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= Tellurium =
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Introduction
======================================================================
Tellurium is a chemical element; it has the symbol Te and atomic
number 52. It is a brittle, mildly toxic, rare, silver-white
metalloid. Tellurium is chemically related to selenium and sulfur, all
three of which are chalcogens. It is occasionally found in its native
form as elemental crystals. Tellurium is far more common in the
universe as a whole than on Earth. Its extreme rarity in the Earth's
crust, comparable to that of platinum, is due partly to its formation
of a volatile hydride that caused tellurium to be lost to space as a
gas during the hot nebular formation of Earth.
Tellurium-bearing compounds were first discovered in 1782 in a gold
mine in Kleinschlatten, Transylvania (now Zlatna, Romania) by Austrian
mineralogist Franz-Joseph Müller von Reichenstein, although it was
Martin Heinrich Klaproth who named the new element in 1798 after the
Latin 'earth'. Gold telluride minerals are the most notable natural
gold compounds. However, they are not a commercially significant
source of tellurium itself, which is normally extracted as a
by-product of copper and lead production.
Commercially, the primary use of tellurium is CdTe solar panels and
thermoelectric devices. A more traditional application in copper
(tellurium copper) and steel alloys, where tellurium improves
machinability, also consumes a considerable portion of tellurium
production.
Tellurium has no biological function, although fungi can use it in
place of sulfur and selenium in amino acids such as tellurocysteine
and telluromethionine. In humans, tellurium is partly metabolized into
dimethyl telluride, (CH3)2Te, a gas with a garlic-like odor exhaled in
the breath of victims of tellurium exposure or poisoning.
Physical properties
=====================
Tellurium has two allotropes, crystalline and amorphous. When
crystalline, tellurium is silvery-white with a metallic luster. The
crystals are trigonal and chiral (space group 152 or 154 depending on
the chirality), like the gray form of selenium. It is a brittle and
easily pulverized metalloid. Amorphous tellurium is a black-brown
powder prepared by precipitating it from a solution of tellurous acid
or telluric acid (Te(OH)6). Tellurium is a semiconductor that shows
greater electrical conductivity in certain directions depending on
atomic alignment; the conductivity increases slightly when exposed to
light (photoconductivity). When molten, tellurium is corrosive to
copper, iron, and stainless steel. Of the chalcogens (oxygen-family
elements), tellurium has the highest melting and boiling points, at
722.66 and, respectively.
Chemical properties
=====================
Crystalline tellurium consists of parallel helical chains of Te atoms,
with three atoms per turn. This gray material resists oxidation by air
and is not volatile.
Isotopes
==========
Naturally occurring tellurium has eight isotopes. Six of those
isotopes, 120Te, 122Te, 123Te, 124Te, 125Te, and 126Te, are stable.
The other two, 128Te and 130Te, are slightly radioactive, with
extremely long half-lives, including 2.2 × 1024 years for 128Te. This
is the longest known half-life among all radionuclides and is about
160 trillion (1012) times the age of the known universe. Electron
capture decay should occur for 123Te, but is still unobserved.
A further 31 artificial radioisotopes of tellurium are known, with
atomic masses ranging from 104 to 142 and with half-lives up to 19.31
days for 121Te. Also, 17 nuclear isomers are known, with half-lives up
to 164.7 days for the same isotope. Except for beryllium-8 and
beta-delayed alpha emission branches in some lighter nuclides,
tellurium (104Te to 109Te) is the lightest element with isotopes known
to undergo alpha decay.
The atomic mass of tellurium () exceeds that of iodine (), the next
element in the periodic table. Such inversions were thought by some to
be paradoxical before atomic number was discovered.
Occurrence
============
With an abundance in the Earth's crust comparable to that of platinum
(about 1 μg/kg), tellurium is one of the rarest stable solid elements.
In comparison, even thulium - the rarest of the stable lanthanides -
has crystal abundances of 500 μg/kg (see Abundance of the chemical
elements).
