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= Thulium =
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Introduction
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Thulium is a chemical element; it has symbol Tm and atomic number 69.
It is the thirteenth element in the lanthanide series of metals. It is
the second-least abundant lanthanide in the Earth's crust, after
radioactively unstable promethium. It is an easily workable metal with
a bright silvery-gray luster. It is fairly soft and slowly tarnishes
in air. Despite its high price and rarity, thulium is used as a dopant
in solid-state lasers, and as the radiation source in some portable
X-ray devices. It has no significant biological role and is not
particularly toxic.
In 1879, the Swedish chemist Per Teodor Cleve separated two previously
unknown components, which he called holmia and thulia, from the
rare-earth mineral erbia; these were the oxides of holmium and
thulium, respectively. His example of thulium oxide contained
impurities of ytterbium oxide. A relatively pure sample of thulium
oxide was first obtained in 1911. The metal itself was first obtained
in 1936 by Wilhelm Klemm and Heinrich Bommer.
Like the other lanthanides, its most common oxidation state is +3,
seen in its oxide, halides and other compounds. In aqueous solution,
like compounds of other late lanthanides, soluble thulium compounds
form coordination complexes with nine water molecules.
Physical properties
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Pure thulium metal has a bright, silvery luster, which tarnishes on
exposure to air. The metal can be cut with a knife, as it has a Mohs
hardness of 2 to 3; it is malleable and ductile. Thulium is
ferromagnetic below 32K, antiferromagnetic between 32 and 56K, and
paramagnetic above 56K.
Thulium has two major allotropes: the tetragonal α-Tm and the more
stable hexagonal β-Tm.
Chemical properties
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Thulium tarnishes slowly in air and burns readily at 150°C to form
thulium(III) oxide:
:
Thulium is quite electropositive and reacts slowly with cold water and
quite quickly with hot water to form thulium hydroxide:
: {{chem2|2Tm_{(s)} + 6 H2O_{(l)} → 2Tm(OH)3_{(aq)} + 3H2_{(g)}|}}
Thulium reacts with all the halogens. Reactions are slow at room
temperature, but are vigorous above 200°C:
: {{chem2|2Tm_{(s)} + 3F2_{(g)} → 2TmF3_{(s)}|}} (white)
: {{chem2|2Tm_{(s)} + 3Cl2_{(g)} → 2TmCl3_{(s)}|}} (yellow)
: {{chem2|2Tm_{(s)} + 3Br2_{(g)} → 2TmBr3_{(s)}|}} (white)
: {{chem2|2Tm_{(s)} + 3I2_{(g)} → 2TmI3_{(s)}|}} (yellow)
Thulium dissolves readily in dilute sulfuric acid to form solutions
containing the pale green Tm(III) ions, which exist as complexes:
: {{chem2|2Tm_{(s)} + 3H2SO4_{(aq)} → 2Tm(3+)_{(aq)} + 3SO4(2-)_{(aq)}
+ 3H2_{(aq)}|}}
Thulium reacts with various metallic and non-metallic elements forming
a range of binary compounds, including , , , , , , , , , and . Like
most lanthanides, the +3 state is most common and is the only state
observed in thulium solutions. Thulium exists as a ion in solution.
In this state, the thulium ion is surrounded by nine molecules of
water. ions exhibit a bright blue luminescence. Because it occurs
late in the series, the +2 oxidation state can also exist, stabilized
by the nearly full 4f electron shell, but occurs only in solids.
Thulium's only known oxide is thulium oxide. This oxide is sometimes
called "thulia". Reddish-purple thulium(II) compounds can be made by
the reduction of thulium(III) compounds. Examples of thulium(II)
compounds include the halides (except the fluoride). Some hydrated
thulium compounds, such as and are green or greenish-white. Thulium
dichloride reacts very vigorously with water. This reaction results in
hydrogen gas and thulium(III) hydroxide exhibiting a fading reddish
color. Combination of thulium and chalcogens results in thulium
chalcogenides.
