Indium (In)

Every screen you look at has a layer of indium in front of the pixels. It is invisible, a few tens of nanometres thick, and without it the display industry as it exists would not work, because a screen needs an electrode that carries current to each pixel and lets the light out afterwards. Those two demands are normally contradictory, and indium tin oxide is the compromise that the entire industry settled on.

An indigo line the discoverer could not see

Ferdinand Reich, professor at the Freiberg Mining Academy in Saxony, was examining local zinc ores in 1863 in the hope of finding thallium, which had been identified two years earlier by its green spectral line. He roasted the ore, treated the residue, and put a sample in front of a spectroscope — and then had to call in his assistant, Hieronymus Theodor Richter, because Reich was colour-blind and could not judge what he was looking at.

Richter reported a brilliant blue line at a position matching nothing known. They named the element indium, after indigo, for that line. The partnership did not survive the discovery: Richter subsequently presented himself as the sole discoverer, and the two men fell out over it. The credit above names Reich, which is the usual convention, and it belongs to both.

Indium is the fourth element in the run of discoveries the spectroscope made possible in the 1860s, after cesium, rubidium and thallium, and like all of them it is named for the colour of a line rather than any property a person could observe by hand.

Transparent and conducting at the same time

Metals conduct because they contain a dense sea of free electrons — and that same electron sea is why metals are opaque and shiny, since the electrons respond to and re-emit visible light. Making something both clear and conductive means threading a genuine physical needle.

Indium tin oxide does it by being a wide-band-gap semiconductor doped so heavily that it behaves like a metal. The band gap is around 4 eV, far above the energy of visible photons, so visible light passes straight through without being absorbed. Tin substituting for indium donates spare electrons, giving a carrier concentration high enough to conduct — but still perhaps a thousand times lower than a real metal's, which pushes the plasma frequency down into the infrared. The material is therefore transparent to visible light and reflective in the infrared, which is why the same coating turns up on low-emissivity architectural glazing and on aircraft windscreens that need to be de-iced electrically.

Roughly seventy per cent of world indium consumption goes into this one compound: liquid crystal and OLED displays, touch panels, and the front contact of thin-film photovoltaic cells. Alternatives exist — silver nanowires, conductive polymers, graphene, fluorine-doped tin oxide — and none has matched the combination of low resistance, high transmission and manufacturability across large areas.

Indium's other electronic role is in compound semiconductors. Indium phosphide is the substrate for the lasers and detectors that operate at the 1.3 and 1.55 micrometre wavelengths where optical fibre is most transparent, so long-haul telecommunications runs on it. Indium gallium arsenide makes the photodiodes at the receiving end. And the light-emitting layer of a blue or green LED is indium gallium nitride, in which the indium fraction sets the colour — raising it shifts the emission towards green, though growing high-indium layers without defects has proved persistently difficult, which is the origin of the well-known "green gap" in LED efficiency.

Soft enough to mark paper

Indium is remarkably soft. It can be dented with a fingernail, and drawn across paper it leaves a grey line the way a soft pencil does. Bending a bar produces a faint crackling sound as crystal twins shift past one another, the same phenomenon as the better-known cry of tin, though quieter.

Unusually, it stays ductile all the way down to cryogenic temperatures, where most metals become brittle. That makes indium wire the standard gasket for vacuum and cryogenic apparatus: a ring of it squeezed between two flanges cold-welds into a seal that survives repeated cooling to liquid helium temperatures. Indium also wets glass and quartz, so it is used for glass-to-metal joints, and it is a component of fusible alloys — Field's metal, a bismuth-indium-tin mixture, melts in hot water — and of low-temperature solders used where a component cannot survive normal soldering heat.

The middle metal in a control rod

The standard control rod in a pressurised water reactor is an alloy of about 80% silver, 15% indium and 5% cadmium, and each metal is there for a different part of the neutron energy spectrum.

Cadmium is superb at capturing the slowest neutrons and then falls off a cliff. Indium has strong absorption resonances in the epithermal range just above where cadmium stops, and silver covers a broad band above that. Together the three give a rod that absorbs across a far wider energy range than any of them alone, which is what a control rod actually needs, because a reactor's neutron spectrum is not monoenergetic. It is one of the neater examples of alloying for a nuclear rather than a mechanical property.

Most indium atoms are radioactive

Indium has two natural isotopes, and the abundant one is not stable. Indium-115 makes up about 96% of the element and beta decays with a half-life of roughly 4.4 × 10^14 years — some thirty thousand times the age of the universe, so the activity of a lump of indium is negligible, but the nuclide is genuinely unstable. Only indium-113, the remaining 4%, is stable.

Indium therefore belongs to a very short list of elements whose most common isotope is radioactive; tellurium and rhenium are the other classic cases. The decay is so strongly suppressed — it requires a large change in nuclear spin — that it was only measured directly in the twentieth century, and the same transition has since been studied as a possible probe of whether nuclear decay rates can be altered by the atom's chemical environment.

A manufactured isotope, indium-111, is a mainstay of nuclear medicine. With a half-life of about 2.8 days and two useful gamma emissions, it is long-lived enough to label a patient's own white blood cells, reinject them, and image where they gather over the following days — which is how occult infections and inflammatory bowel disease are localised.

Recovered from zinc, and from itself

Indium is about as abundant in the crust as silver, and it forms almost no minerals of its own. It substitutes for zinc in sphalerite, and essentially all production comes from residues generated when zinc is refined — principally in China, South Korea, Japan and Canada.

The other significant source is the industry's own waste. When indium tin oxide is sputtered onto glass, only a minority of the target material ends up on the substrate; the rest coats the inside of the chamber and is scraped out and reprocessed, and spent targets are returned for refining. Recycled indium accounts for a large share of world supply, which makes the element unusual: for most metals the recycling stream comes from end-of-life products, and for indium it mostly comes from the factory floor.