The rarity of tellurium in the Earth's crust is not a reflection of
its cosmic abundance. Tellurium is more abundant than rubidium in the
cosmos, though rubidium is 10,000 times more abundant in the Earth's
crust. The rarity of tellurium on Earth is thought to be caused by
conditions during preaccretional sorting in the solar nebula, when the
stable form of certain elements, in the absence of oxygen and water,
was controlled by the reductive power of free hydrogen. Under this
scenario, certain elements that form volatile hydrides, such as
tellurium, were severely depleted through the evaporation of these
hydrides. Tellurium and selenium are the heavy elements most depleted
by this process.
Tellurium is sometimes found in its native (i.e., elemental) form, but
is more often found as the tellurides of gold such as calaverite and
krennerite (two different polymorphs of AuTe2), petzite, Ag3AuTe2, and
sylvanite, AgAuTe4. The town of Telluride, Colorado, was named in the
hope of a strike of gold telluride (which never materialized, though
gold metal ore was found). Gold itself is usually found uncombined,
but when found as a chemical compound, it is often combined with
tellurium.
Although tellurium is found with gold more often than in uncombined
form, it is found even more often combined as tellurides of more
common metals (e.g. melonite, NiTe2). Natural tellurite and tellurate
minerals also occur, formed by the oxidation of tellurides near the
Earth's surface. In contrast to selenium, tellurium does not usually
replace sulfur in minerals because of the great difference in ion
radii. Thus, many common sulfide minerals contain substantial
quantities of selenium and only traces of tellurium.
In the gold rush of 1893, miners in Kalgoorlie discarded a pyritic
material as they searched for pure gold, and it was used to fill in
potholes and build sidewalks. In 1896, that tailing was discovered to
be calaverite, a telluride of gold, and it sparked a second gold rush
that included mining the streets.
In 2023 astronomers detected the creation of tellurium during
collision between two neutron stars.
History
======================================================================
Tellurium (Latin 'tellus' meaning "earth") was discovered in the 18th
century in a gold ore from the mines in Kleinschlatten (today Zlatna),
near today's city of Alba Iulia, Romania. This ore was known as
"Faczebajer weißes blättriges Golderz" (white leafy gold ore from
Faczebaja, German name of Facebánya, now Fața Băii in Alba County) or
'antimonalischer Goldkies' (antimonic gold pyrite), and according to
Anton von Rupprecht, was 'Spießglaskönig' ('argent molybdique'),
containing native antimony. In 1782 Franz-Joseph Müller von
Reichenstein, who was then serving as the Austrian chief inspector of
mines in Transylvania, concluded that the ore did not contain antimony
but was bismuth sulfide. The following year, he reported that this was
erroneous and that the ore contained mostly gold and an unknown metal
very similar to antimony. After a thorough investigation that lasted
three years and included more than fifty tests, Müller determined the
specific gravity of the mineral and noted that when heated, the new
metal gives off a white smoke with a radish-like odor; that it imparts
a red color to sulfuric acid; and that when this solution is diluted
with water, it has a black precipitate. Nevertheless, he was not able
to identify this metal and gave it the names 'aurum paradoxum'
(paradoxical gold) and 'metallum problematicum' (problem metal),
because it did not exhibit the properties predicted for antimony.
In 1789, a Hungarian scientist, Pál Kitaibel, discovered the element
independently in an ore from Deutsch-Pilsen that had been regarded as
argentiferous molybdenite, but later he gave the credit to Müller. In
1798, it was named by Martin Heinrich Klaproth, who had earlier
isolated it from the mineral calaverite.Klaproth (1798)
[
https://books.google.com/books?id=8ws_AAAAcAAJ&pg=PA95 "Ueber die
siebenbürgischen Golderze, und das in selbigen enthaltene neue
Metall"] (On the Transylvanian gold ore, and the new metal contained
in it), 'Chemische Annalen für die Freunde der Naturlehre,
Arzneygelahrtheit, Haushaltungskunst und Manufacturen' (Chemical
Annals for the Friends of Science, Medicine, Economics, and
Manufacturing), 1 : 91-104. From
[
https://books.google.com/books?id=8ws_AAAAcAAJ&pg=PA100-IA4 page
100:] " '… ; und welchem ich hiermit den, von der alten Muttererde
entlehnten, Namen 'Tellurium' beylege.'" ( … ; and to which I hereby
bestow the name 'tellurium', derived from the old Mother of the
Earth.)
