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.
Characteristics
Physical properties 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.
Chemical properties Terbium metal is an electropositive element and oxidizes in the presence of most acids (such as sulfuric acid), all of the halogens, and water.
2 Tb (s) + 3 H2SO4 → 2 Tb3+ + 3 SO2−4 + 3 H2↑ 2 Tb + 3 X2 → 2 TbX3 (X = F, Cl, Br, I) 2 Tb (s) + 6 H2O → 2 Tb(OH)3 + 3 H2↑ Terbium oxidizes readily in air to form a mixed terbium(III,IV) oxide:
8 Tb + 7 O2 → 2 Tb4O7 The most common oxidation state of terbium is +3 (trivalent), such as in TbCl3. In the solid state, tetravalent terbium is also known, in compounds such as terbium oxide (TbO2) and terbium tetrafluoride. In solution, terbium typically forms trivalent species, but can be oxidized to the tetravalent state with ozone in highly basic aqueous conditions. The coordination and organometallic chemistry of terbium is similar to other lanthanides. In aqueous conditions, terbium can be coordinated by nine water molecules, which are arranged in a tricapped trigonal prismatic molecular geometry. Complexes of terbium with lower coordination number are also known, typically with bulky ligands like bis(trimethylsilyl)amide, which forms the three-coordinate tris[N,N-bis(trimethylsilyl)amide]terbium(III) (Tb[N(SiMe3)2]3) complex. Most coordination and organometallic complexes contain terbium in the trivalent oxidation state. Divalent Tb2+ complexes are also known, usually with bulky cyclopentadienyl-type ligands. A few coordination compounds containing terbium in its tetravalent state are also known.
Oxidation states Like most rare-earth elements and lanthanides, terbium is usually found in the +3 oxidation state. Like cerium and praseodymium, terbium can also form a +4 oxidation state, although it is unstable in water. It is possible for terbium to be found in the 0, +1, and +2 oxidation states.
Compounds
Terbium combines with nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon and arsenic at elevated temperatures, forming various binary compounds such as TbH2, TbH3, TbB2, TbSi2, TbN, TbP, TbS, Tb2S3, TbSe, and TbTe. In these compounds, terbium mainly exhibits the oxidation state +3, with the +2 state appearing rarely. Terbium(II) halides are obtained by annealing terbium(III) halides in presence of metallic terbium in tantalum containers. Terbium also forms the sesquichloride Tb2Cl3, which can be further reduced to terbium(I) chloride (TbCl) by annealing at 800 °C; this compound forms platelets with layered graphite-like structure. Terbium(IV) fluoride (TbF4) is the only halide that tetravalent terbium can form. It has strong oxidizing properties and is a strong fluorinating agent, emitting relatively pure atomic fluorine when heated, rather than the mixture of fluoride vapors emitted from cobalt(III) fluoride or cerium(IV) fluoride. It can be obtained by reacting terbium(III) chloride or terbium(III) fluoride with fluorine gas at 320 °C:
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