There are three forms of Tio2 in the natural environment, including rutile, anatase and brookite with different properties, among which rutile Tio2 is the most stable one of tio2 and it will never convert and decompose even though at the high temperature.
The crystal form of rutile titanium dioxide belongs to tetragonal system. The center of character is a titanium atom, surrounded with six oxygen atoms which lie in the edge of octahedron. Two tio2 molecular compose a structure cell and its constant of character is a=0.4584nm,c=0.2953nm.
The crystal form of anatase tio2 is also tetragonal system. The structure cell is made of four tio2 and its constant of character is a=0.3776nm,c=0.9486nm. The anatase type titanium dioxide is stable only at the low temperature while the temperature reaches 60℃ it begins to convert into rutile form gradually. When it is 730℃, this conversion can be at a rather high speed. When it is 915℃, the anatase type titanium dioxide can convert into rutile form completely.
The crytal form of tio2 as brookite belongs to rhomnic system. Six tio2 molecular can make a structure cell whose constant of character is a＝0．545nm，b＝0．918nmc＝0．515nm. Tio2 as brookite is unstable compound which will convert into rutile type when the temperature is over 650℃.
Rutile and anatase titanium dioxide:
they belong to tetragonal system with different crystal lattices. The rutile tio2 is long and thin, presenting prismatic, and it is twin crystal generally. The anatase tio2 is similar to regular octahedron commonly.
The rutile type titanium dioxide is composed of two oxygen tio2 molecular and the anatase is four tio2 molecular. Thus, the rutile possesses rather high stability and relative density, high refractive index and dielectric constant, and low thermal conductivity because of its small and close unit lattice.