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.

The largest consumer of the world's terbium supply is in "green" phosphors (which are usually yellow). Terbium oxide is in fluorescent lamps and 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 "trichromatic" lighting technology, a high-efficiency white light for standard illumination uses 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 as compared to other lanthanides. 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 unusual antiferromagnetism transforms into a disordered paramagnetic state at 230 K.

Chemical properties

The most common valence state of terbium is +3, as in . The +4 state is known in TbO₂ and TbF₄.

:2 Tb (s) + 3 H₂SO₄ (aq) → 2 Tb³⁺ (aq) + 3 (aq) + 3 H₂ (g)

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.

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 rather than the mixture of fluoride vapors emitted from CoF₃ or CeF₄.

Isotopes

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 (t_½ 24.4 hours), ^154m2Tb (t_½ 22.7 hours) and ^154m1Tb (t_½ 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.

Occurrence

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.

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.

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.

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.

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.

References

External links

WebElements.com – Terbium

It's Elemental – Terbium

Category:Chemical elements Category:Lanthanides Category:Terbium Category:Reducing agents