Indium

• 1. Introduction

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  Indium is so soft that you can bite into it with your teeth. Rub it on paper and it leaves a line, just like a pencil. Bend a piece and it cries out, a high-pitched crackle that comes from the metal's crystals twisting against each other. This silvery-white metal carries the symbol In and the atomic number 49. It hides almost everywhere and reveals itself almost nowhere. It has no job to do inside any living thing, yet a screen you may be looking at right now likely depends on it. So how did a metal too soft to hold an edge end up coating the world's flat-panel displays? Why is the radioactive form of indium more common in nature than the stable one? And what made two German chemists, one of them color-blind, decide they had found a new element when a single blue line flashed in front of them?

• 2. A Color In The Spectrum

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  In 1863, Ferdinand Reich and Hieronymus Theodor Richter were testing ores from the mines around Freiberg, in Saxony. They dissolved minerals like pyrite, arsenopyrite, galena and sphalerite in hydrochloric acid and distilled raw zinc chloride. Reich was color-blind, so he relied on Richter to read the colored spectral lines for him. Knowing that local ores sometimes held thallium, the pair went looking for that element's green emission lines. Instead, a bright blue line appeared. It matched no known element, so they reasoned a new one must be hidden in the minerals. They named it indium for the indigo color in its spectrum, drawing on the Latin word indicum, which means 'of India'. The name traces back further still. Indicum also means 'Indian', because the dye indigo was first exported to Europe from India. Richter isolated the metal itself in 1864. An ingot weighing half a kilogram went on display at the World Fair of 1867. The collaboration did not end well; Reich and Richter later fell out when Richter claimed to be the sole discoverer.

• 3. The Metal You Can Bite

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  A Mohs hardness of 1.2 makes indium one of the softest elements there is. It can be cut with a knife, and it wets glass the way its lighter relative gallium does. Its melting point sits at 156.60 degrees Celsius, higher than gallium's but lower than thallium's and lower than tin. Its boiling point reaches 2072 degrees Celsius, which is higher than thallium's but lower than gallium's, an inversion of the usual trend explained by weak metallic bonding with few electrons spread around. The density of indium is 7.31 grams per cubic centimeter, again greater than gallium but less than thallium. Cool the metal below 3.41 kelvin and it becomes a superconductor. Indium also has an unusual appetite for liquid mercury; it dissolves in it more readily than any other metal, with more than 50 percent indium by mass at zero degrees Celsius. The metal flows in a ductile, viscoplastic way, and that flow is size-independent under tension and compression. Yet in bending and indentation it shows a size effect tied to a length-scale of 50 to 100 micrometers, large compared with other metals.

• 4. Caught Between Neighbors

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  Indium sits in group 13 of the periodic table, wedged between gallium above it and thallium below. Its chemistry mostly falls between the two. With 49 electrons, it usually gives up its three outermost to become indium(III). Sometimes it keeps the pair of 5s-electrons and forms indium(I) instead. That monovalent state owes its stability to the inert pair effect, where relativistic effects lower the energy of the 5s-orbital in heavier elements. Thallium shows this effect even more strongly, which is why thallium(I) is so common, while gallium almost never appears in the +1 state. So although thallium(III) is a moderately strong oxidizing agent, indium(III) is not, and many indium(I) compounds are powerful reducing agents. Indium metal will not react with water, but stronger oxidizers like the halogens will turn it into indium(III) compounds. Chlorination, bromination, and iodination give colorless indium trichloride and tribromide and yellow indium triiodide, all of them Lewis acids akin to the aluminium trihalides. Indium(I) compounds are far less common; their chloride, bromide and iodide are deeply colored, unlike the trihalides they come from. Heat indium(III) oxide to 700 degrees Celsius and it decomposes into a black powder of indium(I) oxide.

