tellurium

antimony ← tellurium → iodine Se ↑ Te ↓ Po 52Te Periodic table
Appearance
silvery lustrous gray
General properties
Name, symbol, number	tellurium, Te, 52
Element category	metalloid
Group, period, block	16, 5, p
Standard atomic weight	127.60 g·mol−1
Electron configuration	[Kr] 5s2 4d10 5p4
Electrons per shell	2, 8, 18, 18, 6 (Image)
Physical properties
Phase	solid
Density (near r.t.)	6.24 g·cm−3
Liquid density at m.p.	5.70 g·cm−3
Melting point	722.66 K, 449.51 °C, 841.12 °F
Boiling point	1261 K, 988 °C, 1810 °F
Heat of fusion	17.49 kJ·mol−1
Heat of vaporization	114.1 kJ·mol−1
Specific heat capacity	(25 °C) 25.73 J·mol−1·K−1
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k at T/K     (775) (888) 1042 1266
Atomic properties
Oxidation states	6, 5, 4, 2, -2 (mildly acidic oxide)
Electronegativity	2.1 (Pauling scale)
Ionization energies	1st: 869.3 kJ·mol−1
	2nd: 1790 kJ·mol−1
	3rd: 2698 kJ·mol−1
Atomic radius	140 pm
Covalent radius	138±4 pm
Van der Waals radius	206 pm
Miscellanea
Crystal structure	hexagonal
Magnetic ordering	diamagnetic[1]
Thermal conductivity	(300 K) (1.97–3.38) W·m−1·K−1
Speed of sound (thin rod)	(20 °C) 2610 m/s
Young's modulus	43 GPa
Shear modulus	16 GPa
Bulk modulus	65 GPa
Mohs hardness	2.25
Brinell hardness	180 MPa
CAS registry number	13494-80-9
Most stable isotopes
Main article: Isotopes of tellurium
iso NA half-life DM DE (MeV) DP 120Te 0.09% >2.2×1016y ε ε 1.701 120Sn 121Te syn 16.78 d ε 1.040 121Sb 122Te 2.55% 122Te is stable with 70 neutrons 123Te 0.89% >1.0×1013 y ε 0.051 123Sb 124Te 4.74% 124Te is stable with 72 neutrons 125Te 7.07% 125Te is stable with 73 neutrons 126Te 18.84% 126Te is stable with 74 neutrons 127Te syn 9.35 h β− 0.698 127I 128Te 31.74% 2.2×1024 y β−β− 0.867 128Xe 129Te syn 69.6 min β− 1.498 129I 130Te 34.08% 7.9×1020 y β−β− 2.528 130Xe
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Tellurium (pronounced /tɪˈlʊəriəm, tɛ-/ te-LOOR-ee-əm) is a chemical element that has the symbol Te and atomic number 52. A brittle, mildly toxic, silver-white metalloid which looks similar to tin, tellurium is chemically related to selenium and sulfur. Tellurium is primarily used in alloys and as a semiconductor.

Characteristics

Tellurium belongs to the same chemical family as oxygen, sulfur, selenium, and polonium: the chalcogen family.

When crystalline, tellurium is silvery-white and when it is in pure state it has a metallic luster. It is a brittle and easily pulverized metalloid. Amorphous tellurium is found by precipitating it from a solution of tellurous or telluric acid (Te(OH)₆). However, there is some debate whether this form is really amorphous or made of minute crystals.

Compounds

Tellurium is in the same group as sulfur and selenium and forms similar compounds. It exhibits the oxidation states −2, +2, +4, and +6, with the +4 state being most common.^[2]

Tellurides

The −2 oxidation state is exhibited in binary compounds with many metals, such as zinc telluride, ZnTe, formed by heating tellurium with zinc.^[3] Decomposition of ZnTe with hydrochloric acid yields hydrogen telluride, H₂Te, the tellurium analogue of the other chalcogen hydrides, H₂O, H₂S, and H₂Se:

ZnTe + 2 HCl → ZnCl₂ + H₂Te

H₂Te reacts with many metals to form tellurides, containing the Te²⁻ anion.^[3] Gold and silver tellurides are considered good ores.

