# Tellurium

properties
[ Kr ] 4 d 10 5 s 2 5 p 4
52 te
General
Name , symbol , atomic number Tellurium, Te, 52
Element category Semi-metals
Group , period , block 16 , 5 , p
Appearance silver-white, shiny metallic
CAS number 13494-80-9
EC number 236-813-4
ECHA InfoCard 100.033.452
Mass fraction of the earth's envelope 0.01 ppm
Atomic
Atomic mass 127.60 (3) u
Atomic radius (calculated) 140 (123) pm
Van der Waals radius 206 pm
Electron configuration [ Kr ] 4 d 10 5 s 2 5 p 4
1. Ionization energy 9.009 808 (6) eV 869.3 kJ / mol
2. Ionization energy 18th.6 (4) eV1 795 kJ / mol
3. Ionization energy 27.84 (4) eV2 686 kJ / mol
4. Ionization energy 37.4155 (12) eV3 610.05 kJ / mol
5. Ionization energy 59.3 (9) eV5 722 kJ / mol
6. Ionization energy 69.1 (2.0) eV6 667 kJ / mol
Physically
Physical state firmly
Modifications crystalline and amorphous
high pressure modifications (crystalline)
Crystal structure trigonal
density 6.24 g / cm 3
Mohs hardness 2.25
magnetism diamagnetic ( Χ m = −2.4 10 −5 )
Melting point 722.66 K (449.51 ° C)
boiling point 1263 K (990 ° C)
Molar volume 20.46 · 10 −6 m 3 · mol −1
Heat of evaporation 114 kJ / mol
Heat of fusion 17.5 kJ mol −1
Speed ​​of sound 2610 m s −1 at 293.15 K.
Electric conductivity 1 · 10 4 A · V −1 · m −1
Thermal conductivity 3 W m −1 K −1
Chemically
Oxidation states −2 , (± 1), 2, 4 , 6
Normal potential −1.143 V (Te + 2 e - → Te 2− )
Electronegativity 2.1 ( Pauling scale )
Isotopes
isotope NH t 1/2 ZA ZE (M eV ) ZP
120 te 0.096% 2.2 x 10 16 a ε ε 1.701 120 Sn
121 te {syn.} 16.78 d ε 1.040 121 Sb
121 meta Te {syn.} 154 d IT 0.294 121 te
ε 1.334 121 Sb
122 te 2.603% Stable
123 te 0.908% > 1 · 10 13 a ε 0.051 123 Sb
124 te 4.816% Stable
125 te 7.139% Stable
126 te 18,952% Stable
127 te {syn.} 9.35 h β - 0.698 127 I.
127 meta Te {syn.} 109 d IT 0.088 127 te
β - 0.786 127 I.
128 te 31.687% 7.2 x 10 24 a β - β - 0.867 128 Xe
129 te {syn.} 69.6 min β - 1.498 129 I.
129 meta Te {syn.} 33.6 d IT 0.106 129 te
β - 1.604 129 I.
130 te 33.799  % 7.9 x 10 20 a β - β - 2.528 130 Xe
For other isotopes see list of isotopes
NMR properties
Spin
quantum
number I
γ in
rad · T −1 · s −1
E r  ( 1 H) f L at
B = 4.7 T
in MHz
123 te 1 / 2 −7.059 10 7
125 te 1 / 2 −8.510 10 7
safety instructions
GHS labeling of hazardous substances

Caution

H and P phrases H: 332-317-360-412
P: 201-261-280-308 + 313
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .

Tellurium [ tʰɛˈluːɐ̯ ] ( Latin tellus "earth") is a rare chemical element with the element symbol Te and the atomic number 52. In the periodic table it is in the sixth main group , or the 16th  IUPAC group , and the 5th period and counts thus to the chalcogens . Its frequency roughly corresponds to that of gold , with which it also forms various compounds that occur naturally as minerals . Crystalline tellurium is a silvery-white, metallic, shiny semi-metal that is similar in appearance to tin and antimony . It reacts brittle to mechanical stress and can therefore be easily pulverized. In chemical compounds with non-metals , its behavior is close to sulfur and selenium , in alloys and intermetallic compounds , however, it shows very pronounced (semi) metallic properties.

