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.