Terbium

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 SO₄²⁻ + 3 H₂(g)

2 Tb + 3 X₂ → 2 TbX₃ (X = F, Cl, Br, I)

2 Tb(s) + 6 H₂O → 2 Tb(OH)₃ + 3 H₂(g)

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:

2 TbF₃ + F₂ → 2 TbF₄

When TbF₄ and caesium fluoride (CsF) is mixed in a stoichiometric ratio in a fluorine gas atmosphere, caesium pentafluoroterbate (CsTbF₅) is obtained. It is an orthorhombic crystal with space group Cmca and a layered structure composed of TbF₈⁴⁻ and 11-coordinated Cs⁺. The compound barium hexafluoroterbate (BaTbF₆), an orthorhombic crystal with space group Cmma, can be prepared in a similar method. The terbium fluoride ion TbF₈⁴⁻ also exists in the structure of potassium terbium fluoride crystals.

Terbium(III) oxide or terbia is the main oxide of terbium, and appears as a dark brown water-insoluble solid. It is slightly hygroscopic and is the main terbium compound found in rare earth-containing minerals and clays.

Other compounds include:

• Chlorides: TbCl₃
• Bromides: TbBr₃
• Iodides: TbI₃
• Fluorides: TbF₃, TbF₄

Isotopes

Naturally occurring terbium is composed of its only stable isotope, terbium-159; the element is thus mononuclidic and monoisotopic. Thirty-nine radioisotopes have been characterized from ¹³⁵Tb to ¹⁷⁴Tb. The most stable synthetic radioisotopes of terbium are ¹⁵⁸Tb, with a half-life of 180 years, and ¹⁵⁷Tb, with a half-life of 71 years. All of the remaining radioactive isotopes have half-lives that are less than three months, and the majority of these have half-lives that are less than half a minute. The primary decay mode before the most abundant stable isotope, ¹⁵⁹Tb, is electron capture, which results in production of gadolinium isotopes, and the primary mode after is beta minus decay, resulting in dysprosium isotopes. It is also the heaviest element where every stable nuclide has no confirmed energetically allowed decay mode, though the heaviest element with any such isotopes is dysprosium.

The element also has 31 nuclear isomers, with masses of 141–154, 156, 158, 162, and 164–168 (not every mass number corresponds to only one isomer). The most stable of them are terbium-156m2, with a half-life of 24.4 hours, and terbium-154m2, with a half-life of 22.7 hours; this is more stable than ground states of terbium isotopes, except outside the mass range 155–161.

Terbium-149, with its half-life of 4.1 hours, is a promising candidate in targeted alpha therapy and positron emission tomography.