Tellurium
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 by Austrian mineralogist Franz-Joseph Müller von Reichenstein, although it was Martin Heinrich Klaproth who named the new element in 1798 after the Latin tellus '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 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, 2Te, a gas with a garlic-like odor exhaled in the breath of victims of tellurium exposure or poisoning.
Characteristics
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, 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. Tellurium is a semiconductor that shows greater electrical conductivity in certain directions depending on atomic alignment; the conductivity increases slightly when exposed to light. When molten, tellurium is corrosive to copper, iron, and stainless steel. Of the chalcogens, tellurium has the highest melting and boiling points, at, 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 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 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, 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.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 form, but is more often found as the tellurides of gold such as calaverite and krennerite, petzite, Ag3AuTe2, and sylvanite, AgAuTe4. The town of Telluride, Colorado, was named in the hope of a strike of gold telluride. 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. 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 was discovered in the 18th century in a gold ore from the mines in Kleinschlatten, near today's city of Alba Iulia, Romania. This ore was known as "Faczebajer weißes blättriges Golderz" or antimonalischer Goldkies, and according to Anton von Rupprecht, was Spießglaskönig, 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 and metallum problematicum, 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.
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, 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
Most Te 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 of tellurium.The anode sludges contain the selenides and tellurides of the noble metals in compounds with the formula M2Se or M2Te. 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 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 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, Russia, Japan, Canada, Uzbekistan, Sweden and the United States.