*
In the early 1920s, Thomas Midgley Jr. found tellurium prevented
engine knocking when added to fuel, but ruled it out due to the
difficult-to-eradicate smell. Midgley went on to discover and
popularize the use of tetraethyl lead.
The 1960s brought an increase in thermoelectric applications for
tellurium (as bismuth telluride), and in free-machining steel alloys,
which became the dominant use. These applications were overtaken by
the growing importance of CdTe in thin-film solar cells in the 2000s.
Production
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Most Te (and Se) is obtained from porphyry copper deposits, where it
occurs in trace amounts. The element is recovered from anode sludges
from the electrolytic refining of blister copper. It is a component of
dusts from blast furnace refining of lead. Treatment of 1000 tons of
copper ore yields approximately 1 kg of tellurium.
The anode sludges contain the selenides and tellurides of the noble
metals in compounds with the formula M2Se or M2Te (M = Cu, Ag, Au). At
temperatures of 500 °C the anode sludges are roasted with sodium
carbonate under air. The metal ions are reduced to the metals, while
the telluride is converted to sodium tellurite.
Tellurites can be leached from the mixture with water and are normally
present as hydrotellurites HTeO3− in solution. Selenites are also
formed during this process, but they can be separated by adding
sulfuric acid. The hydrotellurites are converted into the insoluble
tellurium dioxide while the selenites stay in solution.
The metal is produced from the oxide (reduced) either by electrolysis
or by reacting the tellurium dioxide with sulfur dioxide in sulfuric
acid.
Commercial-grade tellurium is usually marketed as 200-mesh powder but
is also available as slabs, ingots, sticks, or lumps. The year-end
price for tellurium in 2000 was US$30 per kilogram. In recent years,
the tellurium price was driven up by increased demand and limited
supply, reaching as high as US$220 per pound in 2006. The average
annual price for 99.99%-pure tellurium increased from $38 per kilogram
in 2017 to $74 per kilogram in 2018. Despite the expectation that
improved production methods will double production, the United States
Department of Energy (DoE) anticipates a supply shortfall of tellurium
by 2025.
In the 2020s, China produced ca. 50% of world's tellurium and was the
only country that mined Te as the main target rather than a
by-product. This dominance was driven by the rapid expansion of solar
cell industry in China. In 2022, the largest Te providers by volume
were China (340 tonnes), Russia (80 t), Japan (70 t), Canada (50 t),
Uzbekistan (50 t), Sweden (40 t) and the United States (no official
data).
Compounds
======================================================================
Tellurium belongs to the chalcogen (group 16) family of elements on
the periodic table, which also includes oxygen, sulfur, selenium and
polonium: Tellurium and selenium compounds are similar. Tellurium
exhibits the oxidation states −2, +2, +4 and +6, with +4 being most
common.
Tellurides
============
Reduction of Te metal produces the tellurides and polytellurides,
Ten2−. The −2 oxidation state is exhibited in binary compounds with
many metals, such as zinc telluride, , produced by heating tellurium
with zinc. Decomposition of with hydrochloric acid yields hydrogen
telluride (), a highly unstable analogue of the other chalcogen
hydrides, Water (molecule), Hydrogen sulfide and Hydrogen selenide:
Halides
=========
The +2 oxidation state is exhibited by the dihalides, , and . The
dihalides have not been obtained in pure form, although they are known
decomposition products of the tetrahalides in organic solvents, and
the derived tetrahalotellurates are well-characterized:
where X is Cl, Br, or I. These anions are square planar in geometry.
Polynuclear anionic species also exist, such as the dark brown , and
the black .
With fluorine Te forms the mixed-valence and Tellurium hexafluoride.
