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Home , Tellurium

in your element Tellurium in a twist

Jim Ibers takes a look at the intriguing structures and bonding found in tellurium and its compounds, and considers their uses in a diversity of fi elds ranging from to electronics.

ramatic changes in physical properties Basic research into the synthesis, compounds in which S−S bonding occurs, occur on descending 16 of the structures and properties of tellurium- the S are separated by single bonds. Dperiodic table. , of course, is a containing compounds has increased In contrast, in -state compounds where gas, a , and the in-between markedly in recent years. As one there is Te−Te bonding, in addition to normal elements, , and tellurium, measure, we can consider Te- Te–Te single bonds about 2.74 Å in length, are all that show increasing metallic containing compounds there may well be Te−Te separations of up to properties. In fact, tellurium (Te), which I that have been structurally 3.6 Å or more, but still less than the van der fi nd to be the most interesting of the group, characterized, most oft en by Waals separation of 4.1 Å. Th is oft en makes acts very much like most . X-ray diff raction techniques. the assignment of formal oxidation states Tellurium is not an abundant metal. In the past decade the number of impossible. Th e only allotrope of tellurium Present at about 1 ppb in the Earth’s crust, inorganic Te compounds has increased that is stable at room temperature is an it is less abundant than , by about 40%; the increase in organic infi nite helix (pictured). In that helix, the Te– or the so-called ‘rare-earth’ elements. and organometallic Te compounds Te distances are about 2.83 Å, but there are So why should one care about such an is nearly 70%. Th is last fi gure is some short nearest-neighbour interactions at obscure element? One answer, of course, surprising given that tellurium 3.49 Å. Th e bonding in Te-containing solid- is ‘because it is there’, if one can borrow is not an essential element for state compounds, which has engendered the phrase that George Mallory used to living systems and that many numerous theoretical studies, is far more justify climbing Everest. However, in these of the synthons and products interesting than it is in its lighter congeners. days, where instant gratifi cation of those in organotellurium chemistry make Th e major industrial use of tellurium is in who provide the funds is advisable, other their sulfur analogues smell like roses! metallurgy, where it is an important alloying reasons include important current industrial From a personal perspective, agent. It is added to and to to applications and the potential for more. For when we started our research into make them more machinable, to cast to such applications to be realized, tellurium ternary solid-state tellurides minimize heat shock, and to to reduce must be available for basic research in the about 25 years ago it was fatigue. Another signifi cant industrial use of laboratory and for use in industry. Th at not known whether these tellurium is as an accelerator and vulcanizer us to a discussion of its sources. compounds would resemble in the rubber industry. Th ese uses do not Mining solely for tellurium is not ternary or not. Some do require high-purity tellurium. However, profi table because it is rare and so widely and some do not. Dissimilarity even the electronics industry increasingly makes distributed. However, it is most oft en found extends to the binaries. For example, use of high-purity tellurium in a variety in nature in combination with gold, silver, NaTe exists whereas NaSe is not known. Ask of new and evolving applications. For copper and other commercially valuable a fi rst-year chemistry student to describe example, tellurium is used in the newly metals. In fact, gold tellurides are the most the structure and formal oxidation states of developed -change memory chips as common gold-containing minerals. As an NaTe — the answer is that the well-known well as in rewritable CD, DVD and Blu-ray aside, tellurium was discovered in 1782 propensity of sulfur, selenium and tellurium, discs. Bismuth is widely used in by the Austrian mineralogist Franz Josef as opposed to oxygen, to form chains and thermoelectric cooling devices. Such devices Müller von Reichenstein in gold taken rings reaches its extreme at tellurium. fi nd many applications in electronics and

from a mine in Hungary. Consequently, NaTe is, in fact, Na6Te(Te5), which contains consumer products, the latest being their tellurium is a by-product in the refi ning the Na+ cation together with the Te2− and increased use for portable food coolers and, 4− of these other metals. Th e major source is the Z-shaped Te5 anions. Th is Z-shaped believe it or not, even for car-seat cooling in the sludges produced during the anion possesses Te−Te distances of 2.82 and systems. Perhaps the greatest impact of electrolytic refi ning of copper. Th ese sludges 3.08 Å, and therein lies one of the exciting tellurium on our lives might come from average about 2 wt% Te. In 2007, total world diff erences between Te and S. Structurally, solar panels — still at an production of tellurium was about 500 the mineral (sometimes called ‘fool’s early stage, they are among the most effi cient metric tons — a signifi cant increase over gold’ owing to its golden lustre), of formula for electric power generation. It is possible ❐ previous years. Moreover, as demand for FeS2, possesses S atoms separated by about that tellurium may yet save mankind! tellurium has increased, so has its cost. 2.08 Å, the length of a S−S single-bond. Th e formal of Fe is thus +II, JIM IBERS is in the Department of Chemistry not +IV as one might conclude from the at Northwestern University in the USA. formula. In fact, in essentially all solid-state e-mail: [email protected] In Sn SbTe I Xe Cs Ba La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm

508 NATURE CHEMISTRY | VOL 1 | SEPTEMBER 2009 | www.nature.com/naturechemistry

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