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 that expands and contracts when exposed to
magnetic fields more than any other alloy), terbium is of use in
actuators, in naval sonar systems and in 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
used for standard illumination in indoor lighting.
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. 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 specifically
for research. Single Tb atoms have been isolated by implanting them
into fullerene molecules. 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.
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 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", 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.