CLOSE PACKED STRUCTURES

 
Face Centered Cubic (fcc)
or
Cubic Close Packed (ccp)
 
These are two different names for the same lattice.
 
We can think of this cell as being made by inserting another atom into each face of the simple cubic lattice - hence the "face centered cubic" name.

The reason for the various colors is to help point out how the cells stack in the solid.  Remember that the atoms are all the same.

The unit cell is:
Click on the unit cell above to view it rotating.
 
 

The unit cell is again shown expanded for visibility.   Actually, the corner atoms touch the one in the center of the face. The name "close packed" refers to the packing efficiency of 74.05%. No other packing can exceed this efficiency (although there are others with the same packing efficiency).

If we stack the cells into a lattice we notice that the atoms form diagonal layers - the reason for the colors is to make these stand out.  Note that diagonal layers also form along our line of sight. Since these cut across the other layers, each layer will contain all three colors.

 

 
 

A different and better way of looking at this structure focuses on those layers.

We start with a hexagonal array of spheres (the blue "A" layer). Notice that this is a close-packed arrangement - there is no way to pack more spheres into a given area.

 

 
 

We then place a second close-packed layer (the gold "B" layer)  atop the the first, so they nestle into the left-pointing holes in the first.

All spheres are actually the same atom, the colors are to help you keep track of the layers.
 

Notice that there are 2 separate choices for the second layer; in the animation below, we have arbitrarily chosen to cover the left-pointing holes.  We can put them over either all the "left-pointing" interstices or all the "right-pointing" interstices.

If we put them over the right-pointing interstices we generate a different layer, labeled the green or "C" layer.
   
Remember there are twice as many interstitial sites as spheres.  (One left-pointing and one right-pointing).
 
We can continue to stack these layers in any order, providing that no 2 identical layers are adjacent.
 

 
The cubic close packed structure can be constructed from the
A - B - C - A - B - C . . . . .  sequence.
 
An alternate sequence might be B - A - C - B - A - C ...
 
 
 
The resulting  structure is a 3-D analog of the hexagonal packing in a plane - it is the most efficient way to pack spheres.
 
Click on the  images below to view the structure rotating.
 
 
horizontal           vertical
 
 

However, the internal structures and relationships can be examined by moving the spheres apart slightly.
 

Click on the images below to view the open lattice rotating.
 
horizontal         vertical
 
 
There are two types of interstitial sites in an fcc lattice.   Let us consider a pair of layers - blue and gold.

Remember, the gold atoms cover all the left-pointing interstices in the blue layer, and none of the right-pointing ones.

 

Under each gold atom is a small space surrounded by 4 atoms in a tetrahedral arrangement. This is a 4 - coordinate tetrahedral interstitial site.
 

 
 
If we look at the right-pointing holes in the blue layer, we see that the gold layer does not nestle into them as it does the left-pointing ones. These cavities are surrounded by 6 atoms in octahedral geometry. This is a 6 - coordinate octahedral interstitial site. 
 
   

Go back and look at the lattice again and locate the tetrahedral and octahedral interstitial sites.
Are there any cubic sites?
 

Examples of fcc / ccp metals include nickel, silver, gold, copper, and aluminum.
 



 

Another way of stacking these layers is to omit the "C" layers altogether - simply alternate "A" and "B". This is also a close-packed array, but the symmetry is different.    It is called:
 

Hexagonal Close Packed (hcp)
 
The hexagonal close packed structure can be made by piling layers in the
A - B - A - B - A - B . . . . .  sequence.
 
An alternative sequence would be A - C - A - C - A ...
 
 
 
The resulting HCP structure is shown below.
 
Click on the images below to view the lattice structure rotating.
 
 horizontal        vertical
  
 
The movie below is the same structure slightly expanded to improve visibility.
 
 horizontal         vertical
  
 
The unit cell for hcp is shown below.   Remember that the different colors are used only to make the spatial relationships clearer.   In the solid, the atoms are all identical.
Click on the unit cell above to view it rotating.
 
 
These unit cells stack together like this:
 
 
 
Examples of hcp metals include zinc, titanium, and cobalt.
 

Comparison of fcc/ccp and hcp
Since both structures are composed of stacked hexagonal layers, we expact them to be similar - and they are. There are some subtle differences however.

The coordination geometry about each atom is shown below. Note that while both structures have CN = 12 the arrangements are slightly different. In hcp, the top and bottom three are directly above one another. In ccp, they are staggered.

hcpccp
 
 
Another difference arises from the packing order. In the hcp, there are never any atoms over the "right-pointing" holes. Thus, if we look directly down on the structure we can see tiny channels throught the hcp structure. These are absent in ccp. 
Click on each image to find the channels.
hcp ccp
 
 

Below you see an actual STM image of a Ni surface. This is NOT a simulation, but a genuine picture. Note the hexagonal arrangement of atoms.

 
 
 Nickel
 This image is the property of IBM Corporation.
 
Click here to go to the next page.
 
 Structure of Crystals
Crystal Lattices
Unit Cells
From Unit Cell to Lattice
From Lattice to Unit Cell
Stoichiometry
Packing & Geometry
Simple Cubic Metals
Close Packed Structures
Body Centered Cubic
Cesium Chloride
Sodium Chloride
Rhenium Oxide
Niobium Oxide
 
 
Except as otherwise noted, all images, movies and VRMLs are owned and copyright  by
Barbara L. Sauls and Frederick C. Sauls  1998.
Contact the owners  for individual permission to use.    blsauls@rs01.kings.edu