Terbium

Terbium is a chemical element; it has symbol Tb and atomic number 65. It is a silvery-white, rare earth metal that is malleable and ductile. The ninth member of the lanthanide series, terbium is a fairly electropositive metal that reacts with water, evolving hydrogen gas. Terbium is never found in nature as a free element, but it is contained in many minerals, including cerite, gadolinite, monazite, xenotime and euxenite.

Swedish chemist Carl Gustaf Mosander discovered terbium as a chemical element in 1843. He detected it as an impurity in yttrium oxide (Y₂O₃). Yttrium and terbium, as well as erbium and ytterbium, are named after the village of Ytterby in Sweden. Terbium was not isolated in pure form until the advent of ion exchange techniques.

Terbium is used to dope calcium fluoride, calcium tungstate and strontium molybdate in solid-state devices, and as a crystal stabilizer of fuel cells that operate at elevated temperatures. As a component of Terfenol-D (an alloy that expands and contracts when exposed to magnetic fields more than any other alloy), terbium is of use in actuators, in naval sonar systems and in sensors. Terbium is considered non-hazardous, though its biological role and toxicity have not been researched in depth.

Most of the world's terbium supply is used in green phosphors. Terbium oxide is used in fluorescent lamps and television and monitor cathode-ray tubes (CRTs). Terbium green phosphors are combined with divalent europium blue phosphors and trivalent europium red phosphors to provide trichromatic lighting technology, a high-efficiency white light used in indoor lighting.

Terbium is a silvery-white rare earth metal that is malleable, ductile and soft enough to be cut with a knife. It is relatively stable in air compared to the more reactive lanthanides in the first half of the lanthanide series. Terbium exists in two crystal allotropes with a transformation temperature of 1289 °C between them. The 65 electrons of a terbium atom are arranged in the electron configuration [Xe]4f⁹6s². The eleven 4f and 6s electrons are valence. Only three electrons can be removed before the nuclear charge becomes too great to allow further ionization, but in the case of terbium, the stability of the half-filled [Xe]4f⁷ configuration allows further ionization of a fourth electron in the presence of very strong oxidizing agents such as fluorine gas.

The terbium(III) cation (Tb³⁺) is brilliantly fluorescent, in a bright lemon-yellow color that is the result of a strong green emission line in combination with other lines in the orange and red. The yttrofluorite variety of the mineral fluorite owes its creamy-yellow fluorescence in part to terbium. Terbium easily oxidizes, and is therefore used in its elemental form specifically for research. Single terbium atoms have been isolated by implanting them into fullerene molecules. Trivalent europium (Eu³⁺) and Tb³⁺ ions are among the lanthanide ions that have garnered the most attention because of their strong luminosity and great color purity.

Terbium has a simple ferromagnetic ordering at temperatures below 219 K. Above 219 K, it turns into a helical antiferromagnetic state in which all of the atomic moments in a particular basal plane layer are parallel and oriented at a fixed angle to the moments of adjacent layers. This antiferromagnetism transforms into a disordered paramagnetic state at 230 K.

Terbium metal is an electropositive element and oxidizes in the presence of most acids (such as sulfuric acid), all of the halogens, and water.