Tellurium

Tellurium is a chemical element with symbol Te and atomic number 52. It is a brittle, mildly toxic, rare, silver-white metalloid. Tellurium is chemically related to selenium and sulfur. It is occasionally found in 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 high atomic number, but also to its formation of a volatile hydride that caused it to be lost to space as a gas during the hot nebular formation of the planet.

Tellurium-bearing compounds were first discovered in 1782 in a gold mine in 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 word for "earth", tellus. 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 copper and steel alloys, where it improves machinability. Applications in CdTe solar panels and semiconductors also consume 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.[6] 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.

Tellurium,  52Te
Tellurium2
Tellurium
Pronunciation/tɪˈljʊəriəm/ (tə-LEWR-ee-əm)
Appearancesilvery lustrous gray (crystalline),
brown-black powder (amorphous)
Standard atomic weight Ar, std(Te)127.60(3)[1]
Tellurium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Se

Te

Po
antimonytelluriumiodine
Atomic number (Z)52
Groupgroup 16 (chalcogens)
Periodperiod 5
Blockp-block
Element category  metalloid
Electron configuration[Kr] 4d10 5s2 5p4
Electrons per shell
2, 8, 18, 18, 6
Physical properties
Phase at STPsolid
Melting point722.66 K ​(449.51 °C, ​841.12 °F)
Boiling point1261 K ​(988 °C, ​1810 °F)
Density (near r.t.)6.24 g/cm3
when liquid (at m.p.)5.70 g/cm3
Heat of fusion17.49 kJ/mol
Heat of vaporization114.1 kJ/mol
Molar heat capacity25.73 J/(mol·K)
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K)   (775) (888) 1042 1266
Atomic properties
Oxidation states−2, −1, +1, +2, +3, +4, +5, +6 (a mildly acidic oxide)
ElectronegativityPauling scale: 2.1
Ionization energies
  • 1st: 869.3 kJ/mol
  • 2nd: 1790 kJ/mol
  • 3rd: 2698 kJ/mol
Atomic radiusempirical: 140 pm
Covalent radius138±4 pm
Van der Waals radius206 pm
Color lines in a spectral range
Spectral lines of tellurium
Other properties
Natural occurrenceprimordial
Crystal structurehexagonal
Hexagonal crystal structure for tellurium
Speed of sound thin rod2610 m/s (at 20 °C)
Thermal expansion18 µm/(m·K)[2] (at r.t.)
Thermal conductivity1.97–3.38 W/(m·K)
Magnetic orderingdiamagnetic[3]
Magnetic susceptibility−39.5·10−6 cm3/mol (298 K)[4]
Young's modulus43 GPa
Shear modulus16 GPa
Bulk modulus65 GPa
Mohs hardness2.25
Brinell hardness180–270 MPa
CAS Number13494-80-9
History
Namingafter Roman Tellus, deity of the Earth
DiscoveryFranz-Joseph Müller von Reichenstein (1782)
First isolationMartin Heinrich Klaproth
Main isotopes of tellurium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
120Te 0.09% stable
121Te syn 16.78 d ε 121Sb
122Te 2.55% stable
123Te 0.89%[5] stable
124Te 4.74% stable
125Te 7.07% stable
126Te 18.84% stable
127Te syn 9.35 h β 127I
128Te 31.74% 2.2×1024 y ββ 128Xe
129Te syn 69.6 min β 129I
130Te 34.08% 7.9×1020 y ββ 130Xe

Characteristics

Physical properties

Tellurium has two allotropes, crystalline and amorphous. When crystalline, tellurium is silvery-white with a metallic luster. 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).[7] Tellurium is a semiconductor that shows a greater electrical conductivity in certain directions depending on atomic alignment; the conductivity increases slightly when exposed to light (photoconductivity).[8] 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 K (841.12 °F) and 1,261 K (1,810 °F), respectively.[9]

Chemical properties

Tellurium adopts a polymeric structure consisting of zig-zag chains of Te atoms. 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, have been found to be slightly radioactive,[10][11][12] with extremely long half-lives, including 2.2 × 1024 years for 128Te. This is the longest known half-life among all radionuclides[13] and is about 160 trillion (1012) times the age of the known universe. Stable isotopes comprise only 33.2% of naturally occurring tellurium.

