Swedish chemistCarl Gustaf Mosander discovered terbium as a chemical element in 1843. He detected it as an impurity inyttrium oxide (Y2O3).Yttrium and terbium, as well aserbium andytterbium, are named after the village ofYtterby in Sweden. Terbium was not isolated in pure form until the advent ofion exchange techniques.
Terbium is used todopecalcium fluoride, calciumtungstate andstrontiummolybdate insolid-state devices, and as a crystal stabilizer offuel cells that operate at elevated temperatures. As a component ofTerfenol-D (an alloy that expands and contracts when exposed to magnetic fields more than any other alloy), terbium is of use inactuators, in navalsonar systems and insensors. 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 greenphosphors. Terbiumoxide is used influorescent lamps and television and monitorcathode-ray tubes (CRTs). Terbium green phosphors are combined with divalenteuropium blue phosphors and trivalent europium red phosphors to providetrichromatic lighting technology, a high-efficiency white light used in indoor lighting.
Terbium is a silvery-whiterare earthmetal that ismalleable,ductile and soft enough to be cut with a knife.[10] It is relatively stable in air compared to the more reactive lanthanides in the first half of the lanthanide series.[11] Terbium exists in two crystalallotropes with a transformation temperature of 1289 °C between them.[10] The 65 electrons of a terbium atom are arranged in theelectron configuration [Xe]4f96s2. The eleven 4f and 6s electrons arevalence. 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 asfluorine gas.[10]
The terbium(III) cation (Tb3+) is brilliantlyfluorescent, in a bright lemon-yellow color that is the result of a strong greenemission line in combination with other lines in the orange and red. The yttrofluorite variety of the mineralfluorite 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 intofullerene molecules. Trivalenteuropium (Eu3+) and Tb3+ ions are among the lanthanide ions that have garnered the most attention because of their strong luminosity and great color purity.[12][13]
Terbium has a simpleferromagnetic ordering at temperatures below 219 K. Above 219 K, it turns into ahelical antiferromagnetic state in which all of the atomic moments in a particularbasal plane layer are parallel and oriented at a fixed angle to the moments of adjacent layers. This antiferromagnetism transforms into a disorderedparamagnetic state at 230 K.[14]
The most common oxidation state of terbium is +3 (trivalent), such as inTbCl 3. In the solid state, tetravalent terbium is also known, in compounds such as terbium oxide (TbO2) and terbium tetrafluoride.[16] In solution, terbium typically forms trivalent species, but can be oxidized to the tetravalent state withozone in highly basic aqueous conditions.[17]
The coordination and organometallic chemistry of terbium is similar to other lanthanides. In aqueous conditions, terbium can be coordinated by ninewater molecules, which are arranged in atricapped trigonal prismatic molecular geometry.[18] Complexes of terbium with lower coordination number are also known, typically with bulky ligands likebis(trimethylsilyl)amide, which forms the three-coordinate tris[N,N-bis(trimethylsilyl)amide]terbium(III) (Tb[N(SiMe3)2]3) complex.[19]
Most coordination and organometallic complexes contain terbium in the trivalent oxidation state. Divalent Tb2+ complexes are also known, usually with bulkycyclopentadienyl-type ligands.[20][21][22] A few coordination compounds containing terbium in its tetravalent state are also known.[23][24][25]
Like mostrare-earth elements andlanthanides, terbium is usually found in the +3 oxidation state. Likecerium andpraseodymium, terbium can also form a +4 oxidation state,[26] although it is unstable in water.[27] It is possible for terbium to be found in the 0,[28][29] +1,[30] and +2[26] oxidation states.
Terbium sulfate,Tb2(SO4)3 (top), fluoresces green under ultraviolet light (bottom)
Terbium combines with nitrogen, carbon, sulfur, phosphorus, boron, selenium, silicon and arsenic at elevated temperatures, forming various binary compounds such asTbH2,TbH3,TbB2,TbSi2,TbN,TbP,TbS,Tb2S3,TbSe, andTbTe.[31] In these compounds, terbium mainly exhibits the oxidation state +3, with the +2 state appearing rarely. Terbium(II) halides are obtained byannealing terbium(III) halides in presence of metallic terbium intantalum containers. Terbium also forms the sesquichlorideTb2Cl3, which can be further reduced to terbium(I) chloride (TbCl) by annealing at 800 °C; this compound forms platelets with layered graphite-like structure.[32]
WhenTbF4 andcaesium fluoride (CsF) is mixed in a stoichiometric ratio in a fluorine gas atmosphere, caesium pentafluoroterbate (CsTbF5) is obtained. It is anorthorhombic crystal withspace groupCmca and a layered structure composed of [TbF8]4− and 11-coordinated Cs+.[35] The compound barium hexafluoroterbate (BaTbF6), an orthorhombic crystal with space groupCmma, can be prepared in a similar method. The terbium fluoride ion [TbF8]4−[36] also exists in the structure of potassium terbium fluoride crystals.[37][38]
Terbium(III) oxide or terbia is the main oxide of terbium, and appears as a dark brown water-insoluble solid. It is slightly hygroscopic[39] and is the main terbium compound found in rare earth-containing minerals and clays.[40]
Naturally occurring terbium is composed of its only stableisotope, terbium-159; the element is thusmononuclidic andmonoisotopic.[1] Thirty-nineradioisotopes have been characterized from135Tb to174Tb.[9] The most stablesynthetic radioisotopes of terbium are158Tb, with ahalf-life of 180 years, and157Tb, with a half-life of 71 years. All of the remainingradioactive 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.[9] The primarydecay mode before the most abundant stable isotope,159Tb, iselectron capture, which results in production ofgadolinium isotopes, and the primary mode after isbeta minus decay, resulting indysprosium isotopes.[9]
