# A8.5 Crystal Lattices and Unit Cells

A crystal lattice is the three dimensional array of atoms, ions or molecules adopted by substances when they form crystals.
The same pattern is repeated over and over again throughout the structure – the simplest section of the lattice is called a unit cell.
It is important to realise that the atoms on the faces and corners of a unit cell form part of more than one cell – only the one in the centre of the BCC structure is unique to that unit cell.
Figure 8.3 Two different types if unit cells
Here are two different types of units cell adopted by metal atoms in crystal lattices
Body centre cubic: BCC
Face centre cubic: FCC (note: the atoms on the obscured faces are not shown)
 Position Each unit cell has Fraction of atom contributed to unit cell Inside the unit cell 1 1 On face of the unit cell 2 1/2 On an edge of the unit cell 4 1/4 On a corner of the unit cell 8 1/8
Detailed examination of BCC and FCC cubic structures: (Calculations like this can be done with different unit cells to work out the total number of atoms present in each cell)
 Position BCC FCC Inside the unit cell 1 0 On face of the unit cell 0 6 x ½ = 3 On an edge of the unit cell 0 0 On a corner of the unit cell 8 x 1/8 = 1 8 x 1/8 = 1 Total number of atoms in unit cell 2 4