Indium

Category: post-transition metal Atomic number: 49 Colour: silvery grey Melting point: 157°C (314°F) Boiling point: 2,072°C (3,762°F) First identified: 1863

Indium tin oxide is an extremely useful compound: it is transparent to visible light, electrically conductive, and it bonds strongly to glass. This combination means it can be used in flat-panel displays (LCD screens on computers and other devices – the acronym stands for Liquid Crystal Display). It allows individual pixels to receive signals without light from other pixels being affected.

This has led to a sharp rise in the price of indium in recent years; it is a relatively rare metal, comprising only about 0.1 parts in a million of the Earth’s crust (so it’s about as rare as silver). It previously had few uses, the global production in 1924 being a couple of grams, but now we use over 1,000 tons a year (with half of that coming from recycled sources). Suggestions have been made that the supply of indium might run out over the next decade. This may, though, be slightly alarmist – there is still a lot of the metal available, and rising prices generally mean that suppliers become more ingenious in their extraction techniques.

Indium is a moderately toxic, soft, silvery metal that is unusually sticky (it is used as a solder for this reason; in its pure form, it sticks tightly to other metals) and has many high-tech applications. When you bend a piece of the metal, it gives out a ‘cry’ similar to the noise tin makes – it’s actually more of a crackle and is the sound of the molecules rearranging themselves. It is workable at low temperatures, so it has been used in cryogenic pumps and equipment designed to be used at temperatures close to absolute zero. In alloys it can make significant differences to other metals – for instance, a gold and indium alloy is much harder than gold. The compounds, indium gallium arsenide and copper indium gallium selenide are also used in solar cells.

The metal is named after the colour indigo – when the German chemist Ferdinand Reich discovered it in minerals that were rich in zinc, the tell-tale sign of a new element came when his colleague Hieronymus Richter saw a bright indigo line on the atomic spectroscope. (The two men later fell out because Richter had been claiming that he alone had discovered it.)