## history

Sample of the type material
Martin Heinrich Klaproth

Tellurium was in 1782 by the Austrian chemist and mineralogist Franz Joseph Müller von Reichenstein (1740-1825) for inspections of gold - ore from the pit Mariahilf on the mountain Faczebaja in Zlatna (dt. Small Schlatten , clothes. Zalatna ) near Sibiu (dt Sibiu. , Transylvania , Romania ), which resulted in a lower than expected gold yield. He was by the scientific treatise news of the dignified Spies Glass King in Transylvania of Ignaz von Born hear (1742-1791) to the minerals. Spiesglaskönig denotes genuine antimony , Spiesglas is an old name for the mineral antimonite ( stibnite, gray spike gloss Sb 2 S 3 ). Von Born thought the solid metal in gold ores was antimony and attributed the low yield to a compound of gold with antimony. Müller von Reichenstein contradicted this view and initially considered it " sulphurized bismuth ". After further investigations, the results of which he published in a four-part paper between 1783 and 1785, he also excluded bismuth, since the metal, in contrast to antimony and bismuth, practically did not react with sulfuric acid. He gave the metallic phase the name metallum problematicum (also aurum problematicum or aurum paradoxum ). According to current knowledge, it consists of the minerals Nagyágite ( leaf ore, AuPb (Pb, Sb, Bi) Te 2–3 S 6 ) and sylvanite ( writing tellurium , (Au, Ag) Te 2 ) in addition to the solid tellurium . Müller von Reichenstein suspected that metallum problematicum “... might be a new, hitherto unknown semimetal?”, But first wanted to have his findings confirmed by the Swedish mineralogist and chemist Torben Olof Bergman (1735–1784). In 1783 he sent samples of the ore to Bergman for assessment, but received no definitive responses. Bergman died in 1784 and the investigations on metallum problematicum were stopped for the time being in 1785.

It was not until twelve years later, in 1797, that Martin Heinrich Klaproth (1743–1817) received samples of the ores from Müller von Reichenstein in Berlin. Klaproth confirmed the conclusions from Müller von Reichenstein's investigations and saw enough evidence for the discovery of a new element. In January 1798 Klaproth paid tribute to Müller von Reichenstein's achievements in a lecture and ascribed the discovery of the new element to him. Since Müller von Reichenstein had not given the element a name, Klaproth decided on the name tellurium (Latin tellus : "earth"):

“To fill this previous gap in chemical mineralogy, I present my experiments and experiences made with these precious ores, the main result of which is the discovery and confirmation of a new peculiar metal , to which I use the name tellurium, borrowed from the old mother earth . "

- Martin Heinrich Klaproth

The original hand pieces of the sample material from the type locality Zlatna, which Klaproth had available, are today in the Museum für Naturkunde in Berlin.

Independently of Müller von Reichenstein and Klaproth, the Hungarian chemist and botanist Paul Kitaibel (1757–1817) discovered tellurium in 1789 while investigating gold ores from the mining town of Nagybörzsöny (German Pilsen) in Hungary . However, Klaproth only mentioned Müller von Reichenstein in his published lecture, although he had also been aware of his research from a manuscript from Kitaibel since 1796. In a letter to Kitaibel, Klaproth stated that he had lost the content of the manuscript and that he had seen no connection with his work during the investigation of Müller von Reichenstein's ores. Klaproth finally convinced Kitaibel that the discovery of tellurium should be ascribed to Müller von Reichenstein alone, as he had already made the same observations on the new element a few years earlier.

The element symbol "Te" was proposed by Jöns Jakob Berzelius (1779–1848) in 1814 and is still used today. The first structure determination of crystalline tellurium with the help of X-ray diffraction took place in 1924.

## Occurrence

Tellurium on sylvanite from Tavua (Fiji), image width: 2 mm

Tellurium is a rarely occurring element; its share in the earth's crust is approx. 0.01 ppm ( g / t ). With gold , subordinate also with silver , copper , lead and bismuth as well as the platinum metals it seldom occurs in solid form , i.e. in elemental form in nature.

Dignified tellurium belongs as a mineral to the group of elements, more precisely to the semimetals and non-metals and is listed in the systematics of minerals according to Strunz under number I / B.03-40 (8th edition) or 1.CC.10 (9th edition). Edition), and according to Dana under the number 1.3.4.2.

Traces up to larger amounts of selenium may be in solid tellurium ( selenium tellurium ). Although tellurium is a rare element, a relatively large number of minerals are known, because tellurium forms its own minerals because it is only rarely incorporated into sulfides or selenides or sulfates or selenates ; it is too large for this crystal lattice of the lighter homologues . Conversely, however, the two lighter homologues more frequently represent tellurium in its lattice positions in the crystal structures of minerals containing tellurium.

Of all the elements, tellurium has the highest affinity for gold and is therefore often found in nature in the form of gold tellurides, minerals with telluride (Te 2− ) or ditelluride anions (Te 2 2− ). In addition to gold and other precious metals , lead and bismuth form other natural tellurides, often accompanying ( paragenesis ) to the solid metals and gold ores.