Halogen compounds

The +2 oxidation state is exhibited by the monoxide, TeO, and the dihalides, TeCl₂, TeBr₂, and TeI₂. The dihalides have not been obtained in pure form,^[4]:274 although they are known decomposition products of the tetrahalides in organic solvents, and their derived tetrahalotellurates are well-characterized:

Te + X₂ + 2 X⁻ → TeX2−4

where X is Cl, Br, or I. These anions are square planar in geometry.^[4]:281 Polynuclear anionic species also exist, such as the dark brown Te₂I2−6,^[4]:283 and the black Te₄I2−14.^[4]:285

Fluorine forms two halides with tellurium: the mixed-valence Te₂F₄, and TeF₆. In the +6 oxidation state, the –OTeF₅ structural group occurs in a number of compounds such as HOTeF₅, B(OTeF₅)₃, Xe(OTeF₅)₂, Te(OTeF₅)₄, and Te(OTeF₅)₆.^[5] The square antiprismic anion TeF2−8 is also attested.^[6] The other halogens do not form halides with tellurium in the +6 oxidation state, but only tetrahalides (TeCl₄, TeBr₄, and TeI₄) in the +4 state, and other lower halides (Te₃Cl₂, Te₂Cl₂, Te₂Br₂, Te₂I, and two forms of TeI). In the +4 oxidation state, halotellurate anions are known, such as TeCl2−6 and Te₂Cl2−10. Halotellurium cations are also attested, including TeI+3, found in TeI₃AsF₆.^[7]

Oxygen compounds

Tellurium monoxide was first reported in 1883 as a black amorphous solid formed by the heat decomposition of TeSO₃ in vacuum, disproportionating into tellurium dioxide, TeO₂, and elemental tellurium upon heating.^[8][9] Since then, however, some doubt has been cast on its existence in the solid phase, although it is known as a vapor phase fragment; the black solid may be merely an equimolar mixture of elemental tellurium and tellurium dioxide.^[10]

Tellurium dioxide is formed by heating tellurium in air, causing it to burn with a blue flame.^[3] Tellurium trioxide, β-TeO₃, 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.^[11] Tellurium also exhibits mixed-valence oxides, Te₂O₅ and Te₄O₉.^[11]

The tellurium oxides and hydrated oxides form a series of acids, including tellurous acid (H₂TeO₃), orthotelluric acid (Te(OH)₆), and metatelluric acid ((H₂TeO₄)_n).^[10] 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, it forms red solutions containing the Zintl ion, Te2+4. The oxidation of tellurium by AsF₅ in liquid SO₂ also produces this square planar cation, as well as with the trigonal prismatic, yellow-orange Te4+6:^[6]

4 Te + 3 AsF₅ → Te2+4(AsF−6)₂ + AsF₃

6 Te + 6 AsF₅ → Te4+6(AsF−6)₄ + 2 AsF₃

Other tellurium Zintl cations include the polymeric Te2+7, and the blue-black Te2+8, which consists of two fused 5-membered tellurium rings. The latter cation is formed by the reaction of tellurium with tungsten hexachloride:^[6]

8 Te + 2 WCl₆ → Te2+8(WCl−6)₂

Interchalcogen cations also exist, such as Te₂Se2+6 (distorted cubic geometry) and Te₂Se2+8. These are formed by oxidizing mixtures of tellurium and selenium with AsF₅ or SbF₅.^[6]

Organic compounds

In organic chemistry, tellurium forms analogues of alcohols and thiols, having the functional group –TeH, called tellurols. The –TeH functional group is also referred to with the prefix tellanyl-.

Isotopes

There are 30 known isotopes of tellurium with atomic masses that range from 108 to 137. Naturally found tellurium consists of eight isotopes (listed in the main article); three of them are observed to be radioactive. ¹²⁸Te has the longest known half-life, 2.2×10²⁴ years^[12], among all radioisotopes.^[13] Tellurium is the lightest element known to undergo alpha decay, with isotopes ¹⁰⁶Te to ¹¹⁰Te being able to undergo this mode of decay.

History

Tellurium (Latin tellus meaning "earth") was discovered in 1782 by the Hungarian Franz-Joseph Müller von Reichenstein (Müller Ferenc) in Nagyszeben (now, Sibiu) Transylvania. In 1789, another Hungarian scientist, Pál Kitaibel, also discovered the element independently, but later he gave the credit to Müller. In 1798, it was named by Martin Heinrich Klaproth who earlier isolated it from the mineral calaverite.^[14]

Tellurium was used as a chemical bonder in the making of the outer shell of the first atom bomb. The 1960s brought growth in thermoelectric applications for tellurium, as well as its use in free-machining steel, which became the dominant use.

Occurrence

Tellurium on quartz (Moctezuma, Sonora, Mexico)

With an abundance in the Earth's crust comparable to that of platinum, tellurium is one of the rarest stable solid element in the Earth's crust. Its abundance is about 1 µg/kg.^[15] By comparison, even the rarest of the lanthanides have crustal abundances of 500 µg/kg (see Abundance of elements in Earth's crust).

