| ||In the face centred cubic close packed system, 74% of the space is again occupied by the positive ions. Again, the bottom layer is made of hexagons. The second layer fits in the gaps as before. However, the third layer is different. |
The ions in the third layer layer lie above gaps in both of the previous layers. We say that the layers are arranged as ABCABC. Compare this with ABAB where the third layer lines up with the first. The positive ions have a co-ordination number of 12 as before.
This arrangement is the same as a face centred cubic structure. The main diagonal of the cube runs at right angles to the layers. We need to add a single ion into a layer beneath our bottom layer to see the face centred cube. Notice it is made of one ion in this new layer, one ion on the top layer and three ions in each of the sandwiched layer (arranged in triangles facing opposite ways).
|Picture 1.9b Use the rollovers in the text to build up HCP packing. Note, the colours are to signify the different layers positions.|