Hexagonal close-packed structure unit cell volume

Clearly, there are two choices for placing each plane, and an infinite number of crystal structures can be generated that have the same atomic packing density. The two simplest such structures correspond to the periodic layer sequences abcabcab. . . and abababa.. . . The first of these is the fee structure already discussed, and the second is a close-packed structure in the hexagonal crystal system termed hexagonal close-packed (hep). In each of these simple structures, atoms occupy 74.0% of the unit cell volume, as the following example shows. (Atoms that crystallize in the bcc structure occupy only 68.0% of the crystal volume, and the packing fraction for a simple cubic array is only 52.4%.)... 

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