In the +6 oxidation state, the structural group occurs in a number of
compounds such as Teflic acid, , , and . The square antiprismatic
anion is also attested. The other halogens do not form halides with
tellurium in the +6 oxidation state, but only tetrahalides (Tellurium
tetrachloride, Tellurium tetrabromide and Tellurium tetraiodide) in
the +4 state, and other lower halides (, , , and two forms of ). In
the +4 oxidation state, halotellurate anions are known, such as and .
Halotellurium cations are also attested, including , found in .
Oxocompounds
==============
Tellurium monoxide was first reported in 1883 as a black amorphous
solid formed by the heat decomposition of in vacuum,
disproportionating into tellurium dioxide, and elemental tellurium
upon heating. Since then, however, existence in the solid phase is
doubted and in dispute, although it is known as a vapor fragment; the
black solid may be merely an equimolar mixture of elemental tellurium
and tellurium dioxide.
Tellurium dioxide is formed by heating tellurium in air, where it
burns with a blue flame. Tellurium trioxide, β-, is obtained by
thermal decomposition of . The other two forms of trioxide reported in
the literature, the α- and γ- forms, were found not to be true oxides
of tellurium in the +6 oxidation state, but a mixture of , and .
Tellurium also exhibits mixed-valence oxides, and .
The tellurium oxides and hydrated oxides form a series of acids,
including tellurous acid (), orthotelluric acid () and metatelluric
acid (). The two forms of telluric acid form 'tellurate' salts
containing the TeO and TeO anions, respectively. Tellurous acid forms
'tellurite' salts containing the anion TeO.
Zintl cations
===============
When tellurium is treated with concentrated sulfuric acid, the result
is a red solution of the Zintl ion, . The oxidation of tellurium by
arsenic pentafluoride in liquid sulfur dioxide produces the same
square planar cation, in addition to the trigonal prismatic,
yellow-orange :
Other tellurium Zintl cations include the polymeric and the
blue-black , consisting of two fused 5-membered tellurium rings. The
latter cation is formed by the reaction of tellurium with tungsten
hexachloride:
Interchalcogen cations also exist, such as (distorted cubic geometry)
and . These are formed by oxidizing mixtures of tellurium and selenium
with or antimony pentafluoride.
Organotellurium compounds
===========================
Tellurium does not readily form analogues of alcohols and thiols, with
the functional group -TeH, that are called tellurols. The -TeH
functional group is also attributed using the prefix 'tellanyl-'. Like
H2Te, these species are unstable with respect to loss of hydrogen.
Telluraethers (R-Te-R) are more stable, as are telluroxides.
Organotellurium compounds are mainly of interest in the research
context. Several have been examined such as precursors for
metalorganic vapor phase epitaxy growth of II-VI compound
semiconductors. These precursor compounds include dimethyl telluride,
diethyl telluride, diisopropyl telluride, diallyl telluride, and
methyl allyl telluride. Diisopropyl telluride (DIPTe) is the preferred
precursor for low-temperature growth of CdHgTe by MOVPE. The greatest
purity metalorganics of both selenium and tellurium are used in these
processes. The compounds for semiconductor industry and are prepared
by adduct purification.
Tellurium suboxide is used in the media layer of rewritable optical
discs, including ReWritable Compact Discs (CD-RW), ReWritable Digital
Video Discs (DVD-RW), and ReWritable Blu-ray Discs.
Tellurium is used in the phase change memory chips
developed by Intel. Bismuth telluride (Bi2Te3) and lead telluride are
working elements of thermoelectric devices. Lead telluride exhibits
promise in far-infrared detectors.
Tritelluride quantum materials
================================
Recently, physicists and materials scientists have been discovering
unusual quantum properties associated with layered compounds composed
of tellurium that's combined with certain rare-earth elements, as well
as yttrium (Y).
These novel materials have the general formula of 'R' Te3, where "'R'
" represents a rare-earth lanthanide (or Y), with the full family
consisting of 'R' = Y, lanthanum (La), cerium (Ce), praseodymium (Pr),
neodymium (Nd), samarium (Sm), gadolinium (Gd), terbium (Tb),
dysprosium (Dy), holmium (Ho), erbium (Er), and thulium (Tm).
