging from simple binary salts to coordination complexes, and display properties such as amphoterism, solubility variations, and utility in materials synthesis. Synthesis often involves direct reaction of indium metal with the corresponding acid or oxidizing agent, followed by precipitation or evaporation, while their reactivity includes hydrolysis and coordination with ligands forming octahedral geometries typical for d^{10} In(III) centers. Indium(III) oxide, In₂O₃, is a key compound prepared by calcination of indium(III) hydroxide or carbonate at high temperatures around 800–1000 °C, yielding a yellow to white powder with a cubic bixbyite structure. It is amphoteric, dissolving in acids to form In³⁺ salts and in strong bases to produce indiumate ions like [In(OH)₄]⁻, which underscores its intermediate electronegativity. Widely used as a precursor for other indium compounds and in transparent conductive films, In₂O₃ has a direct band gap of approximately 3.6 eV, making it a wide-bandgap semiconductor suitable for optoelectronic applications. Indium(III) halides, InX₃ where X = Cl, Br, or I, are hygroscopic solids synthesized by direct combination of indium with the or via metathesis reactions. In the solid state, they adopt layered structures with octahedral InX₆ units, but in the gas phase, they form dimeric In₂X₆ species with bridging halides, while monomeric InX₃ units exhibit trigonal planar geometry around indium. These halides are prone to , reacting with water to form indium oxychlorides (e.g., InOCl) or basic salts, a property exploited in analytical separations but requiring conditions for handling. Indium(III) sulfate, In₂(SO₄)₃, is obtained by dissolving indium in and crystallizing the product, resulting in a colorless, highly water-soluble salt ( ~539 g/L at 20 °C) with a monoclinic . Its exceptional and stability in aqueous solutions make it valuable as a hardening agent in baths and as a source for indium salts in synthesis. The compound remains stable under typical processing conditions but decomposes upon strong heating to indium oxide and oxides. Coordination compounds of In(III) often feature six-coordinate octahedral geometries due to the ion's preference for high coordination numbers. A representative example is tris(acetylacetonato)indium(III), [In(acac)₃], where three bidentate acetylacetonate ligands chelate the indium center, forming a propeller-like structure with In–O bond lengths around 2.1 , confirmed by and NMR studies. These complexes are typically prepared by reacting InX₃ with the ligand in the presence of base and serve as volatile precursors for of indium-containing films. A prominent reaction of In(III) compounds is the hydrolysis of the In³⁺ ion in , leading to of indium(III) , In(OH)₃, as a white gelatinous solid:
\ceIn3++3OH−⇌In(OH)3(s)\ce{In^{3+} + 3OH^- ⇌ In(OH)3 (s)}
This process is governed by a very low product constant, Ksp=1.3×10−33K_{sp} = 1.3 \times 10^{-33} at 25 °C, indicating extremely low (~10^{-11} M) and enabling quantitative for purification. The can be dehydrated to In₂O₃ and is amphoteric, redissolving in excess base. In , indium is detected through methods, such as forming In(OH)₃ or indium (In₂S₃) for , or via spectroscopic techniques like (AAS) and (ICP-MS), which offer detection limits down to ppb levels in complex matrices such as alloys or environmental samples. Spectrophotometric methods using chromogenic agents, like 1-(2-pyridylmethylideneamine)-3-(salicylideneamine), provide sensitive colorimetric detection at 450 nm for trace indium in alloys.

Lower oxidation states

Indium in lower oxidation states, particularly +1 and +2, exhibits reduced stability compared to the +3 state due to the , which becomes more pronounced in heavier elements but still renders these states prone to oxidation and in indium. This effect stabilizes the 5s² , favoring lower valences, yet environmental factors like moisture or air often lead to conversion to the more stable trivalent form. Indium(I) compounds, such as InCl, adopt a pyramidal molecular geometry in certain complexes but are highly unstable, readily undergoing disproportionation according to the reaction 3InCl → InCl₃ + 2In. Indium(I) oxide (In₂O) forms as a black powder, typically obtained by thermal decomposition of indium(III) oxide. Preparation of indium(I) halides generally involves the reduction of the corresponding indium(III) halide (InX₃) with metallic indium under controlled conditions to minimize further reaction. Indium(II) compounds are rare in pure form, with InCl₂ actually representing a mixed-valence species best described as [In(I)][In(III)Cl₄], featuring both +1 and +3 indium centers rather than a true +2 state. This mixed-valence character is confirmed through spectroscopic methods, including electron spin resonance (ESR), which reveals interactions between the distinct indium sites. A notable example of an indium(I) compound is In₂S, the indium(I) sulfide, which displays properties suitable for potential optoelectronic applications.

