Indium
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.
Indium is used primarily in the production of flat-panel displays as indium tin oxide, a transparent and conductive coating applied to glass. It is also used in the semiconductor industry, in low-melting-point metal alloys such as solders and soft-metal high-vacuum seals. It is used in the manufacture of blue and white LED circuits, mainly to produce Indium gallium nitride p-type semiconductor substrates. 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.
Indium has no biological role and its compounds are toxic when inhaled or injected into the bloodstream, although they are poorly absorbed following ingestion.
Etymology
The name comes from the Latin word indicum meaning violet or indigo. The word indicum means "Indian", as the naturally based dye indigo was originally exported to Europe from India.Properties
Physical
Indium is a shiny silvery-white, highly ductile post-transition metal with a bright luster. It is so soft that it can be cut with a knife and leaves a visible line like a pencil when rubbed on paper. 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. Like gallium, indium is able to wet glass. Like both, indium has a low melting point, 156.60 °C ; higher than its lighter homologue, gallium, but lower than its heavier homologue, thallium, and lower than tin. The boiling point is 2072 °C, 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.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 : 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. Indium has greater solubility in liquid mercury than any other metal. 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, significantly large when compared with other metals.
Isotopes
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 years, four orders of magnitude greater than the age of the Universe and nearly 30,000 times greater than half-life of thorium-232. The half-life of 115In is very long because the beta decay to 115Sn is spin-forbidden. Indium-115 makes up 95.7% of all indium. Indium is one of three known elements of which the stable isotope is less abundant in nature than the long-lived primordial radioisotopes.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 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.
Chemistry
Indium has 49 electrons, with an electronic configuration of [Kr]4d5s5p. In compounds, indium most commonly donates the three outermost electrons to become indium, In. In some cases, the pair of 5s-electrons are not donated, resulting in indium, 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 shows an even stronger effect, manifested by the pervasiveness of thallium vs thallium, Gallium is only rarely observed in the +1 oxidation state. Thus, although thallium is a moderately strong oxidizing agent, indium is not, and many indium compounds are powerful reducing agents. 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. Indium oxide and hydroxide are more basic and indium oxide and hydroxide are more acidic.A number of standard electrode potentials, depending on the reaction under study, are reported for indium, reflecting the decreased stability of the +3 oxidation state:
Indium metal does not react with water, but it is oxidized by stronger oxidizing agents such as halogens to give indium 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.
Indium(III) compounds
Hydrides and halides
The hydride InH3 has at best a transitory existence in ethereal solutions at low temperatures. It polymerizes in the absence of bases. Lewis bases stabilize a rich collection of indium hydrides of the formula LInH3.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.
Indium halides dissolves in water to give aquo complexes such as 3+ and +. Similar complexes can be prepared from nitrates and acetates. Overall, the pattern is similar to that for aluminium.
Chalcogenides and pnictides
Indium derivatives of chalcogenides are well developed. Indium oxide, In2O3, forms when indium metal is burned in air or when the hydroxide or nitrate is heated. The analogous sesqui-chalcogenides with sulfur, selenium, and tellurium are also known.The chemistry of indium pnictides 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:
Many of these derivatives are prone to hydrolysis.
Indium(I) compounds
Indium 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. Indium oxide black powder is produced when indium oxide decomposes upon heating to 700 °C.Compounds in 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 the halides In2X4 and 2−, and various subchalcogenides such as In4Se3. Several other compounds are known to combine indium and indium, such as InI6Cl3, InI52, and InIInIIIBr4.Organoindium compounds
Organoindium compounds feature In–C bonds. Most are In derivatives, but cyclopentadienylindium is an exception. It was the first known organoindium compound, and is polymeric, consisting of zigzag chains of alternating indium atoms and cyclopentadienyl complexes. Perhaps the best-known organoindium compound is trimethylindium, In3, used to prepare certain semiconducting materials.History
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'.Richter went on to isolate the metal in 1864. An ingot of was presented at the World Fair 1867. Reich and Richter later fell out when the latter claimed to be the sole discoverer.
Occurrence
Indium is created by the long-lasting s-process in low-to-medium-mass stars. 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. 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, 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.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 are known, and none occur at sufficient concentrations for economic extraction. Instead, indium is usually a trace constituent of more common ore minerals, such as sphalerite and chalcopyrite. From these, it can be extracted as a by-product during smelting. 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.
Different estimates exist of the amounts of indium contained within the ores of other metals. However, these amounts are not extractable without mining of the host materials. 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, explaining the paradoxically low depletion times some studies cite.