Terbium – facts and properties

Position in the periodic table

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Occurrence and extraction

Terbium is one of the scarcer lanthanides, but it is still more common in the Earth's crust than gold and silver together. It is obtained from xenotime, monazite and bastnaesite. The basics of the rare earths are on the lanthanum page. Ion exchange does the separating, and the metal is made by reducing the fluoride with calcium.

Ytterby is a place on the island of Resarö near Stockholm. Unusual minerals have been found in its quarry since the late eighteenth century. Four elements carry the name of the place: yttrium, ytterbium, erbium and terbium.

Carl Gustaf Mosander, who had already discovered lanthanum, examined the yttria from Ytterby in 1843 and showed that it is a mixture. Besides white yttrium oxide he found two further oxides, which he called erbia and terbia. In the course of later research the names erbium and terbium were swapped, so today's assignment no longer matches the original one. Pure terbium could not be made until the twentieth century.

Properties and reactions

Terbium is the ninth lanthanide. It sits in period 6, with gadolinium next to it on the left and dysprosium on the right. The electron configuration is [Xe] 4f⁹ 6s². A knife is enough to cut the metal. Terbium becomes liquid at 1356 °C and boils at 3230 °C. With a density of 8.23 g/cm³ it is clearly heavier than the lanthanum at the start of the row, and the atomic radius has shrunk to 177 pm. For a lanthanide it is fairly stable in air.

Besides cerium and praseodymium, terbium also forms compounds in the +4 state, such as TbO₂ and TbF₄. The oxide that forms in air is Tb₄O₇ and is dark brown, because it contains both states. In water only Tb³⁺ exists, since tetravalent terbium would be too strong an oxidising agent there.

The Tb³⁺ ion glows green, with its strongest line near 545 nanometres. The light comes from electrons deep inside the ion, in the 4f subshell, so the colour is almost independent of the material that holds the terbium. The ion itself absorbs light only weakly. In phosphors a second element, usually cerium, takes in the radiation and passes the energy on to the terbium.

Terbium is also magnetically striking. The metal is ferromagnetic only at low temperatures, but in alloys with iron it changes its shape very strongly in a magnetic field.

Uses in everyday life and technology

The green glow was long used in three-band fluorescent lamps, which LEDs are now replacing. The lamp contains a red, a green and a blue material, and terbium supplies the green, to which our eyes are especially sensitive. In X-ray intensifying screens, a terbium phosphor converts X-rays into green light. The film is then exposed with less radiation, and the patient receives a lower dose.

Terfenol-D is built on magnetostriction. Every magnetic material changes its length slightly in a magnetic field, though in iron the change is only a few millionths. Terfenol-D, an alloy of terbium, dysprosium and iron, stretches by up to about two thousandths of its length. The name combines Ter for terbium, Fe for iron, NOL for the American Naval Ordnance Laboratory, where it was developed, and D for dysprosium. It goes into sonar transmitters, ultrasonic transducers and actuators that move very precisely and forcefully, and even into loudspeakers that make a tabletop vibrate.

Terbium gallium garnet crystals rotate the polarisation of light in a magnetic field. They are used to build optical isolators, which let laser light pass in one direction and block light coming back. Magneto-optical storage discs such as the recordable MiniDisc once used a layer of terbium, iron and cobalt. As an additive in heat-resistant neodymium magnets, terbium does a similar job to dysprosium.

Terbium and its compounds are considered to have low to moderate toxicity. Powder of the metal can burn.

Quick question

What is the atomic number of terbium (Tb)?

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