Terbium

Terbium is the second of four elements named after the village of Ytterby, Sweden (along with yttrium, erbium, and ytterbium). Discovered in 1843 by Carl Gustaf Mosander, terbium was incredibly difficult to separate from its neighboring rare earth elements, and truly pure terbium was not available until the development of ion exchange techniques in the 1950s.

Terbium produces a bright green fluorescence and is used in green phosphors for TV screens, LED lights, and fluorescent lamps. Terbium is also a key ingredient in Terfenol-D, an alloy that changes shape in a magnetic field — this property is used in sonar systems, speakers, and precision sensors. Terbium-doped materials are used in solid-state memory devices and naval sonar equipment.

Terbium's symbol 'Tb' is a distinctive two-letter element in Spellement. While 'tb' is uncommon at word beginnings, partial emphasis lets spellers use 't' or 'b' individually — both common consonants that provide strong flexibility for constructing words from element symbols.