Indium

Indium is a soft, shiny post-transition metal that most people have never heard of, but it is inside the touchscreen of every smartphone and tablet computer in the world. It forms transparent, electrically conductive films when alloyed with tin oxide, making touch-sensitive displays possible.

  • Atomic Number4949 protons, 49 electrons
  • Atomic Mass114.818 u49× heavier than hydrogen
  • State at Room TempSolidSolid
  • Density7.31 g/cm³
  • Melting / Boiling156.6°C / 2071.8°C
  • Discovered1863

What is Indium?

Indium is a post-transition metal in Group 13 of the periodic table, sitting below aluminium and gallium. With 49 protons, it is soft (softer than lead), shiny and silvery-white. It adopts primarily the +3 oxidation state. One of indium's most unusual properties is that it emits a high-pitched "shriek" when bent, caused by crystal twinning in its structure. Like gallium, it wets glass easily, making it useful for low-melting alloys.

Indium gets its name from the Latin indicum meaning indigo, because of the brilliant indigo-blue spectral lines it produces: the distinctive colour that led to its discovery. It was found in 1863 by the German chemists Ferdinand Reich and Hieronymus Theodor Richter using spectroscopy, while they were analysing zinc ores from the Freiberg mines in Saxony.

Fact The world produces only approx. 800 tonnes of indium per year, less than the weight of two large commercial aeroplanes. Yet demand from the electronics industry is enormous. Every smartphone touchscreen contains ITO, and the explosive growth of consumer electronics has made indium supply a growing concern for technology manufacturers.

Where you find Indium

In space

Indium is produced in stars and found in the Sun in trace amounts.

On Earth

Indium is one of the rarest stable elements, at just 0.25 parts per million in the Earth's crust.

  • By-product of zinc smelting. Almost all indium is recovered from the flue dust and residues of zinc smelting. China produces approx. 60% of world supply from its large zinc industry.
  • Sphalerite (zinc sulfide). Zinc ore contains small amounts of indium that are concentrated and recovered during zinc processing.

How we use Indium

  • ITO (indium tin oxide).. About 75% of all indium goes into making indium tin oxide (ITO), an electrically conductive, optically transparent material used in LCD screens, touchscreens, smartphones, tablets and flat-panel televisions.
  • Thin-film solar cells.. CIGS (copper indium gallium selenide) solar cells are a high-efficiency thin-film technology that uses indium.
  • Soldering alloys.. Indium-containing solders melt at lower temperatures than conventional tin-lead solders and bond well to difficult substrates like glass and ceramics.
  • Semiconductors.. Indium phosphide (InP) and indium arsenide (InAs) are compound semiconductors used in high-speed transistors, infrared detectors and LEDs.
Did you know? The touchscreen of your smartphone works because a layer of indium tin oxide on the screen surface conducts electricity. When you touch the screen, your finger slightly alters the electrical field at that point, and the phone's processor detects exactly where the disturbance occurred. This technology, called capacitive touch sensing, relies entirely on indium's combination of electrical conductivity and optical transparency.

How it was discovered

Indium was discovered spectroscopically in 1863 by Ferdinand Reich and Hieronymus Richter while they were examining zinc ore samples from Freiberg, Saxony. Reich, who was colourblind, asked Richter to confirm the indigo spectral lines he suspected. Richter confirmed the lines and the new element. The two men later disputed credit for the discovery in an acrimonious priority argument.

Deeper dive: indium chemistry and applications

Indium tin oxide (ITO) works as a transparent conductor because, in the right crystallographic phase, the electronic structure allows both good electrical conductivity and transparency to visible light. Tin atoms (Sn⁴⁺) substituted into the indium oxide (In₂O₃) lattice donate electrons that can carry electrical current, while the bandgap of the material is large enough that visible light photons cannot be absorbed. The combination, hard to achieve, is what makes ITO the go-to material for flat displays.

Indium is considered a "companion metal", it has no ore deposits of its own and is entirely recovered as a by-product of other mining operations. This makes supply inflexible and vulnerable to changes in zinc mining rates. The concentration of production in China (approx. 60%) has prompted significant research into ITO alternatives for touchscreens, with graphene and carbon nanotube films being investigated as potential substitutes.

Moving to 50 protons on the periodic table brings us to Tin.