Thulium reacts with hydrogen chloride to produce hydrogen gas and
thulium chloride. With nitric acid it yields thulium nitrate, or .
Isotopes
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The isotopes of thulium range from {{chem2|^{144}Tm}} to
{{chem2|^{183}Tm}}. The primary decay mode before the most abundant
stable isotope, {{chem2|^{169}Tm}}, is electron capture, and the
primary mode after is beta emission. The primary decay products before
{{chem2|^{169}Tm}} are element 68 (erbium) isotopes, and the primary
products after are element 70 (ytterbium) isotopes.
Thulium-169 is thulium's only primordial isotope and is the only
isotope of thulium that is thought to be stable; it is predicted to
undergo alpha decay to holmium-165 with a very long half-life. The
longest-lived radioisotopes are thulium-171, which has a half-life of
1.92 years, and thulium-170, which has a half-life of 128.6 days. Most
other isotopes have half-lives of a few minutes or less.
In total, 40 isotopes and 26 nuclear isomers of thulium have been
detected. Most isotopes of thulium lighter than decay via electron
capture or beta-plus decay, although some exhibit significant alpha
decay or proton emission. Heavier isotopes undergo beta-minus decay.
History
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Thulium was discovered by Swedish chemist Per Teodor Cleve in 1879 by
looking for impurities in the oxides of other rare earth elements.
This was the same method Carl Gustaf Mosander earlier used to discover
some other rare earth elements.See:
* Cleve named thulium on p. 480: "'Pour le radical de l'oxyde placé
entre l'ytterbine et l'erbine, qui est caractérisé par la bande 'x'
dans la partie rouge du spectre, je propose la nom de 'thulium',
dérivé de Thulé, le plus ancien nom de la Scandinavie.'" (For the
radical of the oxide located between the oxides of ytterbium and
erbium, which is characterized by the 'x' band in the red part of the
spectrum, I propose the name of "thulium", [which is] derived from
'Thule', the oldest name of Scandinavia.)
*
* Cleve started by removing all of the known contaminants of erbia
(). Upon additional processing, he obtained two new substances; one
brown and one green. The brown substance was the oxide of the element
holmium and was named holmia by Cleve, and the green substance was the
oxide of an unknown element. Cleve named the oxide thulia and its
element thulium after Thule, an Ancient Greek place name associated
with Scandinavia or Iceland. Thulium's atomic symbol was initially Tu,
but later changed to Tm.
Thulium was so rare that none of the early workers had enough of it to
purify sufficiently to actually see the green color; they had to be
content with spectroscopically observing the strengthening of the two
characteristic absorption bands, as erbium was progressively removed.
The first researcher to obtain nearly pure thulium was Charles James,
a British expatriate working on a large scale at New Hampshire College
in Durham, USA. In 1911 he reported his results, having used his
discovered method of bromate fractional crystallization to do the
purification. He famously needed 15,000 purification operations to
establish that the material was homogeneous.
High-purity thulium oxide was first offered commercially in the late
1950s, as a result of the adoption of ion-exchange separation
technology. Lindsay Chemical Division of American Potash &
Chemical Corporation offered it in grades of 99% and 99.9% purity. The
price per kilogram oscillated between US$4,600 and $13,300 in the
period from 1959 to 1998 for 99.9% purity, and it was the second
highest for the lanthanides behind lutetium.
Occurrence
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The element is never found in nature in pure form, but it is found in
small quantities in minerals with other rare earths. Thulium is often
found with minerals containing yttrium and gadolinium. In particular,
thulium occurs in the mineral gadolinite. Like many other lanthanides,
thulium also occurs in the minerals monazite, xenotime, and euxenite.
Thulium has not been found in prevalence over the other rare earths in
any mineral yet. Its abundance in the Earth's crust is 0.5 mg/kg by
weight.
