Close packed structures

Re-use of this resource is governed by a Creative Commons Attribution-
NonCommercial-ShareAlike 4.0 International
https://creativecommons.org/licenses/by-nc-sa/4.0/

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

Play

To continue press Next

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