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= Terbium =
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Introduction
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Terbium is a chemical element; it has symbol Tb and atomic number 65.
It is a silvery-white, rare earth metal that is malleable and ductile.
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 ().
Yttrium and terbium, as well as erbium and ytterbium, 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 in solid-state devices, and as a crystal
stabilizer of fuel cells that 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. Terbium is
considered non-hazardous, though its biological role and toxicity have
not been researched in depth.
Most of the world's terbium supply is used in green phosphors. Terbium
oxide is used 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 in indoor lighting.
Physical properties
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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 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. The eleven 4f and 6s electrons are valence. 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 (Tb3+) 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. Trivalent europium (Eu3+) and Tb3+ ions are among the
lanthanide ions that have garnered the most attention because of their
strong luminosity and great color purity.
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 antiferromagnetism transforms into a disordered
paramagnetic state at 230 K.
Chemical properties
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Terbium metal is an electropositive element and oxidizes in the
presence of most acids (such as sulfuric acid), all of the halogens,
and water.
:
:
:
Terbium oxidizes readily in air to form a mixed terbium(III,IV) oxide:
:
The most common oxidation state of terbium is +3 (trivalent), such as
in Terbium trichloride. In the solid state, tetravalent terbium is
also known, in compounds such as terbium oxide () and terbium
tetrafluoride. In solution, terbium typically forms trivalent species,
but can be oxidized to the tetravalent state with ozone in highly
basic aqueous conditions.
The coordination and organometallic chemistry of terbium is similar to
other lanthanides. In aqueous conditions, terbium can be coordinated
by nine water molecules, which are arranged in a tricapped trigonal
prismatic molecular geometry. Complexes of terbium with lower
coordination number are also known, typically with bulky ligands like
bis(trimethylsilyl)amide, which forms the three-coordinate
tris[N,N-bis(trimethylsilyl)amide]terbium(III) () complex.
Most coordination and organometallic complexes contain terbium in the
trivalent oxidation state. Divalent Tb2+ complexes are also known,
usually with bulky cyclopentadienyl-type ligands. A few coordination
compounds containing terbium in its tetravalent state are also known.
Oxidation states
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Like most rare-earth elements and lanthanides, terbium is usually
found in the +3 oxidation state. Like cerium and praseodymium, terbium
can also form a +4 oxidation state, although it is unstable in water.
It is possible for terbium to be found in the 0, +1, and +2 oxidation
states.
Compounds
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Terbium combines with nitrogen, carbon, sulfur, phosphorus, boron,
selenium, silicon and arsenic at elevated temperatures, forming
various binary compounds such as , , , , , and . In these compounds,
terbium mainly exhibits the oxidation state +3, with the +2 state
appearing rarely. Terbium(II) halides are obtained by annealing
terbium(III) halides in presence of metallic terbium in tantalum
containers. Terbium also forms the sesquichloride , which can be
further reduced to terbium(I) chloride () by annealing at 800 °C; this
compound forms platelets with layered graphite-like structure.
Terbium(IV) fluoride () is the only halide that tetravalent terbium
can form. It has strong oxidizing properties and is a strong
fluorinating agent, emitting relatively pure atomic fluorine when
heated, rather than the mixture of fluoride vapors emitted from
cobalt(III) fluoride or cerium(IV) fluoride. It can be obtained by
reacting terbium(III) chloride or terbium(III) fluoride with fluorine
gas at 320 °C:
: 2 TbF3 + F2 → 2 TbF4
When and caesium fluoride (CsF) is mixed in a stoichiometric ratio in
a fluorine gas atmosphere, caesium pentafluoroterbate () is obtained.
It is an orthorhombic crystal with space group 'Cmca' and a layered
structure composed of [TbF8]4− and 11-coordinated Cs+. The compound
barium hexafluoroterbate (), an orthorhombic crystal with space group
'Cmma', can be prepared in a similar method. The terbium fluoride ion
[TbF8]4− also exists in the structure of potassium terbium fluoride
crystals.
Terbium(III) oxide or terbia is the main oxide of terbium, and appears
as a dark brown water-insoluble solid. It is slightly hygroscopic and
is the main terbium compound found in rare earth-containing minerals
and clays.
Other compounds include:
* Chlorides:
* Bromides:
* Iodides:
* Fluorides: ,
Isotopes
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Naturally occurring terbium is composed of its only stable isotope,
terbium-159; the element is thus mononuclidic and monoisotopic.