Thulium makes up approximately 0.5 parts per million of soil, although
this value can range from 0.4 to 0.8 parts per million. Thulium makes
up 250 parts per quadrillion of seawater. In the Solar System, thulium
exists in concentrations of 200 parts per trillion by weight and 1
part per trillion by moles. Thulium ore occurs most commonly in China.
Australia, Brazil, Greenland, India, Tanzania, and the United States
also have large reserves of thulium. In 2001, the total world reserves
of thulium were approximately 100,000 tonnes. Thulium is the least
abundant lanthanide on Earth except for the radioactive promethium.
Production
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Thulium is principally extracted from monazite ores (~0.007% thulium)
found in river sands, through ion exchange. Newer ion-exchange and
solvent-extraction techniques have led to easier separation of the
rare earths, which has yielded much lower costs for thulium
production. The principal sources today are the ion adsorption clays
of southern China. In these, where about two-thirds of the total
rare-earth content is yttrium, thulium is about 0.5% (or about tied
with lutetium for rarity).
The metal can be isolated through reduction of its oxide with
lanthanum metal or by calcium reduction in a closed container. None of
thulium's natural compounds are commercially important. In 2001,
approximately 50 tonnes per year of thulium oxide were produced. In
1996, thulium oxide cost US$20 per gram, and in 2005, 99%-pure thulium
metal powder cost US$70 per gram.
Lasers
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Holmium-chromium-thulium triple-doped yttrium aluminium garnet (, or )
is an active laser medium material with high efficiency. It lases at
2080 nm in the infrared and is widely used in military applications,
medicine, and meteorology. Single-element thulium-doped YAG (Tm:YAG)
lasers operate at 2010 nm. The wavelength of thulium-based lasers is
very efficient for superficial ablation of tissue, with minimal
coagulation depth in air or in water. This makes thulium lasers
attractive for laser-based surgery.
X-ray source
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Despite its high cost, portable X-ray devices use thulium that has
been bombarded with neutrons in a nuclear reactor to produce the
isotope thulium-170, having a half-life of 128.6 days and five major
emission lines of comparable intensity (at 7.4, 51.354, 52.389, 59.4
and 84.253 keV). These radioactive sources have a useful life of about
one year, as tools in medical and dental diagnosis, as well as to
detect defects in inaccessible mechanical and electronic components.
Such sources do not need extensive radiation protectiononly a small
cup of lead.
They are among the most popular radiation sources for use in
industrial radiography.
Thulium-170 is gaining popularity as an X-ray source for cancer
treatment via brachytherapy (sealed source radiation therapy).
Others
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Thulium has been used in high-temperature superconductors similarly to
yttrium. Thulium potentially has use in ferrites, ceramic magnetic
materials that are used in microwave equipment. Thulium is also
similar to scandium in that it is used in arc lighting for its unusual
spectrum, in this case, its green emission lines, which are not
covered by other elements. Because thulium fluoresces with a blue
color when exposed to ultraviolet light, thulium is put into euro
banknotes as a measure against counterfeiting.
The blue fluorescence of Tm-doped calcium sulfate has been used in
personal dosimeters for visual monitoring of radiation. Tm-doped
halides in which Tm is in its 2+ oxidation state are luminescent
materials that are proposed for electric power generating windows
based on the principle of a luminescent solar concentrator.
Biological role and precautions
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Soluble thulium salts are mildly toxic, but insoluble thulium salts
are completely nontoxic. When injected, thulium can cause degeneration
of the liver and spleen and can also cause hemoglobin concentration to
fluctuate. Liver damage from thulium is more prevalent in male mice
than female mice. Despite this, thulium has a low level of toxicity.
In humans, thulium occurs in the highest amounts in the liver,
kidneys, and bones. Humans typically consume several micrograms of
thulium per year. The roots of plants do not take up thulium, and the
dry matter of vegetables usually contains one part per billion of
thulium. Thulium metal has low to moderate toxicity. Thulium dust can
cause explosions and fires.
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Thulium
