Terbium

Terbium is a chemical element with the symbol Tb and atomic number 65. It is a silvery-white rare earth metal that is malleable, ductile and soft enough to be cut with a knife. Terbium is never found in nature as a free element, but it is contained in many minerals, including cerite, gadolinite, monazite, xenotime and euxenite.

Terbium is used to dope 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. As a component of Terfenol-D (an alloy which expands and contracts in magnetic field more than any other alloy), terbium is of use in actuators, in naval sonar systems and sensors. Terbium oxide is used in green phosphors in fluorescent lamps and color TV tubes. Terbium "green" phosphors (which fluoresce a brilliant lemon-yellow) are combined with divalent europium blue phosphors and trivalent europium red phosphors to provide the "trichromatic" lighting technology, which is by far the largest consumer of the world's terbium supply.
Characteristics

Physical

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 as compared to other lanthanides.[1] Terbium exists in two crystal allotropes with a transformation temperature of 1289 °C between them.[2]

The terbium(III) cation 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 only for research purposes. Single Tb atoms have been isolated by implanting them into fullerene molecules.[3]

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 unusual antiferromagnetism transforms into a disordered paramagnetic state at 230 K.[4]

Chemical

The most common valence state of terbium is +3, as in Tb2O3. The +4 state is known in TbO2 and TbF4.[5][6] Terbium burns readily to form a mixed terbium(III,IV) oxide:

8 Tb + 7 O2 → 2 Tb4O7

In solution, terbium forms only trivalent ions. Terbium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form terbium hydroxide:

2 Tb (s) + 6 H2O (l) → 2 Tb(OH)3 (aq) + 3H2 (g)

Terbium metal reacts with all the halogens:

2 Tb (s) + 3 F2 (g) → 2 TbF3 (s) [white]
2 Tb (s) + 3 Cl2 (g) → 2 TbCl3 (s) [white]
2 Tb (s) + 3 Br2 (g) → 2 TbBr3 (s) [white]
2 Tb (s) + 3 I2 (g) → 2 TbI3 (s)

Terbium dissolves readily in dilute sulfuric acid to form solutions containing the pale pink Tb(III) ions, which exist as a [Tb(OH2)9]3+ complexes:[7]

2 Tb (s) + 3 H2SO4 (aq) → 2 Tb3+ (aq) + 3 SO2−4 (aq) + 3 H2 (g)

Compounds
See also: Category:Terbium 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, Tb2S3, TbSe, TbTe and TbN.[6] In those compounds, Tb mostly exhibit oxidation states +3 and sometimes +2. Terbium(II) halogenides are obtained by annealing Tb(III) halogenides in presence of metallic Tb in tantalum containers. Terbium also forms sesquichloride Tb2Cl3, which can be further reduced to TbCl by annealing at 800 °C. This terbium(I) chloride forms platelets with layered graphite-like structure.[8]

Other compounds include

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

Terbium(IV) fluoride is a strong fluorinating agent, emitting relatively pure atomic fluorine when heated[9] rather than the mixture of fluoride vapors emitted from CoF3 or CeF4.

Isotopes
Main article: isotopes of terbium

Naturally occurring terbium is composed of 1 stable isotope, 159Tb. 33 radioisotopes have been characterized, with the most stable being 158Tb with a half-life of 180 years, 157Tb with a half-life of 71 years, and 160Tb with a half-life of 72.3 days. All of the remaining radioactive isotopes have half-lives that are less than 6.907 days, and the majority of these have half-lives that are less than 24 seconds. This element also has 18 meta states, with the most stable being 156m1Tb (t½ 24.4 hours), 154m2Tb (t½ 22.7 hours) and 154m1Tb (t½ 9.4 hours).

The primary decay mode before the most abundant stable isotope, 159Tb, is electron capture, and the primary mode after is beta minus decay. The primary decay products before 159Tb are element Gd (gadolinium) isotopes, and the primary products after are element Dy (dysprosium) isotopes.

