Crystal properties of titanium dioxide

Titanium dioxide has three crystal forms in nature: rutile, anatase and brookite. The brookite belongs to the orthorhombic system and is an unstable crystal form. It will transform into rutile at above 650℃, so it has no practical value in industry. The anatase is stable at room temperature, but it will transform into rutile at high temperature. The intensity of its transformation depends on the manufacturing method and whether inhibitors or promoters are added during the calcination process. It is generally believed that the crystal form transformation hardly occurs below 165℃, and the transformation is very fast when it exceeds 730℃. The rutile form is the most stable crystal form of titanium dioxide, with a dense structure. Compared with the anatase, it has higher hardness, density, dielectric constant and refractive index. Both rutile and anatase belong to the tetragonal system, but have different lattices, so the X-ray images are also different. The diffraction angle of anatase titanium dioxide is 25.5°, and the diffraction angle of rutile is 27.5°. The crystals of rutile type are slender, prismatic, and usually twinned; while anatase type is generally close to a regular octahedron.

Compared with anatase type, rutile type has a smaller and more compact unit lattice because its unit lattice is composed of two titanium dioxide molecules while anatase type is composed of four titanium dioxide molecules, so it has greater stability and relative density, and therefore has a higher refractive index and dielectric constant and lower thermal conductivity.

Among the three isomers of titanium dioxide, only rutile type is the most stable, and only rutile type can be obtained through thermal conversion. Natural brookite is converted to rutile type at above 650℃, and anatase can also be converted to rutile type at around 915℃.