A further 30 artificial radioisotopes of tellurium are known, with atomic masses ranging from 105 to 142 and with half-lives of 19 days or less. Also, 17 nuclear isomers are known, with half-lives up to 154 days. Tellurium (106Te to 110Te ) are among the lightest elements known to undergo alpha decay.[10]

The atomic mass of tellurium (127.60 g·mol−1) exceeds that of iodine (126.90 g·mol−1), the next element in the periodic table.[14]

Occurrence

Tellurium
Tellurium on quartz (Moctezuma, Sonora, Mexico)
Tellurium-89043
Native tellurium crystal on sylvanite (Vatukoula, Viti Levu, Fiji). Picture width 2 mm.

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.[15] In comparison, even the rarest of the stable lanthanides have crustal abundances of 500 µg/kg (see Abundance of the chemical elements).[16]

This 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 the formation of the Earth, 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 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 city of Telluride, Colorado, was named in 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 most 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 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.[17]

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.[18]

History

Martin Heinrich Klaproth
Klaproth named the new element and credited von Reichenstein with its discovery

Tellurium (Latin tellus meaning "earth") was discovered in the 18th century in a gold ore from the mines in 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.[19][20] 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.[21] 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 paradoxium (paradoxical gold) and metallum problematicum (problem metal), because it did not exhibit the properties predicted for antimony.[22][23][24]

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.[25][23][24][26]

The 1960s brought an increase in thermoelectric applications for tellurium (as bismuth telluride), and in free-machining steel alloys, which became the dominant use.[27]

Production

The principal source of tellurium is 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 typically yields one kilogram (2.2 pounds) of tellurium.

World Tellurium Production 2006
Tellurium production 2006

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.[28]

M2Te + O2 + Na2CO3 → Na2TeO3 + 2 M + CO2

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.[28]

HTeO
3
+ OH + H2SO4 → TeO2 + SO2−
4
+ 2 H2O

The metal is produced from the oxide (reduced) either by electrolysis or by reacting the tellurium dioxide with sulfur dioxide in sulfuric acid.[28]

TeO2 + 2 SO2 + 2H2O → Te + 2 SO2−
4
+ 4 H+

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$14 per pound. In recent years, the tellurium price was driven up by increased demand and limited supply, reaching as high as US$100 per pound in 2006.[29][30] 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.[31]

Tellurium is produced mainly in the United States, Peru, Japan and Canada.[32] The British Geological Survey gives the following production numbers for 2009: United States 50 t, Peru 7 t, Japan 40 t and Canada 16 t.[33]

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.[7]

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, ZnTe, produced by heating tellurium with zinc.[34] Decomposition of ZnTe with hydrochloric acid yields hydrogen telluride (H
2
Te
), a highly unstable analogue of the other chalcogen hydrides, H
2
O
, H
2
S
and H
2
Se
:

ZnTe + 2 HCl → ZnCl
2
+ H
2
Te

H
2
Te
is unstable, whereas salts of its conjugate base [TeH] are stable.

Halides

The +2 oxidation state is exhibited by the dihalides, TeCl
2
, TeBr
2
and TeI
2
. The dihalides have not been obtained in pure form,[35]:274 although they are known decomposition products of the tetrahalides in organic solvents, and the derived tetrahalotellurates are well-characterized:

Te + X
2
+ 2 X
TeX2−
4

where X is Cl, Br, or I. These anions are square planar in geometry.[35]:281 Polynuclear anionic species also exist, such as the dark brown Te
2
I2−
6
,[35]:283 and the black Te
4
I2−
14
.[35]:285