Organoindium compounds

Organoindium compounds encompass a class of organometallic characterized by direct indium-carbon bonds, predominantly featuring indium in the +3 . These compounds exhibit diverse reactivity due to the Lewis acidity of the indium center and are valued in synthetic chemistry for their ability to facilitate carbon-carbon bond formations. Representative types include triorganoindium of the general R₃In, which adopt a trigonal planar in the monomeric form (no on In(III)); in the solid state, compounds like trimethylindium form tetramers with bridging methyl groups. A notable example is trimethylindium, (CH₃)₃In, a colorless solid that is highly pyrophoric and ignites spontaneously in air, necessitating inert atmosphere handling. Another important category comprises indium alkoxides, such as homoleptic tris(alkoxides) In(OR)₃ or mixed organoindium alkoxides like R₂InOR', which often form dimeric structures through indium-oxygen bridging. Synthesis of triorganoindium compounds typically proceeds via transmetalation reactions between indium(III) chloride and Grignard reagents, as illustrated by the equation: 3RMgBr+InCl3→R3In+3MgBrCl3 \mathrm{RMgBr + InCl_3 \rightarrow R_3In + 3 MgBrCl} This method yields air-sensitive products that must be isolated under anhydrous conditions. Indium alkoxides are commonly prepared by alcoholysis of indium halides or amides, resulting in compounds that are soluble in organic solvents and prone to oligomerization via In-O bonds. These organoindium species are inherently air-sensitive, decomposing in the presence of oxygen or moisture, and display pronounced Lewis acidity attributable to the vacant p-orbital on trivalent indium, enabling coordination with Lewis bases to form stable adducts. The +3 oxidation state predominates due to the relative stability of In-C bonds, which exhibit moderate strength compared to lighter group 13 analogs. Many of these compounds tend to oligomerize, particularly the alkoxides, forming dimers or higher aggregates to satisfy the coordination preferences of indium. In , organoindium compounds act as effective catalysts or for C-C bond formation, notably in allylation reactions of carbonyl compounds to produce homoallylic alcohols with high selectivity. For instance, trialkylindium promote chemoselective transfer of organic groups in palladium-catalyzed cross-couplings. Additionally, they serve as volatile precursors in (CVD) processes for fabricating indium-containing thin films, such as (hfac)In(CH₃)₂ for indium metal deposition. Post-2000 developments have highlighted indium-mediated Barbier reactions, where allylindium species, generated from indium metal and allyl halides, enable efficient allylations in aqueous media, offering eco-friendly alternatives to traditional organometallic methods with improved stereocontrol.

Applications

Alloys and solders

Indium is widely utilized in low-melting alloys due to its ability to form eutectic compositions with significantly depressed melting points, enabling applications requiring thermal management and mechanical bonding at moderate temperatures. These alloys exploit indium's inherent , which allows for deformation without fracture, and its resistance, which protects against oxidation in humid or cryogenic environments. Additionally, indium's relatively low toxicity compared to traditional fusible metals like or lead makes it suitable for safer handling and end-use. A prominent example of a low-melting indium is , composed of 51% indium, 32.5% , and 16.5% tin, which exhibits a eutectic of 62°C. This serves as a lead- and cadmium-free alternative to historical fusible metals, such as traditional , and is employed in fusible plugs for pressure relief in safety devices, where it melts to release excess heat or pressure. In , indium-based alloys provide reliable seals for vacuum systems and superconducting components, leveraging their low and ability to maintain integrity at temperatures near . Indium is also incorporated into dental amalgams at low concentrations (typically 1-5%) to reduce mercury vapor release during setting and improve long-term stability, enhancing the safety and performance of restorative fillings. In soldering applications, indium forms lead-free alternatives to traditional tin-lead solders, driven by regulatory shifts like the European Union's RoHS directive implemented in 2006, which restricted hazardous substances including lead in manufacturing. A key example is the indium-tin eutectic alloy (52% In, 48% Sn), which melts at 118°C and offers excellent wettability on ceramics and glasses while providing superior for flexible joints. Another notable solder is the indium-silver eutectic (97% In, 3% Ag), with a of 143°C, valued for its sharp melting behavior and resistance to creep under thermal cycling. These phase diagrams highlight indium's role in achieving precise eutectic points, allowing solders to transition from solid to liquid without a pasty phase, which minimizes defects in bonding.

