Terbium is a chemical element with 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. The
ninth member of the lanthanide series, terbium
is a fairly electropositive metal that reacts
with water, evolving hydrogen gas. Terbium
is never found in nature as a free element,
but it is contained in many minerals, including
cerite, gadolinite, monazite, xenotime, and
euxenite.
Swedish chemist Carl Gustaf Mosander discovered
terbium as a chemical element in 1843. He
detected it as an impurity in yttrium oxide,
Y2O3. Yttrium and terbium are named after
the village of Ytterby in Sweden. Terbium
was not isolated in pure form until the advent
of ion exchange techniques.
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.
Most of the world's terbium supply is used
in green phosphors. Terbium oxide is in fluorescent
lamps and television and monitor cathode ray
tubes (CRTs). Terbium green phosphors 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.
== 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 compared to the earlier, more reactive
lanthanides in the first half of the lanthanide
series. Terbium exists in two crystal allotropes
with a transformation temperature of 1289
°C between them. The 65 electrons of a terbium
atom are arranged in the electron configuration
[Xe]4f96s2; normally, only three electrons
can be removed before the nuclear charge becomes
too great to allow further ionization, but
in the case of terbium, the stability of the
half-filled [Xe]4f7 configuration allows further
ionization of a fourth electron in the presence
of very strong oxidizing agents such as fluorine
gas.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 terbium
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.
=== 
Chemical properties ===
The most common oxidation state of terbium
is +3, as in Tb2O3. The +4 state is known
in TbO2 and TbF4.
Terbium burns readily to form a mixed terbium(III,IV)
oxide:
8 Tb + 7 O2 → 2 Tb4O7In 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 + 6 H2O → 2 Tb(OH)3 + 3 H2↑Terbium
metal reacts with all the halogens, forming
white trihalides:
2 Tb + 3 X2 → 2 TbX3 (X = F, Cl, Br, I)Terbium
dissolves readily in dilute sulfuric acid
to form solutions containing the pale pink
terbium(III) ions, which exist 
as [Tb(OH2)9]3+ complexes:
2 Tb (s) + 3 H2SO4 → 2 Tb3+ + 3 SO2−4
+ 3 H2↑
=== 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. In those compounds,
Tb mostly exhibits the 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.Other
compounds include
Chlorides: TbCl3
Bromides: TbBr3
Iodides: TbI3
Fluorides: TbF3, TbF4Terbium(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
its only stable isotope, terbium-159; the
element is thus called mononuclidic and monoisotopic.
Thirty-six radioisotopes have been characterized,
with the heaviest being terbium-171 (with
atomic mass of 170.95330(86) u) and lightest
being terbium-135 (exact mass unknown). The
most stable synthetic radioisotopes of terbium
are terbium-158, with a half-life of 180 years,
and terbium-157, with a half-life of 71 years.
All of the remaining radioactive isotopes
have half-lives that are much less than a
quarter of a year, and the majority of these
have half-lives that are less than half a
minute. The primary decay mode before the
most abundant stable isotope, 159Tb, is electron
capture, which results in production of gadolinium
isotopes, and the primary mode after is beta
minus decay, resulting in dysprosium isotopes.The
element also has 27 nuclear isomers, with
masses of 141–154, 156, and 158 (not every
mass number corresponds to only one isomer).
The most stable of them are terbium-156m,
with half-life of 24.4 hours and terbium-156m2,
with half-life of 22.7 hours; this is longer
than half-lives of most ground states of radioactive
terbium isotopes, except only those with mass
numbers 155–161.
== History ==
Swedish chemist Carl Gustaf Mosander discovered
terbium in 1843. He detected it as an impurity
in yttrium oxide, Y2O3. Yttrium is named after
the village of Ytterby in Sweden. Terbium
was not isolated in pure form until the advent
of ion exchange techniques.Mosander first
separated yttria into three fractions, all
named for the ore: yttria, erbia, and terbia.
"Terbia" was originally the fraction that
contained the pink color, due to the element
now known as erbium. "Erbia" (containing what
we now call terbium) originally was the fraction
that was essentially colorless in solution.
The insoluble oxide of this element was noted
to be tinged brown.
Later workers had difficulty in observing
the minor colorless "erbia", but the soluble
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,
so that the pink fraction referred eventually
to the solution containing erbium (which in
solution, is pink). It is now thought that
workers using double sodium or potassium sulfates
to remove ceria from yttria inadvertently
lost the terbium into the ceria-containing
precipitate. 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 yttrium oxide. Thus, terbium
was a minor component in the original fraction
containing it, where it was 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, the brown oxide
of terbium was obtained in pure form. The
19th century investigators did not have the
benefit of the UV fluorescence technology
to observe the brilliant yellow or green Tb(III)
fluorescence that would have made terbium
easier to identify in solid mixtures or solutions.
== Occurrence ==
Terbium is contained along with other rare
earth elements in many minerals, including
monazite ((Ce,La,Th,Nd,Y)PO4 with up to 0.03%
terbium), xenotime (YPO4) and euxenite ((Y,Ca,Er,La,Ce,U,Th)(Nb,Ta,Ti)2O6
with 1% or more terbium). The crust abundance
of terbium is estimated as 1.2 mg/kg. No terbium-dominant
mineral has yet been found.Currently, the
richest commercial sources of terbium are
the ion-adsorption clays of southern China;
the concentrates with about two-thirds yttrium
oxide by weight have about 1% terbia. Small
amounts of terbium occur in bastnäsite and
monazite; when these are processed by solvent
extraction to recover the valuable heavy lanthanides
as samarium-europium-gadolinium concentrate,
terbium is recovered therein. Due to the large
volumes of bastnäsite processed relative
to the ion-adsorption clays, a significant
proportion of the world's terbium supply comes
from bastnäsite.
== 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 decomposed
to oxides by heating. 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.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.
== Applications ==
Terbium is used as a dopant in 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, terbium is used
in actuators, in naval sonar systems, sensors,
in the SoundBug device (its first commercial
application), and other magnetomechanical
devices. Terfenol-D is a terbium 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.Terbium is also used to detect endospores,
as it acts as an assay of dipicolinic acid
based on photoluminescence.
== 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
