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:
 * Te + O2 → TeO2

An alternative preparation is to dehydrate tellurous acid, H2TeO3, or to thermally decompose basic tellurium nitrate,Te2O4.HNO3 above 400°C.

Chemical properties
TeO2 is highly insoluble in water and completely insoluble in concentrated sulfuric acid. It is also incompatible with strong acids and strong oxidizing agents. It is an amphoteric substance and therefore can act both as an acid or as a base depending on the solution it is in.

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 is 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.

Uses
It is used as an acousto-optic material.

Tellurium dioxide is also a conditional glass former, which means it will form a glass with small molar% additions 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 information
TeO2 is a possible teratogen.

It produces a garlic-like odour on the breath.