Germanium dioxide

Germanium dioxide, also called germanium oxide and germania, is an inorganic compound, an oxide of germanium. Its chemical formula is GeO2. Its other names are germanic acid, G-15, and ACC10380. It forms as a passivation layer on pure germanium in contact with atmospheric oxygen.

Structure
Germanium dioxide is a structural analog of silicon dioxide. Hexagonal GeO2 has structure as β-quartz with coordination number of germanium 4, tetragonal GeO2 has rutile structure of stishovite with coordination number of germanium 6 and amorphous GeO2 is similar to fused silica. It can be prepared both crystalline and amorphous. At very high pressures, germanium dioxide forms unusual octahedral structure.

Reactions
Heating of germanium dioxide with powdered germanium at 1000 °C forms germanium monoxide (GeO). It is not flammable. In contact with hydrochloric acid it releases volatile and corrosive germanium tetrachloride. It is moderately soluble in water, with which it reacts and forms germanic acid.

Uses
Germanium dioxide's refractive index (1.7) and optical dispersion properties make it useful as an optical material for wide-angle lenses and in optical microscope objective lenses. It is transparent in infrared.

A mixture of silicon dioxide and germanium dioxide ("silica-germania") is used as an optical material for optical fibers and optical waveguides. Controlling the ratio of the elements allows precise control of refractive index. Silica-germania glasses have lower viscosity and higher refractive index than pure silica. Germania replaced titania as the silica dopant for silica fiber, eliminating the need for subsequent heat treatment, which made the fibers brittle.

Germanium dioxide is also used as a catalyst in production of polyethylene terephthalate resin, and for production of other germanium compounds. It is used as a feedstock for production of some phosphors and semiconductor materials. In the manufacture of integrated circuits and transistors, germanium dioxide is a rather poor dielectric and is chemically unstable, which is one of the disadvantages of germanium in comparison with silicon.

Toxicity and medical
Germanium dioxide has low toxicity; in higher doses it is nephrotoxic. Germanium dioxide is used as a germanium supplement in some questionable dietary supplements and "miracle cures". High doses of these resulted in several cases of germanium poisonings.