Tellurium dioxide

Tellurium dioxide (TeO₂) is a solid oxide 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.

Preparation

Paratellurite, α-TeO₂, is produced by reacting tellurium with O₂:

Chemical properties

TeO₂ is barely soluble in water and soluble in strong acids 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 tellurates.

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-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 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. or with additions of a small molar fraction of a second compound such as an oxide or halide. TeO₂ glasses have high refractive indices and transmit into the mid-infrared part of the electromagnetic spectrum, therefore they are of technological interest for optical waveguides. Tellurite glasses have also been shown to exhibit Raman gain up to 30 times that of silica, 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
• Safety data for TeO₂