Less common are minerals with Te 4+ - cations in the crystal structure, wherein also the most important oxide of tellurium, the tellurium dioxide TeO 2 in two modifications as tetragonal paratellurite (α-TeO 2 ) and orthorhombic tellurite (β-TeO 2 ) occurs in nature . The other minerals with tellurium (IV) cations are oxotellurates (IV) ( tellurites ), which contain complex [TeO 3 ] 2− or [TeO 4 ] 4− anions. Minerals with Te 6+ cations in the form of octahedral [TeO 6 ] 6− complex anions are extremely rare; 21 minerals are known, most of which contain copper and lead. In addition to the said minerals also exist in nature mixed-valent Tellurminerale, including the calcium - Oxotellurat (IV, VI) Carlfriesit CaTe 3 O 8 with a Te 4+ : Te 6+ ratio of 2: 1. The minerals with Te 4+ and Te 6+ cations are secondary minerals that arose from the weathering of native tellurium and tellurides.

Tellurium-containing minerals are of no importance for the technical extraction of tellurium, as they are too rare and practically no mines worth mining exist. In addition to the type locality Zlatna (Transylvania, Romania), Moctezuma (Mexico), Cripple Creek (Colorado), Kalgoorlie (Australia) and Calaveras (California) are among the known sites of native tellurium or minerals containing tellurium . So far (as of 2012) 154 tellurium-containing minerals are known, of which five (Dilithium, Imgreit, Kurilit, Sztrokayit, Protojoseit) have not yet been recognized or discredited as independent minerals by the International Mineralogical Association (IMA). A selection of known minerals with tellurium in various oxidation states is shown in the table below.

Telluride Ditelluride Mixed chalcogenides Te (IV) minerals

Hessite
Ag 2 Te

Calaverit
AuTe 2  ( monoclinic )

Nagyágit ( leaf
ore ) AuPb (Pb, Sb, Bi) Te 2–3 S 6

Tellurite
β-TeO 2  ( orthorhombic )

Altait
PbTe

Sylvanite ( writing tellurium )
(Au, Ag) Te 2

Bi 2 Te 2 S

Zemannite
Mg 0.5 ZnFe [TeO 3 ] 3 • 4.5 H 2 O

## Extraction and presentation

Annual world production of tellurium in tons
country 2014 2015 2016 2017 2018
China 180 210 279 291 307 253.4
United States 050 050 050 050 050 050.0
Russia 033 034 040 044 046 039.4
Japan 032 034 028 034 055 036.6
Sweden 031 033 039 035 045 036.6
Canada 009 009 018th 017th 017th 014.0
Bulgaria 005 004th 004th 005 004th 004.4
to hum 340 374 458 476 524 434.4

Together with selenium, tellurium is obtained industrially exclusively from by-products of the large-scale electrolytic copper and nickel production. The anode sludge produced contains water-insoluble noble metals - tellurides and selenides of the general formula M 2 Ch (M = Cu, Ag, Au; Ch = Se, Te), which at temperatures above 500 ° C under atmospheric oxygen (O 2 ) with soda ( Sodium carbonate Na 2 CO 3 ) to react. The noble metal cations (M + ) here are to elemental metals (M) is reduced , the telluride anions to Oxotelluraten (IV) (TeO 3 2- ) oxidized :

${\ displaystyle \ mathrm {{\ overset {+ I} {M_ {2}}} {\ overset {-II} {Te}} \ + \ {\ overset {0} {O}} _ {2} \ + \ Na_ {2} C {\ overset {-II} {O}} _ {3} \ \ longrightarrow \ Na_ {2} {\ overset {+ IV} {Te}} {\ overset {-II} {O} } _ {3} + \ 2 \ {\ overset {0} {M}} \ + C {\ overset {-II} {O}} _ {2} \ uparrow}}$

Alternatively, this reaction can also take place with saltpeter ( sodium nitrate NaNO 3 ) in the absence of air and the formation of nitrogen oxides (NO and NO 2 ):

${\ displaystyle \ mathrm {{\ overset {+ I} {M_ {2}}} {\ overset {-II} {Te}} \ + \ 2 \ Na {\ overset {+ V} {N}} O_ { 3} \ \ longrightarrow \ Na_ {2} {\ overset {+ IV} {Te}} O_ {3} + \ 2 \ {\ overset {0} {M}} \ + \ {\ overset {+ IV} { N}} O_ {2} \ uparrow \ + \ {\ overset {+ II} {N}} O \ uparrow}}$

The resulting Natriumtellurat (IV) Na 2 TeO 3 is then dissolved in water where it basically responds Hydrogentellurat and (IV) ions HTeO 3 - forms. The separation of the tellurates (IV) from the selenates (IV) also formed in the basic solution is carried out by neutralization with the addition of sulfuric acid (H 2 SO 4 ), whereby tellurium dioxide TeO 2 , which is almost insoluble in water, precipitates:

${\ displaystyle \ mathrm {2 \ HTeO_ {3} ^ {-} + \ H_ {2} SO_ {4} \ \ longrightarrow \ 2 \ TeO_ {2} \ downarrow \ + \ SO_ {4} ^ {2-} \ + \ 2 \ H_ {2} O}}$