Electronics and optoelectronics

Indium plays a pivotal role in and through its incorporation into transparent conductive oxides and compound semiconductors. Indium tin oxide (ITO), composed of approximately 90% In2O3In_2O_3 and 10% SnO2SnO_2 by weight, serves as a leading transparent conductor due to its high optical transmittance of about 90% in the and electrical conductivity ranging from 10410^4 to 10510^5 S/cm. These properties enable ITO thin films, typically deposited via , to function as electrodes in devices requiring both transparency and conductivity, such as touchscreens and displays (LCDs). In semiconductor applications, indium is essential for III-V compounds like (InP) and (InAs), which exhibit direct band gaps of 1.34 eV and 0.36 eV, respectively, at room temperature. These materials are doped with indium to form high-performance structures used in light-emitting diodes (LEDs) and lasers, where InP-based alloys like AlGaInP enable efficient emission in the visible range, while InAs supports mid-infrared operation. Indium's utility extends to , particularly in (CIGS) thin-film solar cells, where it forms the absorber layer to achieve high efficiency. Record lab-scale efficiencies for CIGS cells reached 23.6% in 2024, facilitated by optimized grading to enhance and fill factor. The majority of global indium consumption—accounting for over 50%—supports ITO production for flat-panel displays, including LCDs and touchscreens, underscoring indium's dominance in . Emerging applications leverage (InP) in for and networks, where InP-based heterojunction bipolar transistors (HBTs) and high-electron-mobility transistors (HEMTs) enable operation at frequencies exceeding 100 GHz due to superior . Additionally, quantum dots (InP QDs) are gaining traction in for displays and LEDs, offering tunable emission from blue to near-infrared with quantum yields approaching unity, positioning them as eco-friendly alternatives to cadmium-based materials. Market demand for indium in and is projected to grow at a (CAGR) of 4.5% from 2024 to 2030, driven by expansions in AI-enabled devices, , and high-resolution displays.

Other industrial uses

Indium oxide and indium-tin oxide (ITO) coatings are applied to surfaces for glazing applications, providing resistance and reflection while maintaining high visible light transmittance. These coatings are particularly used on mirrors and windows to enhance durability against and to deflect heat without obstructing visibility. In mechanical applications, indium plating is employed on bearings to achieve low and improved wear resistance, especially in high-performance environments like engines. The soft metal properties of indium allow it to form a lubricating layer under load, reducing and extending component life in demanding conditions such as systems. Ion-plated indium coatings have demonstrated friction coefficients comparable to while offering superior antigalling protection over traditional electroplated alternatives. As a thermal interface material, indium foil is utilized between heat-generating components and sinks to improve heat dissipation, particularly in LED assemblies where efficient thermal management prevents performance degradation. Its high thermal conductivity—approximately 86 W/m·K—and enable conformal contact without requiring additional adhesives, ensuring reliable operation under varying thermal loads. Indium-based compounds, such as ITO, are incorporated into anti-reflective coatings for optical lenses to minimize surface reflections and maximize light transmission across visible wavelengths. These coatings reduce glare and improve clarity in precision by combining indium's transparency with layers, achieving as low as 0.5% in targeted bands. In niche safety applications, low-melting-point alloys containing indium are used in fire-sprinkler systems to enable precise thermal activation. These fusible alloys, with melting points as low as 47°C, melt in response to fire heat, releasing mechanisms without reliance on electrical power and ensuring rapid response in commercial and industrial settings.

Medical and biological applications

Indium-111 (¹¹¹In) is widely employed in radiopharmaceuticals for diagnostic imaging, particularly in somatostatin receptor scintigraphy, where it is chelated to octreotide (pentetreotide) to visualize neuroendocrine tumors and other somatostatin receptor-positive lesions. This isotope has a physical half-life of 2.8 days and decays by electron capture, emitting gamma photons at 171 keV (90.7% abundance) and 245 keV (94.1% abundance), which are suitable for detection with gamma cameras in single-photon emission computed tomography (SPECT) imaging. The typical administered dose for ¹¹¹In-octreotide scans ranges from 111 to 222 MBq (3 to 6 mCi), enabling high-resolution tumor localization with minimal radiation burden. In cancer therapy, indium isotopes such as ¹¹⁴mIn and ¹¹¹In have been explored in chelate complexes for targeted radionuclide treatment, leveraging their emission profiles for radiotherapy. For instance, ¹¹⁴mIn-labeled lymphocytes have demonstrated antitumor effects in clinical trials for chronic lymphocytic leukemia by delivering beta particles and Auger electrons to malignant cells. Similarly, ¹¹¹In chelates serve as Auger electron emitters in experimental targeted therapies, where the short-range electrons cause DNA damage in tumor cells upon binding to specific ligands like monoclonal antibodies. These approaches aim to enhance specificity and reduce off-target effects compared to traditional external beam radiation. Indium(III) complexes exhibit properties and have been incorporated into wound dressings as alternatives to silver-based agents, particularly in eutectic gallium-indium (EGaIn) alloys integrated into hydrogels. These hybrids disrupt bacterial membranes through ion release and , promoting control in chronic wounds without the risk of associated with silver. Historically, indium was added to dental amalgams in the late to reduce mercury vapor release and improve handling, with concentrations up to 10% lowering during mixing and enhancing restoration durability. Recent advancements include 2024 research on indium nanoparticles and complexes for in , where they facilitate targeted release of chemotherapeutics via to tumor-specific carriers, improving and minimizing systemic toxicity in preclinical models.

