Question
The number of nearest neighbours that each atom touches is called the coordination number for that structure. What is the coordination number for hcp and for fcc structures?
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Consider layers of close-packed atoms. A second layer of atoms can be placed on top of the first, filling one of two sets of the hollows between three atoms beneath.
Important
There are two possible positions for this second layer. Alternate rows of hollows may be filled. The structure is still the same, whichever half of these hollows is filled (not true for the third layer).
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The relative position of a third layer is very important. Call the first layer A, the second layer B, etc. If the third layer of atoms sits above the first we have a hexagonally close-packed structure (hcp) with layers ABABA...
Note
It is also referred to as close-packed hexagonal (cph).
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If the third layer fills the opposite set of interstices to the second, and the layers repeat at the fourth layer, a face-centred cubic structure (fcc) results, with layers ABCABCA...
Note
The face-centred cubic arrangement is sometimes called the cubic closed-packed (ccp) arrangement.
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