The tellurium dioxide can be reduced to elemental tellurium either in alkalis by electrolysis or chemically by dissolving it in concentrated mineral acids and introducing sulfur dioxide SO 2 , the sulfur being derived from the SO 2 molecules (or the sulfite ions formed from them in the solution SO 3 2− ) is oxidized and sulfate ions (SO 4 2− ) are formed:

${\ displaystyle \ mathrm {{\ overset {+ IV} {Te}} O_ {2} + \ 2 \ {\ overset {+ IV} {S}} O_ {2} \ + \ 2 \ H_ {2} O \ \ longrightarrow \ {\ overset {0} {Te}} \ + \ 2 \ {\ overset {+ VI} {S}} O_ {4} ^ {2 -} \ + \ 4 \ H ^ {+}} }$

The zone melting process is used to obtain high-purity tellurium (> 99.9%) .

The annual global production of tellurium rose by 54% from 340  tons in 2014 to 524 tons in 2018, averaging 434.4 tons per year (t / a). The main producers include China (∅ 253.4 t / a), the USA (∅ 50.0 t / a), Russia (∅ 39.4 t / a), Japan (∅ 36.6 t / a), Sweden (∅ 36.6 t / a), Canada (∅ 14.0 t / a) and Bulgaria (∅ 4.4 t / a). An overview of the production quantities in the individual countries is shown in the table. Other industrial nations such as Germany and Belgium probably also produce tellurium, but no figures are available. The United States Geological Survey (USGS) estimates the world's available reserves of tellurium at around 31,000 tons in 2020.

## Modifications

### Crystalline tellurium

Crystallographic Data

Crystalline tellurium, length approx. 2 cm
Crystal system trigonal
Space group P 3 1 21 (No. 152) ( P 3 2 21 (No. 154) )
Lattice parameter
(unit cell )
a  = 446  pm
c  = 592 pm
c / a  = 1.33
Number (Z) of the
formula units
Z = 3
Spiral chain of tellurium atoms along the 3 1 screw axis. Every third atom is congruent (highlighted in blue).
View of the chains along the c-axis.
Crystal structure of tellurium with a distorted octahedral (2 + 4) coordination environment (yellow) of a tellurium atom composed of 2 atoms within the chain and 4 from neighboring chains.

Under standard conditions , only one crystalline modification (Te-I or α-Te) is known of tellurium, which is known as crystalline or metallic tellurium. It is isotypic to α- selenium , that is, it has the same crystal structure . Tellurium crystallizes in the trigonal crystal system in the space group  P 3 1 21 (No. 152) with the lattice parameters a  = 446  pm and c  = 592 pm and three formula units in the unit cell (smallest structural unit of the crystal structure).

The space group P 3 1 21 (No. 152) described  according to the Hermann Mauguin symbolism explains the centering of the unit cell and the existing symmetry elements. P means that the Bravais lattice is primitive . The reference to the centering of the existing symmetry elements follow the space group: 3 1 describes a three-fold screw axis (reproduction of a particle by rotation of 120 ° and displacement ( translation ) to 1 / 3 in the direction of the rotation axis) parallel to the crystallographic c-axis ([001 ]), 2 describes a twofold axis of rotation (multiplication by rotation by 180 °) parallel to the three crystallographic a-axes (<100>), 1 the symmetry element of the single axis of symmetry or identity (multiplication by rotation by 360 °, the particle is formed i.e. on itself) in the direction perpendicular to the a-axes and the c-axis (<120>).

The crystal structure contains only one crystallographic distinguishable tellurium with the position coordinates x = 0.2636, y = 0 and z =  1 / 3 . All other atoms of the crystal structure can be traced back to this one atom through the existing symmetry elements of the space group. Since the tellurium atom coincides in its position with the twofold symmetry axis of the space group ( P 3 1 21 (No. 152) ), it is only reproduced by the threefold screw axis (3 1 ). This creates spiral chains of covalently bound tellurium atoms parallel to the c-axis. The tellurium atoms are 284 pm apart within the chain, the bond angle is 103.1 °. The linkages within the chain are highlighted in red in the figures, in each case one chain is shown for clarity in blue, with the dark blue atom to z = 1 / 3 , the mid-blue to z = 2 / 3 and the light blue to z = 1 and z = 0 is located. Every third atom within the chain is congruent. Each chain is surrounded by six more chains. Van der Waals bonds exist between the chains with Te-Te distances of 349 pm (green dashed line), which are created when the tellurium atoms fall below the van der Waals radius (2 · 206 pm = 412 pm) . For a single tellurium atom, the result is a coordination number of 6, more precisely 2 + 4, since 2 atoms come from the same chain and are therefore closer to each other than the other 4 from neighboring chains. The coordination polyhedron is a distorted octahedron (highlighted in yellow).

