Terbium

Terbium (pronounced /ˈtɝbiəm/) is a chemical element with the symbol Tb and atomic number 65.

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Notable characteristics

Terbium is a silvery-white rare earth metal that is malleable, ductile and soft enough to be cut with a knife. It is reasonably stable in air, and two crystal allotropes exist, with a transformation temperature of 1289 °C. Terbium(III) cation is brilliantly fluorescent, in a beautiful 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.

Applications

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, together with ZrO₂.

Terbium is also used in alloys and in the production of electronic devices. As a component of Terfenol-D (an alloy that expands or contracts to a high degree in the presence of a magnetic field), terbium is of use in actuators, sensors and other magenetomechanical devices.

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.

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. 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.

Terbium is classified as a rare earth element. The term "rare" is misleading because terbium is more common than metals such as silver and mercury. The name "rare earth" meant something else to early chemists. It was used because the rare earth elements were very difficult to separate from each other. They were not "rare" in the Earth, but they were "rarely" used for anything. Also, it was initially believed that they were rare, but actually it was later discovered that it wasn't the case.^{[citation needed]}.

Occurrence

Terbium is never found in nature as a free element, but it is contained in many minerals, including cerite, gadolinite, monazite ((Ce,LaTh,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 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 bastnaesite 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 bastnaesite processed, relative to the richer ion-adsorption clays, a significant proportion of the world's terbium supply comes from bastnaesite.

Compounds

Terbium compounds include:

• Fluorides: TbF₃, TbF₄
• Chlorides: TbCl₃
• Bromides: TbBr₃
• Iodides: TbI₃
• Oxides: Tb₂O₃, Tb₄O₇
• Sulfides: Tb₂S₃
• Nitrides: TbN

See also terbium compounds.

Isotopes

Main article: isotopes of terbium

Naturally occurring terbium is composed of 1 stable isotope, 159-Tb. 33 radioisotopes have been characterized, with the most stable being 158-Tb with a half-life of 180 years, 157-Tb with a half-life of 71 years, and 160-Tb with a half-life of 72.3 days. All of the remaining radioactive isotopes have half-lifes that are less than 6.907 days, and the majority of these have half lifes that are less than 24 seconds. This element also has 18 meta states, with the most stable being 156m1-Tb (t_½ 24.4 hours), 154m2-Tb (t_½ 22.7 hours) and 154m1-Tb (t_½ 9.4 hours).

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

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

• Los Alamos National Laboratory – Terbium

Further reading

• D.M. Gruen, W.C. Koehler, and J.J. Katz (April 1951). "Higher Oxides of the Lanthanide Elements: Terbium Dioxide" (PDF). Journal of the American Chemical Society: 1475.

See also

• Erbium
• Ytterbium
• Yttrium