Fluorine forms two halides with tellurium: the mixed-valence Te
2
F
4
and TeF
6
. In the +6 oxidation state, the –OTeF
5
structural group occurs in a number of compounds such as HOTeF
5
, B(OTeF
5
)
3
, Xe(OTeF
5
)
2
, Te(OTeF
5
)
4
and Te(OTeF
5
)
6
.[36] The square antiprismatic anion TeF2−
8
is also attested.[28] The other halogens do not form halides with tellurium in the +6 oxidation state, but only tetrahalides (TeCl
4
, TeBr
4
and TeI
4
) in the +4 state, and other lower halides (Te
3
Cl
2
, Te
2
Cl
2
, Te
2
Br
2
, Te
2
I
and two forms of TeI). In the +4 oxidation state, halotellurate anions are known, such as TeCl2−
6
and Te
2
Cl2−
10
. Halotellurium cations are also attested, including TeI+
3
, found in TeI
3
AsF
6
.[37]

Oxocompounds
TeO2powder
A sample of tellurium dioxide powder

Tellurium monoxide was first reported in 1883 as a black amorphous solid formed by the heat decomposition of TeSO
3
in vacuum, disproportionating into tellurium dioxide, TeO
2
and elemental tellurium upon heating.[38][39] 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.[40]

Tellurium dioxide is formed by heating tellurium in air, where it burns with a blue flame.[34] Tellurium trioxide, β-TeO
3
, is obtained by thermal decomposition of Te(OH)
6
. 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 Te4+
, OH
and O
2
.[41] Tellurium also exhibits mixed-valence oxides, Te
2
O
5
and Te
4
O
9
.[41]

The tellurium oxides and hydrated oxides form a series of acids, including tellurous acid (H
2
TeO
3
), orthotelluric acid (Te(OH)
6
) and metatelluric acid ((H
2
TeO
4
)
n
).[40] The two forms of telluric acid form tellurate salts containing the TeO2–
4
and TeO6−
6
anions, respectively. Tellurous acid forms tellurite salts containing the anion TeO2−
3
. Other tellurium cations include TeF2+
8
, which consists of two fused tellurium rings and the polymeric TeF2+
7
.

Zintl cations

When tellurium is treated with concentrated sulfuric acid, the result is a red solution of the Zintl ion, Te2+
4
.[42] The oxidation of tellurium by AsF
5
in liquid SO
2
produces the same square planar cation, in addition to the trigonal prismatic, yellow-orange Te4+
6
:[28]

4 Te + 3 AsF
5
Te2+
4
(AsF
6
)
2
+ AsF
3
6 Te + 6 AsF
5
Te4+
6
(AsF
6
)
4
+ 2 AsF
3

Other tellurium Zintl cations include the polymeric Te2+
7
and the blue-black Te2+
8
, consisting of two fused 5-membered tellurium rings. The latter cation is formed by the reaction of tellurium with tungsten hexachloride:[28]

8 Te + 2 WCl
6
Te2+
8
(WCl
6
)
2

Interchalcogen cations also exist, such as Te
2
Se2+
6
(distorted cubic geometry) and Te
2
Se2+
8
. These are formed by oxidizing mixtures of tellurium and selenium with AsF
5
or SbF
5
.[28]

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-.[43] Like H2Te, these species are unstable with respect to loss of hydrogen. Telluraethers (R–Te–R) are more stable, as are telluroxides.

Applications

Metallurgy

The largest consumer of tellurium is metallurgy in iron, stainless steel, copper, and lead alloys. The addition to steel and copper produces an alloy more machinable than otherwise. It is alloyed into cast iron for promoting chill for spectroscopy, where the presence of electrically conductive free graphite tends to interfere with spark emission testing results. In lead, tellurium improves strength and durability, and decreases the corrosive action of sulfuric acid.[27][44]

Semiconductor and electronic industry uses

Tellurium is used in cadmium telluride (CdTe) solar panels. National Renewable Energy Laboratory lab tests of tellurium demonstrated some of the greatest efficiencies for solar cell electric power generators. Massive commercial production of CdTe solar panels by First Solar in recent years has significantly increased tellurium demand.[45][46][47] Replacing some of the cadmium in CdTe by zinc, producing (Cd,Zn)Te, produces a solid-state X-ray detector, providing an alternative to single-use film badges.[48]