Tellurium can also crystallize in the space group  P 3 2 21 (No. 154) instead of P 3 1 21 (No. 152) . The 3 2 screw axis reproduced an atom also by rotation of 120 °, but then it will be at 2 / 3 instead of 1 / 3 the axis of rotation in the direction shifted. This also creates spiral chains, which, however, wind clockwise instead of counterclockwise (with the 3 1 screw axis) along the c-axis. The crystal structure in space group  P 3 2 21 (no. 154) ("left form") is thus the mirror image of the structure in space group  P 3 1 21 (no. 152) ("legal form"). The occurrence of mirror-image crystal forms is called enantiomorphism in crystallography .

The crystal system of tellurium is often given as hexagonal . The hexagonal and trigonal crystal systems are based on the same unit cell, but a hexagonal symmetry would require the existence of a six-fold symmetry axis (6, multiplication of a particle by rotation by 60 °). The crystal structure of tellurium, however, only contains the three-fold screw axis (3 1 ) and therefore undoubtedly belongs to the lower symmetrical trigonal crystal system .

Further modifications were discovered in high pressure experiments with crystalline tellurium (Te-I or α-tellurium). The specified pressure ranges for the stability of the modifications vary in part in the literature:

• Te-II crystallizes in the monoclinic crystal system in the pressure range from 4 to 6.6 GPa . C 2 / m (No. 12) and P 2 1 (No. 4) are mentioned as possible space groups in the literature .
• Te-III crystallizes in the orthorhombic crystal system and is stable in the pressure range above 6.6 GPa. For an orthorhombic modification there is a theoretical calculation in the space group  Imma (No. 74) .
• Te-IV crystallizes in the trigonal crystal system in the space group  R 3 m (No. 166) and corresponds to the structure of β- polonium . It is stable in the pressure range from 10.6 to 27 GPa. The distances between the tellurium atoms within the chains and to neighboring chains are the same in this modification and are each 300 pm, which results in the higher symmetry compared to α-Te.
• Te-V is stable above 27 GPa. A body-centered cubic lattice (space group  Im 3 m (No. 229) ) is assumed for this modification .

### Amorphous tellurium

The inconsistent amorphous modification is a brown powder and can be produced from telluric acid (H 2 TeO 3 ) through reaction with sulphurous acid (H 2 SO 3 ) or sulphite ions (SO 3 2− ). The sulfite ions are oxidized to sulfate ions (SO 4 2− ) while the Te 4+ cations are reduced to elemental tellurium :

${\ displaystyle \ mathrm {H_ {2} TeO_ {3} \ + \ 2 \ SO_ {3} ^ {2 -} \ longrightarrow \ Te \ downarrow \ + \ 2 \ SO_ {4} ^ {2 -} \ + \ H_ {2} O}}$

Amorphous tellurium slowly converts to the crystalline modification under standard conditions .

## properties

### Physical Properties

Crystalline tellurium is an intrinsic direct semiconductor with a band gap of 0.334  eV . The electrical conductivity can be increased as all semiconductors, by increasing the temperature or exposure, but this results in tellurium only a slight increase. The electrical and thermal conductivity of tellurium is directional, i.e. anisotropic . Crystalline tellurium is a soft ( Mohs hardness 2.25) and brittle material that can be easily processed into powder. When the pressure is increased, tellurium changes into further crystalline modifications. Above 450 ° C is tellurium into a red melt at temperatures above 990 ° C is tellurium as a yellow diamagnetic gas from Te 2 - molecules before. At temperatures above 2000 ° C, the Te 2 molecules disintegrate into individual atoms .

### Chemical properties

Crystalline tellurium is insoluble in water and poorly soluble in the mineral acids hydrochloric and sulfuric acid as well as in alkalis . On the other hand, it is readily soluble in nitric acid , as this is a very strong oxidizing agent and oxidizes elemental tellurium to tellurates with the stable oxidation state + IV. Tellurium melts attack copper, iron and stainless steel.

In compounds with non-metals , tellurium behaves like the lighter group member selenium . In air it burns in a green, blue flame to tellurium dioxide TeO 2 :

${\ displaystyle \ mathrm {Te \ + \ O_ {2} \ longrightarrow \ TeO_ {2}}}$

Tellurium reacts spontaneously with halogens to form tellurium halides . It is noteworthy that, in contrast to the lighter homologues selenium and sulfur , tellurium also forms thermodynamically stable iodides , including tellurium iodide TeI with the oxidation state + I. It reacts violently with base metals such as zinc to form the corresponding tellurides.

