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Close packed structures

To build a close packed structure we first consider the
closest packed structures in two dimensions
Either upon sites equivalent to this point
There are two distinct ways we can place a second layer
upon this layer
Or upon sites equivalent to this point
We will now refer to the bottom (red) layer as A and the
second (orange) layer as B
For the third layer, again there are two distinct ways that it
can be placed. However, this time where we place it will
determine which sort of structure we get.
Third layer can be placed on sites equivalent to this point X

Or upon sites equivalent to this point Y
Notice that the X sites are positioned above atoms from layer A
while the Y sites are positioned above gaps in the structure.
This structure has the form ...ABA... That is, every A layer is followed by a B layer and every B layer is followed by an A layer
The unit cell has a hexagonal lattice type so this close packing type is called hexagonal close packing or hcp

This structure is usually represented by the following unit cell.The unit cell has a hexagonal lattice type so this close packing type is called hexagonal close packing or hcp

If the atoms of the third layer are, instead, placed over the Y sites then the atoms will not coincide with any other layer. The third
layer is then labelled C.
The form of this close packed structure is ...ABC... It is a cubic close packed or ccp structure. The unit call used is a face centered
cubic (fcc) unit cell.
For an explanation as to why the fcc unit cell is used press Play

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The form of this close-packed structure is ...ABC.... It is a cubic-close-packed or ccp structure. The unit cell used is a face centred
cubic (fcc) unit cell.
These spheres are arranged in ABC layers
Here the close packed layers are coloured and labelled as before
Here the close packed layers are coloured and labelled as before
Notice that we can apply a cubic unit cell to the structure
Notice that we can apply a cubic unit cell to the structure
The unit cell is face centred cubic