Tellurium dioxide (TeO₂) is a solid

oxide An oxide () is a chemical compound containing at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion (anion bearing a net charge of −2) of oxygen, an O2− ion with oxygen in the oxidation st ...

of tellurium. It is encountered in two different forms, the yellow orthorhombic mineral tellurite, β-TeO₂, and the synthetic, colourless tetragonal (paratellurite), α-TeO₂. Most of the information regarding reaction chemistry has been obtained in studies involving paratellurite, α-TeO₂.W.R.McWhinnie (1995) ''Tellurium - Inorganic chemistry'' Encyclopedia of Inorganic Chemistry Ed. R. Bruce King (1994) John Wiley & Sons

Preparation

Paratellurite, α-TeO₂, is produced by reacting tellurium with O₂: :Te + O₂ → TeO₂ An alternative preparation is to dehydrate tellurous acid, H₂TeO₃, or to thermally decompose basic tellurium nitrate, Te₂O₄·HNO₃, above 400 °C.

Physical properties

The longitudinal speed of sound in Tellurium dioxide is at around room temperature.

Chemical properties

TeO₂ is barely soluble in

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

and soluble in

strong acids Acid strength is the tendency of an acid, symbolised by the chemical formula , to dissociate into a hydron (chemistry), proton, , and an anion, . The Dissociation (chemistry), dissociation or ionization of a strong acid in solution is effectivel ...

and alkali metal hydroxides. It is an amphoteric substance and therefore can act both as an acid or as a base depending on the solution it is in. It reacts with acids to make tellurium salts and bases to make tellurites. It can be oxidized to telluric acid or

tellurate In chemistry, tellurate is a compound containing an oxyanion of tellurium where tellurium has an oxidation number of +6. In the naming of inorganic compounds it is a suffix that indicates a polyatomic ion, polyatomic anion with a central telluri ...

s. The tellurite ion is kinetically inert, but TeO₂ equivalents will oxidize thioates in acid to the diacyl disulfide.

Structure

Paratellurite, α-TeO₂, converts at high pressure into the β-, tellurite form. Both the α-, (paratellurite) and β- (tellurite forms) contain four coordinate Te with the oxygen atoms at four of the corners of a trigonal bipyramid. In paratellurite all vertices are shared to give a

rutile Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest refractive indices at vis ...

-like structure, where the O-Te-O bond angle are 140°. α-TeO₂ In tellurite pairs of trigonal pyramidal, TeO₄ units, sharing an edge, share vertices to then form a layer. The shortest Te-Te distance in tellurite is 317 pm, compared to 374 pm in paratellurite. Similar Te₂O₆ units are found in the mineral denningite. melts at 732.6 °C, forming a red liquid. The structure of the liquid, as well as the glass which can be formed from it with sufficiently rapid cooling, are also based on approximately four coordinate Te. However, compared to the crystalline forms, the liquid and glass appear to incorporate short-range disorder (a variety of coordination geometries) which marks TeO₂ glass as distinct from the canonical single-oxide glass-formers such as SiO₂, which share the same short-range order with their parent liquids.

Uses

It is used as an acousto-optic material. Tellurium dioxide is also a reluctant glass former, it will form a glass under suitable cooling conditions, or with additions of a small

molar fraction In chemistry, the Mole (unit), mole fraction or molar fraction, also called mole proportion or molar proportion, is a quantity (science), quantity defined as the ratio between the amount of substance, amount of a constituent substance, ''ni'' (ex ...

of a second compound such as an oxide or halide. TeO₂ glasses have high

refractive indices In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

and transmit into the mid-

infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...

part of the

electromagnetic spectrum The electromagnetic spectrum is the full range of electromagnetic radiation, organized by frequency or wavelength. The spectrum is divided into separate bands, with different names for the electromagnetic waves within each band. From low to high ...

, therefore they are of technological interest for

optical waveguide An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. Common types of optical waveguides include optical fiber waveguides, transparent dielectric waveguides made of plastic and glass, liquid ligh ...

s. Tellurite glasses have also been shown to exhibit Raman gain up to 30 times that of

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...

, useful in optical fibre amplification.

Safety

TeO₂ is a possible teratogen. Exposure to tellurium compounds produces a garlic-like odour on the breath, caused by the formation of diethyl telluride.

References

External links

TeO₂ properties at Moltech Berlin
{{Oxides Oxides Tellurium(IV) compounds Nonlinear optical materials Interchalcogens