Erbium – facts and properties

Position in the periodic table

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Occurrence and extraction

Erbium is one of the heavy rare earths, whose basics are covered on the lanthanum page. In the Earth's crust it is about as common as tin, so it is far from scarce. It is found in xenotime, in monazite and in the ion-adsorption clays of southern China, there beside yttrium and the other heavy lanthanides. After separation by ion exchange it is available as erbium oxide, from which the metal is made by reduction. Pure metal could not be made until the twentieth century.

The trail leads to the quarry at Ytterby near Stockholm, which also gave yttrium its name. A heavy mineral had been discovered there in the late eighteenth century, and at first only one earth was split off from it, yttria. In 1843 the Swedish chemist Carl Gustaf Mosander showed that it hides two more oxides. One he called erbia, the other terbia. Later the names got muddled, as the terbium page tells.

Properties and reactions

Erbium is the twelfth lanthanide. To its left in period 6 stands holmium, to its right thulium. With the electron configuration [Xe] 4f¹² 6s², its 4f subshell is nearly full. The electronegativity is 1.24. The metal melts at 1529 °C, boils at 2868 °C and reaches a density of 9.07 g/cm³. Its atomic radius is 175 pm. Erbium is soft and pliable, tarnishes only slowly in air and reacts sluggishly with water. In compounds it almost always appears triply charged.

Colour is typical of erbium. Er³⁺ ions turn salts and the oxide pink, and glasses containing erbium are pink as well. The ions absorb light at a few sharply defined wavelengths in the visible range, especially in the green. The remaining light then looks pink to reddish.

For technology, though, a transition in the infrared counts. An excited Er³⁺ ion can give off light at about 1550 nanometres. That is exactly where silica glass is clearest, so light loses the least energy in an optical fibre at this wavelength. The two fit together: the fibre carries this colour especially well, and erbium can amplify it.

Uses in everyday life and technology

The best-known field is fibre-optic technology. Over long distances a cable loses light, and after a few dozen kilometres the signal is too weak. In a fibre amplifier, a short piece of fibre is therefore doped with erbium. A pump laser excites the erbium ions, and the passing signal prompts them to release their energy as extra light of the same wavelength. The signal is boosted with no detour through electronics, and for many colour channels at once. Such amplifiers sit in undersea cables and long-distance networks.

The Er:YAG laser holds erbium in a crystal of yttrium aluminium garnet and emits infrared light at about 2940 nanometres. Water absorbs this wavelength especially strongly, so the beam penetrates only fractions of a millimetre and removes tissue layer by layer, with little heat damage around it. In skin medicine it is used to smooth the skin, and in dentistry to work on tooth substance and decay.

Erbium oxide is a pink colourant for glass, glazes and ceramics, and cubic zirconia, the stand-in for diamonds in jewellery, is tinted pink with erbium too. In metallurgy, a little erbium makes the metal vanadium easier to work. Erbium compounds are hardly toxic.

Quick question

What is the atomic number of erbium (Er)?

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