Hexagonal Closed Packing
There are two types of interstitial sites in an fcc lattice.
Hexagonal Close Packed (hcp)
The hexagonal close packed structure can be made by piling layers in the A – B – A – B – A – B . . . . . sequence. An alternative sequence would be A – C – A – C – A …
Comparison of fcc/ccp and hcp 
Since both structures are composed of stacked hexagonal layers, we expact them to be similar – and they are. There are some subtle differences however.
The coordination geometry about each atom is shown below. Note that while both structures have CN = 12 the arrangements are slightly different. In hcp, the top and bottom three are directly above one another. In ccp, they are staggered.
hcp ccp
Another difference arises from the packing order. In the hcp, there are never any atoms over the “right-pointing” holes. Thus, if we look directly down on the structure we can see tiny channels throught the hcp structure. These are absent in ccp.