## Isotopes

Isotopes with mass numbers between 105 and 142 are known of tellurium . Natural tellurium is a mixed element made up of eight isotopes, five of which ( 122 Te, 123 Te, 124 Te, 125 Te, 126 Te) are stable. The isotope 123 Te should theoretically decay to 123 Sb with electron capture . However, this decay has not yet been observed; the lower limit for its half-life is 9.2 · 10 16 years (92 quadrillion years). The isotope 120 Te is converted directly into 120 Sn via double electron capture . The isotopes 128 Te and 130 Te are converted into 128 Xe and 130 Xe, respectively , through the emission of beta radiation ( double beta decay ) .

The largest proportion of natural tellurium is made up of about one third by the isotope 130 Te with a half-life of 7.9 · 10 20 years, followed by the isotope 128 Te. The average atomic mass of the natural tellurium isotopes is therefore 127.60 and is thus greater than that of the pure element iodine, which follows in the periodic table, with 126.90. 128 Te is considered to be the isotope with the slowest decay of all unstable isotopes of all elements. The extremely slow decay with a half-life of 7.2 · 10 24  years (7 quadrillion years, i.e. one atom in 1 kilogram decays every 18 months) could only be determined on the basis of the detection of the decay product ( 128 Xe) in very old samples of natural tellurium .

Of the other isotopes, the core isomer 121m Te has the longest half-life at 154 days . Even with the isotopes 127 Te and 129 Te, the half-lives of the isomers are longer than those of the ground state. The isotope 127 Te is most frequently used as a tracer , followed by 121 Te. The isotopes 127 Te and 129 Te also occur as fission products in nuclear fission in nuclear reactors .

## use

Tellurium is a technically less important element because it is expensive to manufacture and other elements or compounds are often equivalent in use. In 2016, elementary, polycrystalline and doped tellurium thermoelectric behavior with a high figure of merit in the range between room temperature and 400 ° C was demonstrated. Elemental tellurium is in metal industry, inter alia as an additive (<1%) of steel , cast iron , copper - and lead - alloys and used in stainless steels. It promotes corrosion resistance and improves mechanical properties and machinability. Pure tellurium has so far been used only rarely as a semiconductor ; tellurium is mostly used in II-VI compound semiconductors . Cadmium telluride CdTe is z. B. used in photodiodes and thin-film solar cells to generate electricity from light.

Bismuth telluride Bi 2 Te 3 is used in thermocouples to generate electricity in thermoelectric generators (e.g. in radionuclide batteries ) or in Peltier elements for cooling.

Combinations of germanium -GeTe and antimony- tellurides Sb 2 Te 3 are used in phase change materials as a component of optical storage disks (e.g. CD-RW ) or in novel storage materials such as phase change random access memory .

Glasses made of tellurium dioxide TeO 2 are used in optical waveguides instead of silica glass SiO 2 due to their high refractive index .

In microbiology , agar mixed with colorless potassium tellurate (IV) K 2 TeO 3 is used as a selective nutrient medium for the detection of staphylococci and Corynebacterium diphtheriae . The bacterial colonies appear here as small black spheres, since they are the Te 4+ - cations to elemental tellurium reduce and store in their cells.

Tellurium (or potassium tellurate) was first used medicinally in 1890 for the treatment of night sweats in patients suffering from tuberculosis .

Furthermore, small amounts of tellurium for the vulcanization of rubber , in detonators and for the dyeing of glass and ceramic used. The salts of tellurium are sometimes used to create a grass-green color in fireworks .

## Precautions and Toxicity

Dimethyl telluride Me 2 Te (H 3 C − Te − CH 3 )

In soluble form, tellurium is a poisonous element for the human organism and was therefore classified as poisonous in the past. However, since elemental tellurium is very poorly soluble in water and the body's own acids, it has been downgraded to harmful . Studies by the Netherlands Organization for Applied Scientific Research (TNO) showed that the LD 50 (oral) value for rats is> 5000 mg / kg. The value of 83 mg / kg given in many safety data sheets from the book Toxicometric Parameters of Industrial Toxic Chemicals under single Exposure by NF Ismerow, which dates from 1982, only applies to easily soluble tellurium compounds. In spite of this, various manufacturers continue to use the old LD 50 value for elemental tellurium (powder) and the classification toxic in connection with the H-phrase 301 ("Toxic if swallowed").

Tellurium is not as toxic as selenium . This is in analogy to the neighboring elements of the 5th main group, where the antimony is also less toxic than the arsenic . Passes tellurium, especially in the form of easily soluble tellurium compounds such as alkali metal - tellurates (for example, Na 2 TeO 3 ) by ingestion ( per os ) in the body, is formed by reducing toxic Dimethyltellurid (Me 2 Te: H 3 C-Te-CH 3 ) which can damage the blood , liver , heart and kidneys . Since easily soluble tellurium compounds release far more tellurium, they are also classified as more dangerous. Tellurium poisoning is noticeable through an intense garlic odor in the air that was first described by Christian Gottlob Gmelin in 1824 (during his first investigations into the effects of tellurium on living beings), which is caused by the dimethyl telluride. This only disappears after several weeks and unfolds even in very small amounts that do not yet cause serious poisoning. This garlic odor, unlike real garlic, cannot be removed by brushing your teeth. This also gets stuck in a room and only moves away after several hours. It is also slowly excreted through the skin.

