The NaCl Structure

The CsCl Structure

The ZnS (or Diamond, or Sphalerite) Structure

The CaF 2 or ZrO 2 Structure

Perovskite Structure

Spinel Structure

The spinel structure (sometimes called garnet structure) is named after the mineral spinel (MgAl 2 O 4 ); the general composition is AB 2 O 4 . It is essentially cubic , with the O - ions forming a fcc lattice. The cations (usually metals) occupy 1/8 of the tetrahedral sites and 1/2 of the octahedral sites and there are 32 O-ions in the unit cell.

This sounds complicated, but it is not as bad as it could be; look at the drawing. We "simply" have two types of cubic building units inside a big fcc O-ion lattice, filling all 8 octants.

The spinel structure is very flexible with respect to the cations it can incorporate; there are over 100 known compounds. In particular, the A and B cations can mix! In other words, the composition with respect to one unit cell can be

(A 8 ) (B 16 )O 32 , or

, or A 8 (B 8 A 8 )O 32 = A(AB)O 4 in regular chemical spelling, or

in regular chemical spelling, or (A 8/3 B 16/3 ) (A 16/3 B 32/3 )O 32 and so on, with the atoms in the brackets occupying the respective site at random.

A few examples (in regular chemical symbols) Magnetite; Fe 3+ ( Fe 2+ Fe 3+ )O 4

Spinel; Mg 2+ ( Al 2 3+ )O 4

Chromite; Fe 3+ (Cr 2 3+ )O 4

Jacobsite; Fe3+( Mn2+ Fe3+)O 4