Infrared sensitive semiconductor material is formed by alloying tellurium with cadmium and mercury to form mercury cadmium telluride.[49]

Organotellurium compounds such as dimethyl telluride, diethyl telluride, diisopropyl telluride, diallyl telluride and methyl allyl telluride are precursors for synthesizing metalorganic vapor phase epitaxy growth of II-VI compound semiconductors.[50] Diisopropyl telluride (DIPTe) is the preferred precursor for low-temperature growth of CdHgTe by MOVPE.[51] 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.[52][53]

Tellurium, as 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.[54][55]

Tellurium dioxide is used to create acousto-optic modulators (AOTFs and AOBSs) for confocal microscropy.

Tellurium is used in the new phase change memory chips[56] developed by Intel.[57] Bismuth telluride (Bi2Te3) and lead telluride are working elements of thermoelectric devices. Lead telluride is used in far-infrared detectors.

Other uses

  • Tellurium compounds are used as pigments for ceramics.[58]
  • Selenides and tellurides greatly increase the optical refraction of glass widely used in glass optical fibers for telecommunications.[59][60]
  • Mixtures of selenium and tellurium are used with barium peroxide as an oxidizer in the delay powder of electric blasting caps.[61]
  • Organic tellurides have been employed as initiators for living radical polymerization and electron-rich mono- and di-tellurides possess antioxidant activity.
  • Rubber can be vulcanized with tellurium instead of sulfur or selenium. The rubber produced in this way shows improved heat resistance.[62]
  • Tellurite agar is used to identify members of the corynebacterium genus, most typically Corynebacterium diphtheriae, the pathogen responsible for diphtheria.[63]
  • Tellurium is a key constituent of high performing mixed oxide catalysts for the heterogeneous catalytic selective oxidation of propane to acrylic acid.[64][65] The surface elemental composition changes dynamically and reversibly with the reaction conditions. In the presence of steam the surface of the catalyst is enriched in tellurium and vanadium which translates into the enhancement of the acrylic acid production.[66][67]
  • Neutron bombardment of tellurium is the most common way to produce iodine-131.[68] This in turn is used to treat some thyroid conditions, and as a tracer compound in hydraulic fracturing, among other applications.

Biological role

Tellurium has no known biological function, although fungi can incorporate it in place of sulfur and selenium into amino acids such as telluro-cysteine and telluro-methionine.[6][69] Organisms have shown a highly variable tolerance to tellurium compounds. Many bacteria, such as Pseudomonas aeruginosa, take up tellurite and reduce it to elemental tellurium, which accumulates and causes a characteristic and often dramatic darkening of cells.[70] In yeast, this reduction is mediated by the sulfate assimilation pathway.[71] Tellurium accumulation seems to account for a major part of the toxicity effects. Many organisms also metabolize tellurium partly to form dimethyl telluride, although dimethyl ditelluride is also formed by some species. Dimethyl telluride has been observed in hot springs at very low concentrations.[72][73]

Precautions

Tellurium
Hazards
GHS pictograms The exclamation-mark pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)The health hazard pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)
GHS signal word Danger
H317, H332, H360, H412
P201, P261, P280, P308+313[74]
NFPA 704
Flammability code 0: Will not burn. E.g., waterHealth code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroformReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogenSpecial hazards (white): no codeNFPA 704 four-colored diamond
0
2
0

Tellurium and tellurium compounds are considered to be mildly toxic and need to be handled with care, although acute poisoning is rare.[75] 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.[75]

Humans exposed to as little as 0.01 mg/m3 or less in air exude a foul garlic-like odor known as "tellurium breath".[58][76] This is caused by the body converting tellurium from any oxidation state to dimethyl telluride, (CH3)2Te. This is a volatile compound with a pungent garlic-like smell. 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.[77][78][79]

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.[80]

See also

References

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External links

AgInSbTe

AgInSbTe, or Silver-Indium-Antimony-Tellurium, is a phase change material from the group of chalcogenide glasses, used in rewritable optical discs (such as rewritable CDs) and phase-change memory applications. It is a quaternary compound of silver, indium, antimony, and tellurium.

