The hexagonal close packed structure is based on repeating planes ABABABAB, each of which are closed packed. that is they
sit as close together as possible-like balls in a snooker triangle.
Having arranged one plane of balls, or ions in a crystal structure, there is only one obvious place for the second plane to go for close packing to be seen. The second plane sits in the 'gaps' made by the lower layer.clearly the balls could be stacked directly above the last layer, but in an ionic solid the attractive forces are such that the ion are pulled as close together so 'fall' into the gaps. When the third plane is then added, there is a choice regarding where to put the plane. It can go above the first plane, directly above the second(which as mentioned will be unfavourable), or above neither of these. the diagram shows these positions as A B and C respectively
In the case of hexagonal close packing, the third layer occupies the a position. In contrast the C position is occupied in the cubic close packed structure.
The unit cell for the hexagonal close packed structure, is not cubic, which makes it somewhat more difficult to appreciate where the planes are. the unit cell for the structure is shown below,
and i have attempted to represent the oblique view.
Please note that the unit cell should be a rhombic shape, with an atom at each corner. The the 9th atom should be 1/4 of the way down depth of the cell, and
is contained completely within the cell
Coordination number- this is the number of nearest neighbours. In Hexagonal Close packing the coordination is 12-which is quite easy to appreciate if you think of the packing, there are three atoms forming a triangle on plane A then an atom in the gap, then 3 above in the same manner-totaling 6. Then surrounding the atom are 6 atoms forming a hexagon. The total nearest neighbours is therefore 6.
Metals which adopt this close packed structure are the group 4 transition metals- Ti, Zr and Hf. Also Tc, Ru, Re and Os from the series have the hexagonal close packing. Calculating the fractional volume occupied for this structure is somewhat more difficult than it is for those with a cubic unit cell. I have covered the method of finding occupied volumes for each of the cubic unit cells, but here i shall just say that the value for HCP is exactly the same as for the other close packed arrangement-CCP. 74.01% of the unit cell is occupied by atoms. in terms of interactions between ions and atoms there is no really obvious preference between hcp and ccp. However the HCP and CCP array are used in defining other more complicated structures such the perovskite, wurtzite, spinel and fluorite structure-in which interstitial sites are occupied by different anions and cations.