The element also has 31nuclear 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.[9]
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 aserbium. "Erbia", the oxide containing what is now known as terbium, originally was the fraction that was yellow or dark orange in solution.[43][45] The insoluble oxide of this element was noted to be tinged brown,[48][49][39] and soluble oxides after combustion were noted to be colorless.[50] Until the advent of spectral analysis, arguments went back and forth as to whether erbia even existed. Spectral analysis byMarc Delafontaine allowed the separate elements and their oxides to be identified,[47] but in his publications, the names of erbium and terbium were switched,[51] following a brief period where terbium was renamed "mosandrum", after Mosander.[52] The names have remained switched ever since.[45]
The early years of preparing terbium (as terbium oxide) were difficult. Metal oxides fromgadolinite andsamarskite were dissolved innitric acid, and the solution was further separated usingoxalic acid andpotassium sulfate. There was great difficulty in separating erbia from terbia; in 1881, it was noted that there was no satisfactory method to separate the two.[50] By 1914, different solvents had been used to separate terbium from its host minerals, but the process of separating terbium from its neighbor elements -gadolinium anddysprosium - was described as "tedious" but possible.[53] Modern terbium extraction methods are based on theliquid–liquid extraction process developed by Werner Fischer et al., in 1937.[54]
Xenotime, a mineral source of rare earth elements including terbium
Terbium occurs with other rare earth elements in many minerals, including monazite ((Ce,La,Th,Nd,Y)PO4 with up to 0.03% terbium),xenotime (YPO4) andeuxenite ((Y,Ca,Er,La,Ce,U,Th)(Nb,Ta,Ti)2O6 with 1% or more terbium). The crust abundance of terbium is estimated as 1.2 mg/kg.[31] No terbium-dominant mineral has yet been found.[55]
Currently, the richest commercial sources of terbium are the ion-adsorptionclays ofsouthern China;[40] the concentrates with about two-thirds yttrium oxide by weight have about 1% terbia. Small amounts of terbium occur in bastnäsite and monazite; when these are processed by solvent extraction to recover the valuable heavy lanthanides assamarium-europium-gadolinium concentrate, terbium is recovered therein. Due to the large volumes of bastnäsite processed relative to the ion-adsorption clays, a significant proportion of the world's terbium supply comes from bastnäsite.[10]
In 2018, a rich terbium supply was discovered off the coast ofJapan'sMinamitori Island, with the stated supply being "enough to meet the global demand for 420 years".[57]
Crushed terbium-containing minerals are treated with hot concentratedsulfuric 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 withammonium oxalate to convert rare earths into their insolubleoxalates. The oxalates are decomposed to oxides by heating. The oxides are dissolved innitric acid that excludes one of the main components, cerium, whose oxide is insoluble inHNO3. Terbium is separated as adouble salt withammonium nitrate by crystallization.[31]
The most efficient separation routine for terbium salt from the rare-earth salt solution ision exchange. In this process, rare-earth ions aresorbed onto suitable ion-exchange resin by exchange with hydrogen, ammonium or cupric ions present in the resin. The rare earth ions are then selectively washed out by suitablecomplexing agents. As with other rare earths, terbium metal is produced by reducing the anhydrous chloride or fluoride with calcium metal. Calcium andtantalum impurities can be removed by vacuum remelting, distillation, amalgam formation orzone melting.[31][47]
In 2020, the annual demand for terbium was estimated at 340 tonnes (750,000 lb).[40] Terbium is not distinguished from other rare earths in theUnited States Geological Survey's Mineral Commodity Summaries, which in 2024 estimated the global reserves of rare earth minerals at 110,000,000 tonnes (2.4×1011 lb).[58]
Terbium is also used inalloys and in the production of electronic devices. As a component ofTerfenol-D, terbium is used inactuators, in navalsonar systems,sensors, and other magnetomechanical devices. Terfenol-D is a terbium alloy that expands or contracts in the presence of a magnetic field.[60] It has the highestmagnetostriction of anyalloy.[61] It is used to increase verdet constant in long-distance fiber optic communication.[62][63] Terbium-doped garnets are also used in optical isolators, which prevents reflected light from traveling back along the optical fiber.[64]
Terbium oxides are used in greenphosphors in fluorescent lamps, color TV tubes,[10] and flat screen monitors.[65] Terbium, along with all otherlanthanides exceptlanthanum andlutetium, isluminescent in the 3+ oxidation state.[66] The brilliant fluorescence allows terbium to be used as aprobe in biochemistry, where it somewhat resemblescalcium 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 providetrichromatic 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 doesincandescent lighting.[10]
In 2023, terbium compounds were used to create a lattice with a singleiron atom that was then examined bysynchrotronx-ray beam. This was the first successful attempt to characterize a single atom at sub-atomic levels.[67]
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[70] and the lack of market demand forestalling evidence of toxicity.[71]
Some studies demonstrate environmental accumulation of terbium as hazardous to fish and plants.[72][73] High exposures of terbium may enhance the toxicity of other substances causingendocytosis inplant cells.[74]
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