Compounds containing promethium (Pm), europium (Eu), ytterbium (Yb),
and lutetium (Lu) have not yet been observed. These materials have a
two-dimensional character within an orthorhombic crystal structure,
with slabs of 'R' Te separated by sheets of pure tellurium.
It is thought that this 2-D layered structure is what leads to a
number of interesting quantum features, such as charge-density waves,
high carrier mobility, superconductivity under specific conditions,
and other peculiar properties whose natures are only now emerging.
For example, in 2022, a small group of physicists at Boston College in
Massachusetts led an international team that used optical methods to
demonstrate a novel axial mode of a Higgs-like particle in 'R' Te3
compounds that incorporate either of two rare-earth elements ('R'
= La, Gd). This long-hypothesized, axial, Higgs-like particle also
shows magnetic properties and may serve as a candidate for dark
matter.
Applications
======================================================================
In 2022, the major applications of tellurium were thin-film solar
cells (40%), thermoelectrics (30%), metallurgy (15%), and rubber (5%),
with the first two applications experiencing a rapid increase owing to
the worldwide tendency of reducing dependence on the fossil fuel. In
metallurgy, tellurium is added to iron, stainless steel, copper, and
lead alloys. It improves the machinability of copper without reducing
its high electrical conductivity. It increases resistance to vibration
and fatigue of lead and stabilizes various carbides and in malleable
iron.
Heterogeneous catalysis
=========================
Tellurium oxides are components of commercial oxidation catalysts.
Te-containing catalysts are used for the ammoxidation route to
acrylonitrile (CH2=CH-C≡N):
Related catalysts are used in the production of tetramethylene glycol:
Niche
=======
*Synthetic rubber vulcanized with tellurium shows mechanical and
thermal properties that in some ways are superior to sulfur-vulcanized
materials.
* Tellurium compounds are specialized pigments for ceramics.
* Selenides and tellurides greatly increase the optical refraction of
glass widely used in glass optical fibers for telecommunications.
* Mixtures of selenium and tellurium are used with barium peroxide as
an oxidizer in the delay powder of electric blasting caps.
* Neutron bombardment of tellurium is the most common way to produce
iodine-131. This in turn is used to treat some thyroid conditions, and
as a tracer compound in hydraulic fracturing, among other
applications.
Semiconductor and electronic
==============================
Cadmium telluride (CdTe) solar panels exhibit some of the greatest
efficiencies for solar cell electric power generators.
In 2018, China installed thin-film solar panels with a total power
output of 175 GW, more than any other country in the world; most of
those panels were made of CdTe. In June 2022, China set goals of
generating 25% of energy consumption and installing 1.2 billion
kilowatts of capacity for wind and solar power by 2030. This proposal
will increase the demand for tellurium and its production worldwide,
especially in China, where the annual volumes of Te refining increased
from 280 tonnes in 2017 to 340 tonnes in 2022.
is an efficient material for detecting X-rays. It is being used in
the NASA space-based X-ray telescope NuSTAR.
Mercury cadmium telluride is a semiconductor material that is used in
thermal imaging devices.
Photocathodes
===============
Tellurium shows up in a number of photocathodes used in solar blind
photomultiplier tubes and for high brightness photoinjectors driving
modern particle accelerators. The photocathode Cs-Te, which is
predominantly Cs2Te, has a photoemission threshold of 3.5 eV and
exhibits the uncommon combination of high quantum efficiency (>10%)
and high durability in poor vacuum environments (lasting for months
under use in RF electron guns). This has made it the go to choice for
photoemission electron guns used in driving free electron lasers. In
this application, it is usually driven at the wavelength 267 nm which
is the third harmonic of commonly used Ti-sapphire lasers. More Te
containing photocathodes have been grown using other alkali metals
such as rubidium, potassium, and sodium, but they have not found the
same popularity that Cs-Te has enjoyed.
