Face Centered Cubic
In the Face Centered Cubic (FCC) unit
cell there is one host atom at each corner and one host atom in each face.
Since each corner atom contributes one eighth of its volume to the cell
interior, and each face atom contributes one half of its volume to the
cell interior (and there are six faces), then Z = 1/8.8
+ 1/2.6 = 4.
(crossed stereo pair)
The corner and face atoms touch along the
face
diagonal, and it is easy to show that the cube edge (a) is about 2.8r.
Thus, the corner atoms do not touch one another.
The packing efficiency is about 74%.
This is the maximum packing efficiency for spheres of equal radius
and is call closest packing. Thus a face centered lattice
of atoms is also called Cubic Closest Packing (CCP).
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