Tellurium

Tellurium is a chemical element that has the symbol Te and atomic number 52. A brittle silver-white metalloid which looks like tin, tellurium is chemically related to selenium and sulfur. Tellurium is primarily used in alloys and as a semiconductor.

Notable characteristics
Tellurium is extremely rare, one of the nine rarest elements on earth. It is in the same chemical family as oxygen, sulfur, selenium, and polonium (the chalcogens).

When crystalline, tellurium is silvery-white and when it is in its pure state it has a metallic luster. This 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.

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 (photoelectric effect).

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
 * 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 also used in blasting caps
 * Organic tellurides have also been employed as initiators for living radical polymerisation and electron-rich mono- and di-tellurides possess antioxidant activity.

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

Semiconductor and electronic industry uses:


 * Tellurium is used in the media layer of ReWritable Compact Discs (CD-RW).


 * Tellurium is used in the media layer of ReWritable Digital Video Discs (DVD-RW).


 * Tellurium is used in the new phase change memory chips developed by Intel. See phase change memory. Also see here.


 * Tellurium is used in the media layer of ReWritable Blu-Ray DVD. See here.


 * Bismuth telluride (Bi2Te3) has found use in thermoelectric devices.


 * Tellurium has potential applications in cadmium telluride (CdTe) solar panels. Some of the highest efficiencies for solar cell electric power generation have been obtained by using this material. First Solar Inc. 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.

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.

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
With an abundance in the Earth's crust even lower than platinum, tellurium is, apart from the precious metals, the rarest stable solid element in the earth's crust. Its abundance by mass is less than 0.001 ppm, compare with 0.037 ppm for platinum. By comparison, even the rarest of the lanthanides have crustal abundances of 0.5 ppm.

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. Rather, the extraordinarily low abundance of tellurium on Earth results from the fact that, during the formation of the Earth, 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 most depleted in the Earth's crust by this process.

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 only chemical compounds of gold found in nature, but tellurium itself (unlike gold) is also found combined with other elements (in metallic salts). 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 of tellurium. Tellurium is produced mainly in the US, Canada, Peru, and Japan. See here.

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. See also here.


 * See also: Telluride, Colorado, category:Telluride minerals



Compounds
Tellurium is in the same series as sulfur and selenium and forms similar compounds. A compound with metal or hydrogen and similar ions is called a telluride. Gold and silver tellurides are considered good ores. Compounds with tellurate ions complexes TeO42- or TeO66- are known as tellurates. Also tellurites TeO32-. Also tellurols –TeH, named with prefix tellanyl- or suffix -tellurol.
 * See also: Category:Tellurium compounds

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 table to the right); three of them are observed to be radioactive. 128Te has the longest known half-life, 2.2&times;1024 years, among all radioactive isotopes.

Precautions
Tellurium and tellurium compounds should be considered to be mildly toxic and need to be handled with care.

Acute poisoning is rare. Tellurium is not reported to be carcinogenic.

Humans exposed to as little as 0.01 mg/m3 or less in air develop "tellurium breath", which has a garlic-like odor. 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. Dimethyl telluride is volatile and produces the garlic-like smell.