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]
The name comes from theLatin wordindicum meaningviolet orindigo.[20] The wordindicum means "Indian", as the naturally based dyeindigo was originally exported to Europe fromIndia.
Indium is a shiny silvery-white, highlyductilepost-transition metal with a brightluster.[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 ofgroup 13 on theperiodic table and its properties are mostly intermediate between its vertical neighborsgallium andthallium. As withtin, a high-pitchedcry is heard when indium is bent – a crackling sound due tocrystal twinning.[21] Like gallium, indium is able towet glass. Like both, indium has a lowmelting 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 fewelectrons delocalized.[24]
The density of indium, 7.31 g/cm3, is also greater than gallium, but lower than thallium. Below thecritical temperature, 3.41 K, indium becomes asuperconductor. Indium crystallizes in the body-centeredtetragonal crystal system in thespace groupI4/mmm (lattice parameters: a = 325 pm,c = 495 pm):[23] this is a slightly distortedface-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 ductileviscoplastic response, found to be size-independent in tension and compression. However it does have asize effect in bending and indentation, associated to a length-scale of order 50–100 μm,[27] significantly large when compared with other metals.
Indium has 39 knownisotopes, ranging inmass number from 97 to 135. Only two isotopes occur naturally asprimordial nuclides: indium-113, the onlystable isotope, and indium-115, which has ahalf-life of 4.41×1014 years, four orders of magnitude greater than theage of the Universe and nearly 30,000 times greater than half-life ofthorium-232.[28] The half-life of115In is very long because thebeta decay to115Sn isspin-forbidden.[29] Indium-115 makes up 95.7% of all indium. Indium is one of three known elements (the others beingtellurium andrhenium) of which the stable isotope is less abundant in nature than the long-lived primordial radioisotopes.[30]
The stablestartificial isotope isindium-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 byisomeric transition. The indium isotopes lighter than113In predominantly decay throughelectron capture orpositron emission to formcadmium isotopes, while the indium isotopes heavier than113In predominantly decay through beta-minus decay to form tin isotopes.[28]
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 themonovalent state is attributed to theinert pair effect, in whichrelativistic effects lowers the energy of the 5s-orbital, observed in heavier elements. Thallium (indium's heavierhomolog) 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 strongoxidizing agent, indium(III) is not, and many indium(I) compounds are powerfulreducing 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 ashalogens to give indium(III) compounds. It does not form aboride,silicide, orcarbide. Indium is rather basic in aqueous solution, showing only slightamphoteric characteristics, and unlike its lighter homologs aluminium and gallium, it is insoluble in aqueous alkaline solutions.[35]
Chlorination, bromination, and iodination of In produce colorlessInCl3,InBr3, and yellow InI3. The compounds areLewis 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]
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 withsulfur,selenium, andtellurium are also known.[39]
The chemistry of indium pnictides (N, P, As, Sb) is also well known, motivated by their relevance tosemiconductor technology. Direct reaction of indium metal with thepnictogens For applications in microelectronics, the P, As, and Sb derivatives are made by reactions oftrimethylindium:
In(CH3)3 + H3E → InE + 3 CH4 (E = P, As, Sb)
Many of these derivatives are prone to hydrolysis.[40]
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]
Less frequently, indium forms compounds in oxidation state +2 and even fractional oxidation states. Usually such materials feature In–In bonding, most notably in thehalides 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 feature In–C bonds. Most are In(III) derivatives, butcyclopentadienylindium(I) is an exception. It was the first known organoindium(I) compound,[46] and is polymeric, consisting of zigzag chains of alternating indium atoms andcyclopentadienyl complexes.[47] Perhaps the best-known organoindium compound istrimethylindium, In(CH3)3, used to prepare certain semiconducting materials.[48][49]
In 1863, German chemistsFerdinand Reich andHieronymus Theodor Richter were testing ores from the mines aroundFreiberg, Saxony. They dissolved the mineralspyrite,arsenopyrite,galena andsphalerite inhydrochloric acid and distilled rawzinc chloride. Reich, who wascolor-blind, employed Richter as an assistant for detecting the colored spectral lines. Knowing that ores from that region sometimes containthallium, 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 theindigo color seen in its spectrum, after the Latinindicum, meaning 'ofIndia'.[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 theWorld Fair 1867.[55] Reich and Richter later fell out when the latter claimed to be the sole discoverer.[53]
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 10solar masses). When a silver-109 atom captures a neutron, it transmutes into silver-110, which then undergoesbeta decay to become cadmium-110. Capturing further neutrons, it becomes cadmium-115, which decays to indium-115 by anotherbeta decay. This explains why the radioactive isotope is more abundant than the stable one.[56] The stable indium isotope, indium-113, is one of thep-nuclei, the origin of which is not fully understood; although indium-113 is known to be made directly in the s- andr-processes (rapid neutron capture), and also as the daughter of very long-lived cadmium-113, which has a half-life of about eightquadrillion years, this cannot account for all indium-113.[57][58]
Indium is the68th most abundant element in Earth's crust at approximately 50ppb. This is similar to the crustal abundance ofsilver,bismuth andmercury. It very rarely forms its own minerals, or occurs in elemental form. Fewer than 10 indium minerals such asroquesite (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 assphalerite andchalcopyrite.[60][61] From these, it can be extracted as aby-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 therate 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]
Indium is produced exclusively as aby-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 theroast-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 byelectrolysis.[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 materialsper year under current market conditions (i.e. technology and price).[68] Reserves and resources are not relevant for by-products, since theycannot 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 wasUS$240/kg, down fromUS$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] TheTeck Resources refinery inTrail, 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 isLCD 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 theUNEP, indium's end-of-life recycling rate is less than 1%.[72]
Indium wire is used as avacuum seal and a thermal conductor incryogenics andultra-high-vacuum applications, in such manufacturing applications asgaskets that deform to fill gaps.[80] Owing to its great plasticity and adhesion to metals, Indium sheets are sometimes used for cold-soldering inmicrowave circuits andwaveguide joints, where direct soldering is complicated. Indium is an ingredient in the gallium–indium–tin alloygalinstan, which is liquid at room temperature and replacesmercury in somethermometers.[81] Other alloys of indium withbismuth,cadmium,lead, andtin, which have higher but still low melting points (between 50 and 100 °C), are used infire sprinkler systems and heat regulators.[67]
Indium is one of many substitutes for mercury inalkaline batteries to prevent thezinc from corroding and releasinghydrogen gas.[82] Indium is added to somedental amalgam alloys to decrease thesurface 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 incontrol rods fornuclear reactors, typically in an alloy of 80%silver, 15% indium, and 5%cadmium.[84] In nuclear engineering, the (n,n') reactions of113In and115In are used to determine magnitudes of neutron fluxes.[85]
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