Tellurium dusts can self-ignite in air and, finely distributed in the appropriate concentration, can also react explosively, with tellurium dioxide TeO 2 being formed in each case . Like other metal dusts, tellurium powder can also react explosively with interhalogen compounds such as bromopentafluoride BrF 5 . A maximum workplace concentration (MAK) for tellurium is not specified.

## proof

Tetratellur dication Te 4 2+ .
Positive evidence: Te 4 2+ cations in concentrated sulfuric acid

Elemental tellurium may in hot concentrated sulfuric acid (H 2 SO 4 by) oxidation of the tellurium to form the red Te 4 2+ - cation ( Tetratellur dication be detected). Part of the sulfuric acid is reduced to sulfuric acid (H 2 SO 3 ) during the reaction , which, due to the high temperatures, breaks down into water (H 2 O) and its anhydride sulfur dioxide (SO 2 ), which escapes as a gas:

${\ displaystyle \ mathrm {4 \ Te \ + \ 3 \ H_ {2} SO_ {4} \ longrightarrow \ Te_ {4} ^ {2 +} \ + \ 2 \ H_ {2} O + \ SO_ {2} \ uparrow \ + \ 2 \ HSO_ {4} ^ {-}}}$

The color of the square-planar Te 4 2+ cation is created by six delocalized π electrons that absorb part of the visible light . The other, unabsorbed wavelengths of light result in the complementary color red.

Tellurate and tellurite can be specified by polarography , i.e. H. can be determined selectively side by side. While the level of tellurate is −1.66 V, that of tellurite appears at −1.22 V (against SCE , 0.1 M sodium hydroxide solution). Both tellurium species are reduced to telluride in one step . Traces of 0.03% tellurate or 0.003% tellurite can be detected in this way. The methods of atomic spectroscopy are much more reliable . While a detection limit of 20 µg / l is achieved with the flame AAS , this value is significantly lower with the graphite tube AAS (0.2 µg / l) and the hydride technology (0.02 µg / l).

## Tellurium compounds

In compounds, tellurium occurs most frequently in the oxidation states −II ( telluride ) and + IV (tetrahalide, tellurium dioxide and tellurate (IV) , tellurite out of date ). The oxidation states + VI (tellurates (VI)) and + II (dihalides) as well as −I (ditellurides) and + I (monohalides, only known as TeI) are rarer.

### Hydrogen compounds

Tellurium hydrogen H 2 Te is a colorless, very poisonous gas that is produced by the reaction of tellurides (M x Te y ) with strong acids, e.g. hydrochloric acid HCl. From the elements (hydrogen and tellurium), the compound can only be represented as a strongly endothermic compound at temperatures above 650 ° C. When dissolved in water ( hydro telluric acid) it reacts acidic, whereby the acid strength corresponds roughly to that of phosphoric acid. In the air, the aqueous solution immediately decomposes into water and elemental tellurium.

### Oxygen compounds

Tellurium dioxide ( tellurium (IV) oxide ) TeO 2 is a colorless crystalline solid and the most important oxide of tellurium. It is created when elemental tellurium is burned with air. It is the anhydride of the weakly amphoteric and unstable telluric acid H 2 TeO 3 . Tellurium dioxide exists in an orthorhombic ( tellurite ) and a tetragonal ( paratellurite ) modification , which also occur naturally as minerals .

Tellurium trioxide ( tellurium (VI) oxide ) TeO 3 is a yellow, trigonal / rhombohedral crystallizing solid and the anhydride of orthotelluric acid H 6 TeO 6 . It arises from the dehydration of the orthotelluric acid through a strong increase in temperature. The yellow color comes about through the transfer of electrons from the oxygen to the tellurium ( “charge transfer” ).

Tellurium monoxide ( tellurium (II) oxide ) TeO is another oxide of tellurium that is unstable under standard conditions. It is described as a black amorphous solid and reacts in moist air with oxygen to form the more stable tellurium dioxide TeO 2 .

Ditellurium pentoxide ( Tellurium (IV) -Tellurium (VI) oxide ) is a mixed tellurium oxide with Te 4+ and Te 6+ cations. In addition to tellurium trioxide, it is another product in the thermal decomposition of orthotelluric acid and crystallizes in the monoclinic crystal system .

Tellurates are the salts of orthotelluric acid H 6 TeO 6 and metatelluric acid H 2 TeO 4 with the anions [TeO 6 ] 6− and [TeO 4 ] 2−, respectively . The salts of the telluric acid H 2 TeO 3 with the anion [TeO 3 ] 2− are called tellurates (IV) (outdated tellurites ).