During writing, the material is first erased, initialized into its crystalline state, with long, lower-intensity laser irradiation. The material heats up to its crystallization temperature, but not up to its melting point, and crystallizes in a metastable face-centered cubic structure. Then the information is written on the crystalline phase, by heating spots of it with short (<10 ns), high-intensity laser pulses; the material locally melts and is quickly cooled, remaining in the amorphous phase. As the amorphous phase has lower reflectivity than the crystalline phase, the bitstream can be recorded as "dark" amorphous spots on the crystalline background. At low linear velocities, clusters of crystalline material can exist in the amorphous spots.

Another similar material is GeSbTe, offering a lower linear density, but with higher overwrite cycles by 1-2 orders of magnitude. It is used in pit-and-groove recording formats, often in rewritable DVDs.

Chalcogen

The chalcogens () are the chemical elements in group 16 of the periodic table. This group is also known as the oxygen family. It consists of the elements oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and the radioactive element polonium (Po). The chemically uncharacterized synthetic element livermorium (Lv) is predicted to be a chalcogen as well. Often, oxygen is treated separately from the other chalcogens, sometimes even excluded from the scope of the term "chalcogen" altogether, due to its very different chemical behavior from sulfur, selenium, tellurium, and polonium. The word "chalcogen" is derived from a combination of the Greek word khalkόs (χαλκός) principally meaning copper (the term was also used for bronze/brass, any metal in the poetic sense, ore or coin), and the Latinised Greek word genēs, meaning born or produced.Sulfur has been known since antiquity, and oxygen was recognized as an element in the 18th century. Selenium, tellurium and polonium were discovered in the 19th century, and livermorium in 2000. All of the chalcogens have six valence electrons, leaving them two electrons short of a full outer shell. Their most common oxidation states are −2, +2, +4, and +6. They have relatively low atomic radii, especially the lighter ones.Lighter chalcogens are typically nontoxic in their elemental form, and are often critical to life, while the heavier chalcogens are typically toxic. All of the chalcogens have some role in biological functions, either as a nutrient or a toxin. The lighter chalcogens, such as oxygen and sulfur, are rarely toxic and usually helpful in their pure form. Selenium is an important nutrient but is also commonly toxic. Tellurium often has unpleasant effects (although some organisms can use it), and polonium is always extremely harmful, both in its chemical toxicity and its radioactivity.

Sulfur has more than 20 allotropes, oxygen has nine, selenium has at least five, polonium has two, and only one crystal structure of tellurium has so far been discovered. There are numerous organic chalcogen compounds. Not counting oxygen, organic sulfur compounds are generally the most common, followed by organic selenium compounds and organic tellurium compounds. This trend also occurs with chalcogen pnictides and compounds containing chalcogens and carbon group elements.

Oxygen is generally extracted from air and sulfur is extracted from oil and natural gas. Selenium and tellurium are produced as byproducts of copper refining. Polonium and livermorium are most available in particle accelerators. The primary use of elemental oxygen is in steelmaking. Sulfur is mostly converted into sulfuric acid, which is heavily used in the chemical industry. Selenium's most common application is glassmaking. Tellurium compounds are mostly used in optical disks, electronic devices, and solar cells. Some of polonium's applications are due to its radioactivity.

Ditellurium bromide

Ditellurium bromide is the inorganic compound with the formula Te2Br. It is one of the few stable lower bromides of tellurium. Unlike sulfur and selenium, tellurium forms families of polymeric subhalides where the chalcogen/halide ratio is less than 2.

