Tin dioxide

Tin dioxide, SnO2, also tin(IV) oxide, stannic oxide, tin peroxide, stannic anhydride, and flowers of tin, is an oxide of tin, with tin in oxidation state +4. It has the appearance of a white powder with melting point of 1127 &deg;C and specific gravity 6.95 g/cm3. Its CAS number is and its EINECS number is 242-159-0. It is insoluble in water and soluble in concentrated sulfuric acid. It can be prepared by reaction of tin and concentrated nitric acid at high temperature.

The naturally occurring mineral is called cassiterite.

When hydrated, it may be called stannic acid.

Tin dioxide is used as a catalyst and a polishing powder for steels and other metals, glasses, and metallic dental restorations. 

When suitably doped (e.g. with CuO, or with cobalt+aluminum+niobium, or with cobalt+manganese, etc.), polycrystalline tin dioxide becomes semiconductive. The resulting material can be used in e.g. high voltage varistors. It is an n-type semiconductor with rutile structure.

Tin dioxide doped with iron or manganese is being investigated as a magnetic semiconductor.

Semiconductive tin dioxide is also used in sensors of combustible gases. The sensor area is heated to constant temperature (low 100s &deg;C); in presence of a combustible gas its electrical resistivity drops.