Health, safety, and environmental impact

Biological role

Indium has no known biological role in living organisms and is not considered an essential element for , animals, or humans. It does not participate in any enzymatic processes or serve as a cofactor in biochemical pathways, distinguishing it from trace elements like or iron that are vital for metabolic functions. Studies in model organisms, such as rats and , have shown no observable deficiency symptoms upon exclusion of indium from their environments, further confirming its non-essential status. Bioavailability of indium is limited, with oral absorption rates typically below 1% in animal models, primarily due to its poor in gastrointestinal fluids. Once absorbed, indium tends to accumulate in soft tissues, particularly the liver and kidneys, where it distributes relatively evenly among major organs but at low concentrations. This uptake pattern reflects its ionic behavior rather than any functional integration into biological systems. Due to chemical similarities in and coordination preferences, indium can potentially mimic or at certain metal-binding sites in proteins, acting as a disruptor by competing for these positions without fulfilling physiological roles. Its rarity in the stems from low crustal abundance, estimated at 0.25 parts per million, which limits natural exposure and evolutionary incorporation into organisms. Trace levels occur in at typically 0.05–0.15 pmol/L (approximately 0.006–0.017 ng/L) in the .

Toxicity and precautions

Indium compounds demonstrate low acute oral , with reported LD50 values exceeding 3000 mg/kg in rats, indicating minimal risk from under normal circumstances. In contrast, acute of indium fumes or dust can lead to severe respiratory irritation and potentially , necessitating immediate medical intervention if exposure occurs. Chronic exposure to respirable indium compounds, particularly (ITO) dust, is linked to indium lung disease, a condition involving lung , , and , with cases emerging among industrial workers since the early 2000s. This progressive disorder can impair lung function irreversibly, highlighting the importance of monitoring long-term occupational exposure. Regarding carcinogenicity, the International Agency for Research on Cancer (IARC) classifies (ITO), of which indium oxide (In₂O₃) is the primary component, as Group 2B—possibly carcinogenic to humans—based on sufficient evidence in experimental animals for lung tumors via , though human data remain limited. To mitigate risks, occupational exposure limits for indium compounds (measured as In) are set at 0.1 mg/m³ as an 8-hour time-weighted average by the National Institute for Occupational Safety and Health (NIOSH). Safety precautions include handling materials in fume hoods or well-ventilated areas, using such as NIOSH-approved respirators, gloves, and protective clothing, and implementing to minimize dust generation. In case of exposure, affected individuals should be moved to , given oxygen if breathing is difficult, and receive prompt evaluation; for ingestion, avoid inducing vomiting and seek poison control assistance immediately. Notable case studies from Japanese ITO production facilities in the 2010s documented clusters of workers developing severe indium lung disease, including progressive that progressed to and necessitated in advanced instances. These outbreaks underscored the link between cumulative ITO dust and irreversible damage, prompting enhanced regulatory surveillance in high-risk industries.

Environmental considerations

Indium, primarily occurring as the soluble In³⁺ in acidic conditions, exhibits moderate mobility in aquatic environments, where it can in organisms such as and at low concentrations typically ranging from 0.01 to 15 pmol/kg in rivers and oceans. This potential is heightened in areas affected by (AMD), where indium is mobilized from minerals during oxidative weathering, though overall environmental levels remain dilute due to its geochemical scarcity. Major pollution sources include tailings, which release indium through processes, and from displays, with displays (LCDs) containing approximately 50–100 mg of indium per panel in the form of (ITO) coatings. Improper disposal of e-waste exacerbates localized contamination in landfills and water bodies, while operations contribute to broader risks near extraction sites. Recycling efforts focus on recovering indium from ITO scrap via hydrometallurgical acid leaching, achieving efficiencies exceeding 95% under optimized conditions such as using hydrochloric or sulfuric acid. Urban mining from e-waste holds significant potential, estimated to yield several hundred tonnes annually globally, helping to offset reliance on primary production. Recent 2024 advances, including integrated membrane separation and bioleaching techniques reviewed in scientific literature, enhance recovery rates from LCDs and reduce the need for virgin indium supplies. Regulatory frameworks address these issues, with the European Union's REACH regulation requiring registration and evaluation of indium compounds like ITO used in electronics to mitigate environmental releases. The U.S. Geological Survey designates indium as a , emphasizing supply risks and prompting policies for sustainable sourcing. Overall, indium's global environmental footprint is low due to limited production volumes (approximately 1,080 metric tons per year as of 2024), but localized risks from persist near areas, necessitating targeted remediation.

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