Chapters / Chapter 1 · Solid state / Topic 10 of 14

Fluorite, CaF₂, 8:4

Ca²⁺ ions are arranged at the corners and at the centres of the faces of a cubic unit cell, and F⁻ ions are at the centres of the eight small cubes into which the cell may be divided. Each Ca²⁺ is coordinated by 8 F⁻ neighbours at the corners of a cube, while each F⁻ has 4 Ca²⁺ neighbours.

This is the only structure in the notes with 8:4 coordination. The reverse structure, antifluorite, is taken by oxides such as Li₂O and Na₂O, with the cation and anion positions exchanged.

Rutile, TiO₂, 6:3

The cation is octahedrally surrounded by six anions, while the anion (O²⁻) is trigonally surrounded by three cations. Rutile has a low-symmetry anion sublattice that fits neither a simple cubic nor an fcc lattice. The cation sublattice is a distorted body-centred cube, because one side of the cube differs from the others, the cation sublattice is actually body-centred tetragonal. The unit cell is tetragonal.