History

Terbium was discovered in 1843 by Swedish chemist Carl Gustaf Mosander, who detected it as an impurity in Yttrium oxide, Y2O3, and named after the village Ytterby in Sweden. It was not isolated in pure form until the recent advent of ion exchange techniques.[10]

When Mosander first partitioned "yttria" into three fractions, "terbia" was the fraction that contained the pink color (due to what is now known as erbium), and "erbia" was the fraction that was essentially colorless in solution, but gave a brown-tinged oxide. Later workers had difficulty in observing the latter, but the pink fraction was impossible to miss. Arguments went back and forth as to whether "erbia" even existed. In the confusion, the original names got reversed, and the exchange of names stuck. It is now thought that those workers who used the double sodium or potassium sulfates to remove "ceria" from "yttria" inadvertently lost the terbium content of the system into the ceria-containing precipitate. In any case, what is now known as terbium was only about 1% of the original yttria, but that was sufficient to impart a yellowish color to the oxide. Thus, terbium was a minor component in the original terbium fraction, dominated by its immediate neighbors, gadolinium and dysprosium. Thereafter, whenever other rare earths were teased apart from this mixture, whichever fraction gave the brown oxide retained the terbium name, until at last it was pure. The 19th century investigators did not have the benefit of fluorescence technology, wherewith to observe the brilliant fluorescence that would have made this element much easier to track in mixtures.[10]

Occurrence
Xenotime

Terbium is never found in nature as a free element, but it is contained in many minerals, including cerite, gadolinite, monazite ((Ce,La,Th,Nd,Y)PO4, which contains up to 0.03% of terbium), xenotime (YPO4) and euxenite ((Y,Ca,Er,La,Ce,U,Th)(Nb,Ta,Ti)2O6, which contains 1% or more of terbium). The crust abundance of terbium is estimated as 1.2 mg/kg.[6]

The richest current commercial sources of terbium are the ion-adsorption clays of southern China. The high-yttrium concentrate versions of these are about two-thirds yttrium oxide by weight, and about 1% terbia. However, small amounts occur in bastnäsite and monazite, and when these are processed by solvent-extraction to recover the valuable heavy lanthanides in the form of "samarium-europium-gadolinium concentrate" (SEG concentrate), the terbium content of the ore ends up therein. Due to the large volumes of bastnäsite processed, relative to the richer ion-adsorption clays, a significant proportion of the world's terbium supply comes from bastnäsite.[2]

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. After that the solution is treated with ammonium oxalate to convert rare earths into their insoluble oxalates. The oxalates are converted to oxides by annealing. The oxides are dissolved in nitric acid that excludes one of the main components, cerium, whose oxide is insoluble in HNO3. Terbium is separated as a double salt with ammonium nitrate by crystallization.[6]

The most efficient separation routine for terbium salt from the rare-earth salt solution is ion exchange. In this process, rare-earth ions are sorbed 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 suitable complexing agent. As with other rare earths, terbium metal is produced by reducing the anhydrous chloride or fluoride with calcium metal. Calcium and tantalum impurities can be removed by vacuum remelting, distillation, amalgam formation or zone melting.[6]

Applications

Terbium is also used in alloys and in the production of electronic devices. As a component of Terfenol-D, terbium is of use in actuators, in naval sonar systems, sensors, in the SoundBug device (its first commercial application), and other magnetomechanical devices. Terfenol-D is an alloy that expands or contracts in the presence of a magnetic field. It has the highest magnetostriction of any alloy.[11]

Terbium oxide is used in green phosphors in fluorescent lamps and color TV tubes. Sodium terbium borate is used in solid state devices. 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 the "trichromatic" lighting technology, 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.[2]

Precautions

As with the other lanthanides, terbium compounds are of low to moderate toxicity, although their toxicity has not been investigated in detail. Terbium has no known biological role.[2]