the rutile structure
/ ROO-teel /
The rutile structure, named after a mineral form of titanium dioxide (TiO2), is another common home for an AX2 ceramic, but unlike fluorite it is not built on a close-packed cube. It is tetragonal, meaning its unit cell is a rectangular box that is square in cross-section but stretched along one axis, and it packs its ions less symmetrically.
Each cation sits at the centre of an octahedron of six anions (coordination 6), while each anion is shared between three cations lying in a flat triangle around it (coordination 3), a 6:3 structure with two formula units per cell. A vivid way to see it: the TiO6 octahedra share their edges to form straight chains running along the long axis, and neighbouring chains link by sharing corners, a framework of linked octahedra rather than a simple close-packed stack.
TiO2 rutile is everywhere: it is the brilliant white pigment in paint, paper and sunscreen (thanks to its very high refractive index) and a workhorse photocatalyst. Many other dioxides share the structure, including tin oxide (SnO2, used in gas sensors and transparent conductors), manganese dioxide (MnO2), and the fluoride MgF2. Honest caveat: rutile is only one of three natural forms of TiO2; the others, anatase and brookite, are also octahedral but link their octahedra differently, and anatase is often the more active photocatalyst.
The whiteness of most paint comes from micron-sized grains of rutile TiO2. Its 6:3 structure of edge-sharing titania octahedra gives it one of the highest refractive indices of any common oxide, so it scatters light superbly.
Cation in an octahedron of six anions, each anion shared by three: 6:3, tetragonal.
Rutile is not a close-packed AX2 structure like fluorite. Its oxygen array is only approximately hexagonal, and it is better understood as chains of edge- and corner-sharing octahedra.