Thirty-nine radioisotopes have been characterized, with the heaviest
being terbium-174 and lightest being terbium-135 (both with unknown
exact mass). 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 less than three months, 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, (159)Tb, 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 31 nuclear isomers, with masses of 141-154, 156,
158, 162, and 164-168 (not every mass number corresponds to only one
isomer). The most stable of them are terbium-156m, with a half-life of
24.4 hours, and terbium-156m2, with a half-life of 22.7 hours; this is
longer than half-lives of most ground states of radioactive terbium
isotopes, except those with mass numbers 155-161.
Terbium-149, with a half-life of 4.1 hours, is a promising candidate
in targeted alpha therapy and positron emission tomography.
History
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Swedish chemist Carl Gustaf Mosander discovered terbium in 1843. He
detected it as an impurity in yttrium oxide, , then known as yttria.
Yttrium, erbium, and terbium are all 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", the oxide containing what is now known as terbium,
originally was the fraction that was yellow or dark orange in
solution. The insoluble oxide of this element was noted to be tinged
brown, and soluble oxides after combustion were noted to be colorless.
Until the advent of spectral analysis, arguments went back and forth
as to whether erbia even existed. Spectral analysis by Marc
Delafontaine allowed the separate elements and their oxides to be
identified, but in his publications, the names of erbium and terbium
were switched, following a brief period where terbium was renamed
"mosandrum", after Mosander. The names have remained switched ever
since.
The early years of preparing terbium (as terbium oxide) were
difficult. Metal oxides from gadolinite and samarskite were dissolved
in nitric acid, and the solution was further separated using oxalic
acid and potassium sulfate. There was great difficulty in separating
erbia from terbia; in 1881, it was noted that there was no
satisfactory method to separate the two. By 1914, different solvents
had been used to separate terbium from its host minerals, but the
process of separating terbium from its neighbor elements - gadolinium
and dysprosium - was described as "tedious" but possible. Modern
terbium extraction methods are based on the liquid-liquid extraction
process developed by Werner Fischer et al., in 1937.
Occurrence
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alt=A sample of the mineral xenotime at the Mineralogical Museum,
Bonn, Germany
Terbium occurs with other rare earth elements in many minerals,
including monazite ( with up to 0.03% terbium), xenotime () and
euxenite ( 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.
Terbium (as the species Tb II) has been detected in the atmosphere of
KELT-9b, a hot-Jupiter planet outside the Solar System.
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.
In 2018, a rich terbium supply was discovered off the coast of Japan's
Minamitori Island, with the stated supply being "enough to meet the
global demand for 420 years".
Production
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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.
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 . 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 agents. 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.
In 2020, the annual demand for terbium was estimated at . Terbium is
not distinguished from other rare earths in the United States
Geological Survey's Mineral Commodity Summaries, which in 2024
estimated the global reserves of rare earth minerals at .
Applications
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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 zirconium dioxide ().
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, 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. It is used to increase verdet constant in long-distance
fiber optic communication. Terbium-doped garnets are also used in
optical isolators, which prevents reflected light from traveling back
along the optical fiber.
Terbium oxides are used in green phosphors in fluorescent lamps, color
TV tubes, and flat screen monitors. Terbium, along with all other
lanthanides except lanthanum and lutetium, is luminescent in the 3+
oxidation state. 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 trichromatic lighting,
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.
In 2023, terbium compounds were used to create a lattice with a single
iron atom that was then examined by synchrotron x-ray beam. This was
the first successful attempt to characterize a single atom at
sub-atomic levels.
Safety
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Terbium, along with many of the other rare earth elements, is poorly
studied in terms of its toxicology and environmental impacts. Few
health-based guidance values for safe exposure to terbium are
available. No values are established in the United States by the
Occupational Safety and Health Administration or American Conference
of Governmental Industrial Hygienists at which terbium exposure
becomes hazardous, and it is not considered a hazardous substance
under the Globally Harmonized System of Classification and Labelling
of Chemicals.
Reviews of the toxicity of the rare earth elements place terbium and
its compounds as "of low to moderately toxicity", remarking on the
lack of detailed studies on their hazards and the lack of market
demand forestalling evidence of toxicity.
Some studies demonstrate environmental accumulation of terbium as
hazardous to fish and plants. High exposures of terbium may enhance
the toxicity of other substances causing endocytosis in plant cells.
See also
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* Terbium compounds
* List of elements facing shortage
External links
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* [
http://www.webelements.com/webelements/elements/text/Tb/index.html
WebElements.com - Terbium]
* [
http://education.jlab.org/itselemental/ele065.html It's Elemental -
Terbium]
License
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Original Article:
http://en.wikipedia.org/wiki/Terbium
