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History

Terbium is fourteenth in abundance among the 17 metals usually counted as "rare-earths", present in the earth's crust to the extent of only 0.9 ppm (about 1 teaspoon in every 63 tons of earth).  Terbium was discovered in 1843 by Swedish chemist Carl Gustaf Mosander (along with erbium), 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.

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.

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.  Small amounts of terbium are used in special lasers and some solid state devices. The monazite sand from which terbium is generally extracted contains only about 0.03% by weight of Tb.

Occurrence

Terbium is never found in nature as the 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).

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, its oxide is used in green phosphors in fluorescent lamps and color TV tubes. Sodium terbium borate is used in solid state devices.

Compounds

Isotopes

Naturally occurring terbium is composed of 1 stable isotope, 159-Tb. 36 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 after is beta minus decay.  The primary decay products before 159-Tb are element Gd (gadolinium) isotopes, and the primary products after 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.