Thermoelectric material
=========================
Tellurium itself can be used as a high-performance elemental
thermoelectric material. A trigonal Te with the space group of P3121
can transfer into a topological insulator phase, which is suitable for
thermoelectric material. Though often not considered as a
thermoelectric material alone, polycrystalline tellurium does show
great thermoelectric performance with the thermoelectric figure of
merit, zT, as high as 1.0, which is even higher than some of other
conventional TE materials like SiGe and BiSb.
Telluride, which is a compound form of tellurium, is a more common TE
material. Typical and ongoing research includes Bi2Te3, and La3−xTe4,
etc. Bi2Te3 is widely used from energy conversion to sensing to
cooling due to its great TE properties. The BiTe-based TE material can
achieve a conversion efficiency of 8%, an average zT value of 1.05 for
p-type and 0.84 for n-type bismuth telluride alloys. Lanthanum
telluride can be potentially used in deep space as a thermoelectric
generator due to the huge temperature difference in space. The zT
value reaches to a maximum of ~1.0 for a La3−xTe4 system with x near
0.2. This composition also allows other chemical substitution which
may enhance the TE performance. The addition of Yb, for example, may
increase the zT value from 1.0 to 1.2 at 1275K, which is greater than
the current SiGe power system.
Biological role
======================================================================
Tellurium has no known biological function, although fungi can
incorporate it in place of sulfur and selenium into amino acids such
as tellurocysteine and telluromethionine. Organisms have shown a
highly variable tolerance to tellurium compounds. Many bacteria, such
as 'Pseudomonas aeruginosa' and 'Gayadomonas' sp, take up tellurite
and reduce it to elemental tellurium, which accumulates and causes a
characteristic and often dramatic darkening of cells. In yeast, this
reduction is mediated by the sulfate assimilation pathway. Tellurium
accumulation seems to account for a major part of the toxicity
effects. Some species metabolize tellurium to form dimethyl telluride
or dimethyl ditelluride. Dimethyl telluride has been observed in hot
springs at very low concentrations.
Tellurite agar is used to identify members of the corynebacterium
genus, most typically 'Corynebacterium diphtheriae', the pathogen
responsible for diphtheria.
Precautions
======================================================================
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Tellurium and tellurium compounds are considered to be mildly toxic
and need to be handled with care, although acute poisoning is rare.
Tellurium poisoning is particularly difficult to treat as many
chelation agents used in the treatment of metal poisoning will
increase the toxicity of tellurium. Tellurium is not reported to be
carcinogenic, but it may be fatal if inhaled, swallowed, or absorbed
through skin.
Humans exposed to as little as 0.01 mg/m3 or less in air exude a foul
garlic-like odor known as "tellurium breath".
This is caused by the body converting tellurium from any oxidation
state to dimethyl telluride, (CH3)2Te, a volatile compound with a
pungent garlic-like smell. Volunteers given 15 mg of tellurium still
had this characteristic smell on their breath eight months later. In
laboratories, this odor makes it possible to discern which scientists
are responsible for tellurium chemistry, and even which books they
have handled in the past. Even though the metabolic pathways of
tellurium are not known, it is generally assumed that they resemble
those of the more extensively studied selenium because the final
methylated metabolic products of the two elements are similar.
People can be exposed to tellurium in the workplace by inhalation,
ingestion, skin contact, and eye contact. The Occupational Safety and
Health Administration (OSHA) limits (permissible exposure limit)
tellurium exposure in the workplace to 0.1 mg/m3 over an eight-hour
workday. The National Institute for Occupational Safety and Health
(NIOSH) has set the recommended exposure limit (REL) at 0.1 mg/m3 over
an eight-hour workday. In concentrations of 25 mg/m3, tellurium is
immediately dangerous to life and health.
See also
======================================================================
* The 1862 telluric helix of Alexandre-Émile Béguyer de Chancourtois.
External links
======================================================================
* [
https://minerals.er.usgs.gov/minerals/pubs/commodity/selenium USGS
Mineral Information on Selenium and Tellurium]
* [
http://www.periodicvideos.com/videos/052.htm Tellurium] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
https://www.cdc.gov/niosh/npg/npgd0587.html CDC - NIOSH Pocket
Guide to Chemical Hazards - Tellurium]
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Tellurium
