Consider the most efficient way of packing together equal-sized spheres and stacking close-packed atomic planes in three dimensions. For example, if plane A lies beneath plane B, there are two possible ways of placing an additional atom on top of layer B. If an additional layer was placed directly over plane A, this would give rise to the following series :
This type of crystal structure is known as hexagonal close packing (hcp).
If however, all three planes are staggered relative to each other and it is not until the fourth layer is positioned directly over plane A that the sequence is repeated, then the following sequence arises:
This type of crystal structure is known as cubic close packing (ccp).
The goal is to calculate the packing efficiency of hcp and ccp unit cell and compare to fcc lattice.