Terbium

Terbium was discovered in 1843 by the Swedish chemist Carl Gustaf Mosander, who drew it from the ores of Ytterby along with erbium. Its name, like those of yttrium, erbium and ytterbium, is a fragment of Ytterby, the Swedish village whose quarry yielded them all. Terbium is a soft, silvery rare-earth metal, used in green phosphors for lighting and displays and in magnetostrictive alloys that change shape in a magnetic field. One of the rarer and dearer lanthanides, terbium provides the green in the phosphors of fluorescent lamps and older screens, is added to neodymium magnets to help them resist heat, and forms alloys used in sensors and actuators.

Terbium is a key ingredient of Terfenol-D, an alloy combining terbium, dysprosium and iron that changes shape more dramatically in a magnetic field than almost any other known material, a property exploited in sonar transducers, precision actuators and some loudspeaker designs. Its green phosphor, terbium-doped yttrium compounds, was for decades one of the three primary colours, alongside europium red and a blue phosphor, that made colour television and computer monitor screens possible before liquid-crystal and LED displays largely replaced the cathode-ray tube.

Terbium is the second of the four elements whose names are pieces of Ytterby, that uniquely productive Swedish quarry near Stockholm. The naming of yttrium, terbium, erbium and ytterbium from a single place-name reflects the extraordinary confusion of their discovery, for the rare earths of Ytterby were separated only slowly and in the wrong order more than once, so that the names were reassigned and swapped over the years before settling into their modern form. Terbium has no mythological or alchemical tradition, being a nineteenth-century rare-earth discovery; its identity is geographical, a syllable of a Scandinavian village, and its modern importance lies in the pure green light its compounds emit in fluorescent lamps and screens.

The mix-up between erbium and terbium in particular persisted well into the 1870s, with different chemists using the two names for opposite substances in different countries, until careful comparative work, especially by Marignac, settled the confusion and fixed the modern assignment that holds today. Terbium remains one of the scarcer rare earths in the crust, and its price has tracked the same critical-minerals anxiety now attached to praseodymium and neodymium. Mosander drew both terbium and erbium from a single mineral sample in 1843, a pace of discovery that made him, alongside Auer von Welsbach and Lecoq de Boisbaudran, one of the most productive individual finders in the whole crowded history of the rare earths.