Hydrogen ditelluride

Hydrogen ditelluride or ditellane is an unstable hydrogen dichalcogenide containing two tellurium atoms per molecule, with structure HTeTeH. Hydrogen ditelluride is interesting to theorists because its molecule is simple yet asymmetric (with no centre of symmetry) and is predicted to be one of the easiest to detect parity violation, in which the left handed molecule has differing properties to the right handed one.

Hydrogen telluride

Hydrogen telluride (tellane) is the inorganic compound with the formula H2Te. A hydrogen chalcogenide and the simplest hydride of tellurium, it is a colorless gas. Although unstable in ambient air, the gas can exist at very low concentrations long enough to be readily detected by the odour of rotting garlic at extremely low concentrations; or by the revolting odour of rotting leeks at somewhat higher concentrations. Most compounds with Te–H bonds (tellurols) are unstable with respect to loss of H2. H2Te is chemically and structurally similar to hydrogen selenide, both are acidic. The H–Te–H angle is about 90°. Volatile tellurium compounds often have unpleasant odours, reminiscent of decayed leeks or garlic.

Isotopes of tellurium

There are 39 known isotopes and 17 nuclear isomers of tellurium (52Te), with atomic masses that range from 104 to 142. These are listed in the table below.

Naturally-occurring tellurium on Earth consists of eight isotopes. Two of these have been found to be radioactive: 128Te and 130Te undergo double beta decay with half-lives of, respectively, 2.2×1024 (2.2 septillion) years (the longest half-life of all nuclides proven to be radioactive) and 8.2×1020 (820 quintillion) years. The longest-lived artificial radioisotope of tellurium is 121Te with a half-life of nearly 19 days. Several nuclear isomers have longer half-lives, the longest being 121mTe with a half-life of 154 days.

The very-long-lived radioisotopes 128Te and 130Te are the two most common isotopes of tellurium. Of elements with at least one stable isotope, only indium and rhenium likewise have a radioisotope in greater abundance than a stable one.

It has been claimed that electron capture of 123Te was observed, but the recent measurements of the same team have disproved this. The half-life of 123Te is longer than 9.2 × 1016 years, and probably much longer.124Te can be used as a starting material in the production of radionuclides by a cyclotron or other particle accelerators. Some common radionuclides that can be produced from tellurium-124 are iodine-123 and iodine-124.

The short-lived isotope 135Te (half-life 19 seconds) is produced as a fission product in nuclear reactors. It decays, via two beta decays, to 135Xe, the most powerful known neutron absorber, and the cause of the iodine pit phenomenon.

With the exception of beryllium, tellurium is the lightest element observed to commonly undergo alpha decay, with isotopes 104Te to 109Te being seen to undergo this mode of decay. Some lighter elements, namely those in the vicinity of 8Be, have isotopes with delayed alpha emission (following proton or beta emission) as a rare branch.

Period 5 element

A period 5 element is one of the chemical elements in the fifth row (or period) of the periodic table of the elements. The periodic table is laid out in rows to illustrate recurring (periodic) trends in the chemical behaviour of the elements as their atomic number increases: a new row is begun when chemical behaviour begins to repeat, meaning that elements with similar behaviour fall into the same vertical columns. The fifth period contains 18 elements, beginning with rubidium and ending with xenon. As a rule, period 5 elements fill their 5s shells first, then their 4d, and 5p shells, in that order; however, there are exceptions, such as rhodium.

Polonium

Polonium is a chemical element with symbol Po and atomic number 84. A rare and highly radioactive metal with no stable isotopes, polonium is chemically similar to selenium and tellurium, though its metallic character resembles that of its horizontal neighbors in the periodic table: thallium, lead, and bismuth. Due to the short half-life of all its isotopes, its natural occurrence is limited to tiny traces of the fleeting polonium-210 (with a half-life of 138 days) in uranium ores, as it is the penultimate daughter of natural uranium-238. Though slightly longer-lived isotopes exist, they are much more difficult to produce. Today, polonium is usually produced in milligram quantities by the neutron irradiation of bismuth. Due to its intense radioactivity, which results in the radiolysis of chemical bonds and radioactive self-heating, its chemistry has mostly been investigated on the trace scale only.