### Halogen compounds

Tetrahalides TeX 4 with tellurium in the oxidation state + IV are the most common tellurium halides . These are known to all halogens ( fluorine , chlorine , bromine and iodine ). All compounds are crystalline solids.

TeX 2 dihalides with tellurium in the + II oxidation state are only known with chlorine, bromine and iodine, they only exist in the gas phase.

Monohalides TeX exist from tellurium only with iodine as tellurium iodide TeI. It is the only known thermodynamically stable mono- iodide of the chalcogens and a dark crystalline solid. Tellurium in this compound has the unusual + I oxidation state.

Subhalides contain Te with an oxidation state less than + I. Stable representatives are Te 2 I, Te 2 Br and Te 3 Cl 2 .

Hexahalides TeX 6 with tellurium in the + VI oxidation state are only known as tellurium hexafluoride TeF 6 or tellurium pentafluoride chloride TeF 5 Cl. Both are colorless gases. Tellurium hexafluoride is the most reactive chalcogen hexafluoride (next to sulfur hexafluoride SF 6 and selenium hexafluoride SeF 6 ) and is the only one that is hydrolyzed in water .

Furthermore, there are complex compounds of tellurium in the + IV oxidation state in aqueous solution [TeX 6 ] 2− (X = F - , Cl - , Br - , I - ) with all halide ions. With the exception of the hexafluoro complex, all the others are perfectly octahedral and can also be precipitated from the solution as salts (for example yellow ammonium hexachloridotellurate (IV) (NH 4 ) 2 [TeCl 6 ], red-brown ammonium hexabromidotellurate (IV) ( NH 4 ) 2 [TeBr 6 ] or black cesium hexaiodidotellurate (IV) Cs 2 [TeI 6 ]).

### Organotellurium compounds

Tellurium forms a number of organometallic compounds. However, these are very unstable and rarely used in organic synthesis . Compounds of the form R 2 Te, R 2 Te 2 , R 4 Te and R 6 Te (R each alkyl, aryl) are known as pure tellurium organyls .

In addition, there are also diorganotelluric halides R 2 TeX 2 (R = alkyl, aryl; X = F, Cl, Br, I) and triorganotelluric halides R 3 TeX (R = alkyl, aryl; X = F, Cl, Br, I ) known.

### Tellurium polycations

Polycation Te 8 2+ in Te 8 [U 2 Br 10 ]
Polycation Te 7 2+ in Te 7 [Be 2 Cl 6 ]

By careful oxidation of tellurium, in addition to the already mentioned Te 4 2+, numerous tellurium polycations Te n x + can be produced and crystallized with a suitable counterion. The counterion must be a weak Lewis base , since the tellurium polycations are relatively strong Lewis acids. Suitable oxidizing agents are often halides of the transition metals, which give the desired compound directly at temperatures of typically 200 ° C:

${\ displaystyle \ mathrm {8 \ Te \ + \ 2 \ UBr_ {5} \ longrightarrow \ Te_ {8} [U_ {2} Br_ {10}]}}$

Often the crystallization is successful under the conditions of chemical transport , but sometimes anhydrous solvents such as tin (IV) chloride or silicon tetrabromide have to be used. Salt melts are also suitable reaction media in individual cases. If the metal halide is not a suitable oxidizing agent, as is usually the case with halides of the main group elements, the corresponding tellurium tetrahalides can be used as oxidizing agents:

${\ displaystyle \ mathrm {13 \ Te \ + \ TeCl_ {4} \ + \ 4 \ BeCl_ {2} \ longrightarrow \ 2 \ Te_ {7} [Be_ {2} Cl_ {6}]}}$

By varying the counterion and the reaction medium, a wide variety of polycations could be produced; Mixed selenium-tellurium polycations can also be accessed by appropriate selection of the reactants in the synthesis. In addition to the chain or ribbon-shaped polycations shown, there are also isolated polycations such as Te 6 2+ , Te 6 4+ and Te 8 4+ .

## literature

General and connections
Discovery and History
• Montanhistorischer Verein für Österreich (Ed.): Special issue for the 250th birthday of Franz Joseph Müller von Reichenstein and the discovery of the element tellurium. In: res montanarum. Volume 5, 1992.
• E. Diemann, A. Müller, H. Barbu: The exciting story of the discovery of the tellurium (1782–1798). Importance and complexity of element discoveries. In: Chemistry in Our Time . Volume 36, No. 5, 2002, pp. 334-337.
• Tomas Kron, Eckehard Werner: Brief history of the element tellurium in biology and medicine. In: Würzburg medical history reports. 8, 1990, pp. 279-288.

Wiktionary: Tellurium  - explanations of meanings, word origins, synonyms, translations
Commons : Tellurium  - collection of images, videos and audio files

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