Titanium dioxide CAS#1317-70-0

The lattice structure of anatase is also tetragonal, but the lower packing fraction of the crystal lattice explains why anatase crystal exhibits both a lower hardness and refractive indices than rutile.Nevertheless, because the crystal lattice energies of the two phases are quite similar, anatase remains metastable over long periods of time despite being less thermodynamically stable. However, above 700°C, the irreversible and rapid monotropic conversion of anatase to rutile occurs. From an optical point of view, anatase exhibits a greater transparency in the near-UV than rutile.