Polonium was discovered in 1898 by Marie and Pierre Curie, when it was extracted from uranium ore and identified solely by its strong radioactivity: it was the first element to be so discovered. Polonium was named after Marie Curie's homeland of Poland. Polonium has few applications, and those are related to its radioactivity: heaters in space probes, antistatic devices, and sources of neutrons and alpha particles. Besides the radioactivity polonium is chemically extremely toxic.

Sodium tellurite

Sodium tellurite is an inorganic tellurium compound with formula Na2TeO3. It is a water-soluble white solid and a weak reducing agent. Sodium tellurite is an intermediate in the extraction of the element, tellurium; it is a product obtained from anode slimes and is a precursor to tellurium.

Telluric acid

Telluric acid is a chemical compound with the formula Te(OH)6. It is a white solid made up of octahedral Te(OH)6 molecules which persist in aqueous solution. There are two forms, rhombohedral and monoclinic, and both contain octahedral Te(OH)6 molecules.

Telluric acid is a weak acid which is dibasic, forming tellurate salts with strong bases and hydrogen tellurate salts with weaker bases or upon hydrolysis of tellurates in water.

Tellurite glass

Tellurite glasses contain tellurium oxide (TeO2) as the main component.

Tellurium dioxide

Tellurium dioxide (TeO2) is a solid oxide of tellurium. It is encountered in two different forms, the yellow orthorhombic mineral tellurite, β-TeO2, and the synthetic, colourless tetragonal (paratellurite), α-TeO2. Most of the information regarding reaction chemistry has been obtained in studies involving paratellurite, α-TeO2.

Tellurium hexafluoride

Tellurium hexafluoride is a chemical compound of tellurium and fluorine with the chemical formula TeF6. It is a colorless, highly toxic gas with an extremely unpleasant smell.

Tellurium tetrabromide

Tellurium tetrabromide (TeBr4) is an inorganic chemical compound. It has a similar tetrameric structure to TeCl4. It can be made by reacting bromine and tellurium. In the vapour TeBr4 dissociates:

TeBr4 → TeBr2 + Br2It is a conductor when molten, dissociating into the ions TeBr3+ and Br−

When dissolved in benzene and toluene, TeBr4 is present as the unionized tetramer Te4Br16. In solvents with donor properties such as acetonitrile, CH3CN ionic complexes are formed which make the solution conducting:

TeBr4 + 2CH3CN → (CH3CN)2TeBr3+ + Br−

Tellurium tetrachloride

Tellurium tetrachloride is the inorganic compound with the empirical formula TeCl4. The compound is volatile, subliming at 200 °C at 0.1 mmHg. Molten TeCl4 is ionic, dissociating into TeCl3+ and Te2Cl102−.

Tellurium tetrafluoride

Tellurium tetrafluoride, TeF4, is a stable, white, hygroscopic crystalline solid and is one of two fluorides of tellurium. The other binary fluoride is tellurium hexafluoride. The widely reported Te2F10 has been shown to be F5TeOTeF5 There are other tellurium compounds that contain fluorine, but only the two mentioned contain solely tellurium and fluorine. Tellurium difluoride, TeF2, and ditellurium fluoride, Te2F are not known.

Tellurium trioxide

Tellurium trioxide (TeO3) is an inorganic chemical compound of tellurium and oxygen. In this compound, tellurium is in the +6 oxidation state.

Tellurous acid

Tellurous acid is an inorganic compound with the formula H2TeO3. It is the oxoacid of tellurium(IV). The compound is not well characterized. An alternative way of writing its formula is (HO)2TeO. In principle, tellurous acid would form by treatment of tellurium dioxide with water, that is by hydrolysis. The related conjugate base is well known in the form of several salts such as potassium hydrogen tellurite, KHTeO3.

Tritellurium dichloride

Tritellurium dichloride is the inorganic compound with the formula Te3Cl2. It is one of the more stable lower chlorides of tellurium.

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