Titanium dioxide occurs naturally as the mineral Rutile.
The ionic radius of the titanium(IV) ion is 0.745 angstroms and that of the oxide ion is 1.26 angstroms. The ratio of radii for the cation and anion is thus r+/r- = 0.745/1.26 = 0.591.
With a radius ratio of 0.591, the cubic holes are too large (rhole/r = 0.732) to be suitable. The titanium ions will prefer to occupy octahedral holes in a closest-packed structure. As it happens, the oxide ions in rutile pack in a hexagonal closest-packed structure.
The images below depict the structure of Rutile. The red spheres represent the oxide ions and the blue spheres represent the titanium(IV) ions.
Examine the images and take note of the following points:
1. The oxide ions lie in a hexagonal closest-packed arrangement. The image at the very bottom of the page shows the atoms in four adjoining unit cells. The packing of the oxide ions may be easier to discern in this image.
2. The titanium(IV) ions are smaller than the oxide ions.
3. The insertion of titanium(IV) ions into the octahedral holes causes the structure to expand so that the oxide ions are not in contact with each other.
4. Half of the octahedral holes are occupied by titanium(IV) ions. The ionic solid is electrically neutral and the unit cell itself must also be electrically neutral. Because the oxide ions adopt a hcp structure, there are two oxide ions in the hcp unit cell. The unit cell cell for TiO2, however, is not the hcp unit cell (the presence of the titanium atoms affects the choice of unit cell). Carefully examine the images shown below and determine the number of titanium and oxygen atoms in the unit cell for rutile. Because rutile has a 1:2 ratio of titanium to oxygen atoms, the unit cell must also have this ratio.
5. Rutile has (6,3)-coordination.
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