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 (). 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. Terbium exists in two crystal allotropes with a transformation temperature of 1289 °C between them.
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.
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The most common oxidation state of terbium is +3 (trivalent), such as in Terbium trichloride|. In the solid state, tetravalent terbium is also known, in compounds such as terbium oxide () 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) () complex.
Most coordination and organometallic complexes contain terbium in the trivalent oxidation state. Divalent Tb<sup>2+</sup> 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
Terbium(IV) fluoride () 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 TbF<sub>3</sub> + F<sub>2</sub> → 2 TbF<sub>4</sub>
When and caesium fluoride (CsF) is mixed in a stoichiometric ratio in a fluorine gas atmosphere, caesium pentafluoroterbate () is obtained. It is an orthorhombic crystal with space group Cmca and a layered structure composed of [TbF<sub>8</sub>]<sup>4−</sup> and 11-coordinated Cs<sup>+</sup>. The compound barium hexafluoroterbate (), an orthorhombic crystal with space group Cmma, can be prepared in a similar method. The terbium fluoride ion [TbF<sub>8</sub>]<sup>4−</sup> 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.
History
thumb|right|[[Carl Gustaf Mosander, the scientist who discovered terbium, lanthanum and erbium]]
Swedish chemist Carl Gustaf Mosander discovered terbium in 1843. He detected it as an impurity in yttrium oxide, , then known as yttria. Yttrium, erbium, and terbium are all named after the village of Ytterby in Sweden. Terbium was not isolated in pure form until the advent of ion exchange techniques.
Mosander first separated yttria into three fractions, all named for the ore: yttria, erbia, and terbia. "Terbia" was originally the fraction that contained the pink color, due to the element now known as erbium. "Erbia", the oxide containing what is now known as terbium, originally was the fraction that was yellow or dark orange in solution. Until the advent of spectral analysis, arguments went back and forth as to whether erbia even existed. Spectral analysis by Marc Delafontaine allowed the separate elements and their oxides to be identified, The names have remained switched ever since. Modern terbium extraction methods are based on the liquid–liquid extraction process developed by Werner Fischer et al., in 1937.
Occurrence
thumb|Xenotime, a mineral source of rare earth elements including terbium|alt=A sample of the mineral xenotime at the Mineralogical Museum, Bonn, Germany
Terbium occurs with other rare earth elements in many minerals, including monazite ( with up to 0.03% terbium), xenotime () and euxenite ( with 1% or more terbium). The crust abundance of terbium is estimated as 1.2 mg/kg. No terbium-dominant mineral has yet been found.
Terbium (as the species Tb II) has been detected in the atmosphere of KELT-9b, a hot-Jupiter planet outside the Solar System.
Currently, the richest commercial sources of terbium are the ion-adsorption clays of southern China;
Production
Crushed terbium-containing minerals are treated with hot concentrated sulfuric acid to produce water-soluble sulfates of rare earths. The acidic filtrates are partially neutralized with caustic soda to pH 3–4. Thorium precipitates out of solution as hydroxide and is removed. The solution is treated with ammonium oxalate to convert rare earths into their insoluble oxalates. The oxalates are decomposed to oxides by heating. The oxides are dissolved in nitric acid that excludes one of the main components, cerium, whose oxide is insoluble in . Terbium is separated as a double salt with ammonium nitrate by crystallization. Terbium is not distinguished from other rare earths in the United States Geological Survey's Mineral Commodity Summaries, which in 2024 estimated the global reserves of rare earth minerals at .
Applications
Terbium is used as a dopant in calcium fluoride, calcium tungstate, and strontium molybdate, materials that are used in solid-state devices, and as a crystal stabilizer of fuel cells which operate at elevated temperatures, together with zirconium dioxide ().
Terbium is also used in alloys and in the production of electronic devices. As a component of Terfenol-D, terbium is used in actuators, in naval sonar systems, sensors, and other magnetomechanical devices. Terfenol-D is a terbium alloy that expands or contracts in the presence of a magnetic field. It has the highest magnetostriction of any alloy. It is used to increase verdet constant in long-distance fiber optic communication. Terbium-doped garnets are also used in optical isolators, which prevents reflected light from traveling back along the optical fiber.
Terbium oxides are used in green phosphors in fluorescent lamps, color TV tubes, The brilliant fluorescence allows terbium to be used as a probe in biochemistry, where it somewhat resembles calcium in its behavior. Terbium "green" phosphors (which fluoresce a brilliant lemon-yellow) are combined with divalent europium blue phosphors and trivalent europium red phosphors to provide trichromatic lighting, which is by far the largest consumer of the world's terbium supply. Trichromatic lighting provides much higher light output for a given amount of electrical energy than does incandescent lighting.
Safety
Terbium, along with many of the other rare earth elements, is poorly studied in terms of its toxicology and environmental impacts. Few health-based guidance values for safe exposure to terbium are available. No values are established in the United States by the Occupational Safety and Health Administration or American Conference of Governmental Industrial Hygienists at which terbium exposure becomes hazardous, and it is not considered a hazardous substance under the Globally Harmonized System of Classification and Labelling of Chemicals.
Reviews of the toxicity of the rare earth elements place terbium and its compounds as "of low to moderately toxicity", remarking on the lack of detailed studies on their hazards and the lack of market demand forestalling evidence of toxicity.
Some studies demonstrate environmental accumulation of terbium as hazardous to fish and plants. High exposures of terbium may enhance the toxicity of other substances causing endocytosis in plant cells.
See also
- Terbium compounds
- List of elements facing shortage
References
Bibliography
External links
- WebElements.com – Terbium
- It's Elemental – Terbium