Crystal structure:
Titanium dioxide has three crystalline forms in nature: rutile, anatase and plate titanium. Plate titanium type belongs to the orthorhombic system, which is an unstable crystal type. It is transformed into rutile type at above 650 ℃, so it has no practical value in industry. Anatase is stable at room temperature, but it should be transformed into rutile at high temperature. The conversion strength depends on the manufacturing method and whether inhibition or accelerator is added in the calcination process.
It is generally believed that there is almost no crystal transformation below 165 ℃, and the transformation is fast when it exceeds 730 ℃. Rutile is the most stable crystalline form of titanium dioxide, with dense structure. Compared with anatase, rutile has higher hardness, density, dielectric constant and refractive index. Rutile and anatase belong to tetragonal system, but have different lattice, so the X-ray images are also different. The diffraction angle of anatase titanium dioxide is 25.5 °, and that of rutile is 27.5 °. Rutile crystal is slender and prismatic, usually twin; However, anatase generally approximates regular octahedron.
Compared with anatase, rutile type is composed of two titanium dioxide molecules in its unit lattice, while anatase type is composed of four titanium dioxide molecules, so its unit lattice is small and compact, so it has greater stability and relative density, so it has higher refractive index, dielectric constant and lower thermal conductivity.
Among the three isomers of titanium dioxide, only rutile is the most stable, and only rutile can be obtained by thermal conversion. Natural brookite can be transformed into rutile at 650 ℃, and anatase can also be transformed into rutile at 915 ℃.
