Thulium is a chemical element with symbol
Tm and atomic number 69. It is the thirteenth
and third-last element in the lanthanide series.
Like the other lanthanides, the most common
oxidation state is +3, seen in its oxide,
halides and other compounds; because it occurs
so late in the series, however, the +2 oxidation
state is also stabilized by the nearly full
4f shell that results. In aqueous solution,
like compounds of other late lanthanides,
soluble thulium compounds form coordination
complexes with nine water molecules.
In 1879, the Swedish chemist Per Teodor Cleve
separated from the rare earth oxide erbia
another two previously unknown components,
which he called holmia and thulia; these were
the oxides of holmium and thulium, respectively.
A relatively pure sample of thulium metal
was first obtained in 1911.
Thulium is the second-least abundant of the
lanthanides, after radioactively unstable
promethium which is only found in trace quantities
on Earth. 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 the radiation source in portable X-ray
devices, and in some solid-state lasers. It
has no significant biological role and is
not particularly toxic.
== Properties ==
=== 
Physical properties ===
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 32 K, antiferromagnetic
between 32 and 56 K, and paramagnetic above
56 K.Thulium has two major allotropes: the
tetragonal α-Tm and the more stable hexagonal
β-Tm.
=== Chemical properties ===
Thulium tarnishes slowly in air and burns
readily at 150 °C to form thulium(III) oxide:
4 Tm + 3 O2 → 2 Tm2O3Thulium is quite electropositive
and reacts slowly with cold water and quite
quickly with hot water to form thulium hydroxide:
2 Tm (s) + 6 H2O (l) → 2 Tm(OH)3 (aq) +
3 H2 (g)Thulium reacts with all the halogens.
Reactions are slow at room temperature, but
are vigorous above 200 °C:
2 Tm (s) + 3 F2 (g) → 2 TmF3 (s) (white)
2 Tm (s) + 3 Cl2 (g) → 2 TmCl3 (s) (yellow)
2 Tm (s) + 3 Br2 (g) → 2 TmBr3 (s) (white)
2 Tm (s) + 3 I2 (g) → 2 TmI3 (s) (yellow)Thulium
dissolves readily in dilute sulfuric acid
to form solutions containing the pale green
Tm(III) ions, which exist as [Tm(OH2)9]3+
complexes:
2 Tm (s) + 3 H2SO4 (aq) → 2 Tm3+ (aq) +
3 SO2−4 (aq) + 3 H2 (g)Thulium reacts with
various metallic and non-metallic elements
forming a range of binary compounds, including
TmN, TmS, TmC2, Tm2C3, TmH2, TmH3, TmSi2,
TmGe3, TmB4, TmB6 and TmB12. In those compounds,
thulium exhibits valence states +2 and +3,
however, the +3 state is most common and only
this state has been observed in thulium solutions.
Thulium exists as a Tm3+ ion in solution.
In this state, the thulium ion is surrounded
by nine molecules of water. Tm3+ ions exhibit
a bright blue luminescence.Thulium's only
known oxide is Tm2O3. 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 TmCl3·7H2O and Tm2(C2O4)3·6H2O are
green or greenish-white. Thulium dichloride
reacts very vigorously with water. This reaction
results in hydrogen gas and Tm(OH)3 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 Tm(NO3)3.
=== Isotopes ===
The isotopes of thulium range from 145Tm to
179Tm. The primary decay mode before the most
abundant stable isotope, 169Tm, is electron
capture, and the primary mode after is beta
emission. The primary decay products before
169Tm are element 68 (erbium) isotopes, and
the primary products after are element 70
(ytterbium) isotopes.Thulium-169 is thulium's
longest-lived and most abundant isotope. It
is the only isotope of thulium that is thought
to be stable, although it is predicted to
undergo alpha decay to holmium-165 with a
very long half-life. After thulium-169, the
next-longest-lived isotopes 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. Thirty-five isotopes and 26 nuclear
isomers of thulium have been detected. Most
isotopes of thulium lighter than 169 atomic
mass units decay via electron capture or beta-plus
decay, although some exhibit significant alpha
decay or proton emission. Heavier isotopes
undergo beta-minus decay.
== History ==
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). Cleve started by removing all of
the known contaminants of erbia (Er2O3). 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 once Tu, but this
was 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. 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 has oscillated
between US$4,600 and $13,300 in the period
from 1959 to 1998 for 99.9% purity, and it
was second highest for lanthanides behind
lutetium.
== Occurrence ==
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. However, 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 and 50 parts
per billion by moles. 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. However, Australia, Brazil, Greenland,
India, Tanzania, and the United States also
have large reserves of thulium. Total reserves
of thulium are approximately 100,000 tonnes.
Thulium is the least abundant lanthanide on
earth except for promethium.
== Production ==
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. Approximately
50 tonnes per year of thulium oxide are produced.
In 1996, thulium oxide cost US$20 per gram,
and in 2005, 99%-pure thulium metal powder
cost US$70 per gram.
== Applications ==
Thulium has a few applications:
=== Laser ===
Holmium-chromium-thulium triple-doped yttrium
aluminum garnet (Ho:Cr:Tm:YAG, or Ho,Cr,Tm:YAG)
is an active laser medium material with high
efficiency. It lases at 2080 nm and is widely
used in military applications, medicine, and
meteorology. Single-element thulium-doped
YAG (Tm:YAG) lasers operate at 2,01 μm. 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 ===
Despite its high cost, portable X-ray devices
use thulium that has been bombarded in a nuclear
reactor as a radiation source. These 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 protection – only
a small cup of lead.Thulium-170 is gaining
popularity as an X-ray source for cancer treatment
via brachytherapy. This isotope has 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). Thulium-170 is one of
the four most popular radioisotopes for use
in industrial radiography.
=== Others ===
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 which Tm is in its 2+ valence
state, are promising luminescent materials
that can make efficient electricity generating
windows based on the principle of a luminescent
solar concentrator, possible.
== Biological role and precautions ==
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 weight of vegetables usually contains
one part per billion of thulium. Thulium dust
and powder are toxic upon inhalation or ingestion
and can cause explosions. Radioactive thulium
isotopes can cause radiation poisoning.
== See also ==
Thulium compounds