The extreme rarity of tellurium in the Earth's crust is not a reflection of its cosmic abundance, which is in fact greater than that of rubidium, even though rubidium is ten thousand times more abundant in the Earth's crust. The extraordinarily low abundance of tellurium on Earth is because during the Earth's formation, the stable form of elements in the absence of oxygen and water was controlled by the oxidation and reduction of hydrogen. Under this scenario elements such as tellurium which form volatile hydrides were severely depleted during the formation of the Earth's crust through evaporation. Tellurium and selenium are the heavy elements mostly depleted in the Earth's crust by this process.^{[citation needed]}

Tellurium is sometimes found in its native (elemental) form, but is more often found as the tellurides of gold (calaverite, krennerite, petzite, sylvanite, and others). Tellurium compounds are the most common chemical compounds of gold found in nature (rare non-tellurides such as gold aurostibite and bismuthide are known). Tellurium is also found combined with elements other than gold, in salts of other metals. The principal source of tellurium is from anode sludges produced during the electrolytic refining of blister copper. It is a component of dusts from blast furnace refining of lead. Treatment of 500 tons of copper ore typically yields one pound (0.45 kg) of tellurium. Tellurium is produced mainly in the United States, Canada, Peru, and Japan.

Commercial-grade tellurium is usually marketed as minus 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, tellurium price was driven up by increased demand and limited supply, reaching as high as US$100 per pound in 2006.^[16][17]

Applications

Tellurium is a p-type semiconductor that shows a greater conductivity in certain directions which depends on atomic alignment. Chemically related to selenium and sulfur, the conductivity of this element increases slightly when exposed to light (photoconductivity).

It can be doped with copper, gold, silver, tin, or other metals. When in its molten state, tellurium is corrosive to copper, iron, and stainless steel.

Tellurium gives a greenish-blue flame when burned in normal air and forms tellurium dioxide as a result.

Metal alloys: ^[18]

• It is mostly used in alloys with other metals. It is added to lead to improve its strength and durability, and to decrease the corrosive action of sulfuric acid.
• When added to stainless steel and copper it makes these metals more workable. It is alloyed into cast iron for chill control.

Other uses:

• Used in ceramics.
• It is used in chalcogenide glasses.
• Tellurium is used in blasting caps.
• Organic tellurides have been employed as initiators for living radical polymerization and electron-rich mono- and di-tellurides possess antioxidant activity.
• Tellurite agar is used to identify member of the corynebacterium genus, most typically Corynebacterium diphtheriae, the pathogen responsible for diphtheria.

High purity metalorganics of both selenium and tellurium are used in the semiconductor industry, and are prepared by adduct purification.^[19][20]

Semiconductor and electronic industry uses:

• Tellurium as a tellurium suboxide is used in the media layer of several types of rewritable optical discs, including ReWritable Compact Discs (CD-RW), ReWritable Digital Video Discs (DVD-RW) and ReWritable Blu-ray Discs. ^[21][22]

• Tellurium is used in the new phase change memory chips^[23] developed by Intel.^[24]
• Bismuth telluride (Bi₂Te₃) is used in thermoelectric devices.

• Tellurium is used in cadmium telluride (CdTe) solar panels. NREL lab tests using this material achieved some of the highest efficiencies for solar cell electric power generation. First Solar started massive commercial production of CdTe solar panels in recent years, significantly increased tellurium demand. If some of the cadmium in CdTe is replaced by zinc then CdZnTe is formed which is used in solid-state x-ray detectors.

• Alloyed with both cadmium and mercury, to form mercury cadmium telluride, an infrared sensitive semiconductor material is formed. Organotellurium compounds such as dimethyl telluride, diethyl telluride, diisopropyl telluride, diallyl telluride and methyl allyl telluride are used as precursors for MOVPE growth of II-VI compound semiconductors. Diisopropyl telluride (DIPTe) is employed as the preferred precursor for achieving the low temperature growth of CdHgTe by MOVPE.

Precautions

Tellurium and tellurium compounds are considered to be mildly toxic and need to be handled with care, although acute poisoning is rare.^[25] Tellurium is not reported to be carcinogenic.^[25]

Humans exposed to as little as 0.01 mg/m³ or less in air develop "tellurium breath", which has a garlic-like odor.^[26] The garlic odor that is associated with human intake of tellurium compounds is caused from the tellurium being metabolized by the body. When the body metabolizes tellurium in any oxidation state, the tellurium gets converted into dimethyl telluride, (CH₃)₂Te, which is volatile and is the cause of the 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.

References

External links

Wikimedia Commons has media related to: Tellurium

Look up tellurium in Wiktionary, the free dictionary.

• WebElements.com – Tellurium
• USGS Mineral Information on Selenium and Tellurium