• 5. Born In Dying Stars

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  Indium-115 makes up 95.7 percent of all indium, and it is radioactive. It has a half-life of 4.41 years, a figure that is four orders of magnitude greater than the age of the Universe and nearly 30,000 times the half-life of thorium-232. That extraordinary longevity comes from the fact that its beta decay to tin-115 is spin-forbidden. The stable form, indium-113, is the rarer of the two natural isotopes. This makes indium one of only three elements, alongside tellurium and rhenium, whose stable isotope is less abundant than its long-lived primordial radioisotopes. The reason lies in how the metal is forged. Indium is built by the slow neutron-capture s-process inside low-to-medium-mass stars, those between 0.6 and 10 solar masses, over spans reaching thousands of years. A silver-109 atom captures a neutron to become silver-110, which decays to cadmium-110. Further neutrons carry it to cadmium-115, which decays into indium-115. Indium has 39 known isotopes ranging in mass number from 97 to 135, plus 47 meta states. The most stable artificial isotope is indium-111, which lasts about 2.8 days.

• 6. Hidden In Other Ores

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  At roughly 50 parts per billion, indium is the 68th most abundant element in Earth's crust, on par with silver, bismuth and mercury. It almost never forms minerals of its own. Fewer than 10 indium minerals exist, among them roquesite, and none occur in concentrations worth mining directly. Instead the metal rides along as a trace passenger in common ores like sphalerite and chalcopyrite. Because of that, indium is produced exclusively as a by-product of processing other metals, chiefly the sulfidic zinc ores where sphalerite hosts it. During the roast-leach-electrowinning process of zinc smelting, indium gathers in the iron-rich residues, and electrolysis finishes the purification. This by-product status changes how supply must be understood. The element's availability is set by how fast the host ores are mined, not by how much indium lies underground. Recent estimates put the supply potential at a minimum of 1,300 tonnes a year from sulfidic zinc ores and 20 tonnes a year from copper ores. Those figures far exceed the 655 tonnes produced in 2016. The average price that year was 240 per kilogram, down sharply from 705 per kilogram in 2014. China led production with 290 tonnes, followed by South Korea, Japan and Canada.

• 7. From Aircraft Bearings To Screens

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  In 1924, indium was found to stabilize non-ferrous metals, its first significant use. Its first large-scale job came during World War II, coating bearings in high-performance aircraft engines to guard against damage and corrosion, a role it no longer plays. In the 1950s, tiny beads of indium served as the emitters and collectors of PNP alloy-junction transistors. Then came the shift that defined its modern life. In the middle and late 1980s, indium phosphide semiconductors and indium tin oxide thin films for liquid-crystal displays drew intense interest. By 1992, the thin-film application had become the largest end use. As indium tin oxide, the metal forms a transparent, conductive coating on glass, and LCD production became its primary consumer worldwide. Demand climbed quickly from the late 1990s to 2010 as LCD monitors and televisions spread, and screens now account for half of all indium consumed. The metal earns its keep across electronics. Indium antimonide and indium arsenide go into low-temperature transistors, indium phosphide into high-temperature ones, and the compounds indium gallium nitride and indium gallium phosphide into LEDs and laser diodes. Copper indium gallium selenide drives a kind of thin-film solar cell. Despite all this, the United Nations Environment Programme puts indium's end-of-life recycling rate at less than 1 percent.

• 8. Seals, Reactors, And A New Blue

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  Indium wire makes a vacuum seal and a thermal conductor in cryogenics and ultra-high-vacuum work, deforming into gaskets that fill gaps. Its plasticity and grip on metals also let thin indium sheets cold-solder microwave circuits and waveguide joints where ordinary soldering is awkward. The metal joins gallium and tin in the alloy galinstan, liquid at room temperature, which replaces mercury in some thermometers. Other indium alloys with bismuth, cadmium, lead and tin, melting between 50 and 100 degrees Celsius, run fire sprinkler systems and heat regulators. A high neutron-capture cross-section for thermal neutrons makes indium useful in nuclear reactor control rods, often as an alloy of 80 percent silver, 15 percent indium and 5 percent cadmium. In medicine, radioactive indium-111 acts as a radiotracer, following labeled proteins and white blood cells to diagnose infections, with a biological half-life in humans of about two weeks. Indium's compounds carry real risk; they are toxic when inhaled or injected, though poorly absorbed when swallowed. Yet the metal's most colorful chapter is recent. In 2009, Professor Mas Subramanian and former graduate student Andrew Smith at Oregon State University combined indium with yttrium and manganese to make YInMn blue, an intense, non-toxic, fade-resistant pigment and the first new inorganic blue discovered in 200 years.