MATSE 81
Materials in Today's World

Face Centered Cubic Structure (FCC)

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If, instead of starting with a square, we start with a triangle and continue to add atoms, packing as tightly as we can, we will end up with a layer of atoms as shown in the figure below.

circles in a planar design
First layer of hexagonal structure
Credit: Callister & Rethwisch 5e

Now let me put an atom on top of that first layer over one of the 'B' positions and let it rest down into one of the valleys. I can now place two more atoms in nearby 'B' positions so that each will rest in their own valley in such a way that all three atoms will touch and form a triangle. Now let me add more atoms to the second layer, packing them in as tightly as possible. These two layers are shown in the figure below. If you look closely, you should be able to see that the second layer only covers half of the valleys produced by the first layer. The 'C' valleys are left uncovered. In fact, half of the valleys of the second layer lineup with the unoccupied 'C' valleys of the first layer.

two layers of circles
First and second layer of hexagonal structure
Credit: Callister & Rethwisch 5e

Now let’s put a third layer where the atoms are placed where the unoccupied valleys of the first two layers lineup, the 'C' valleys. It is a little difficult to visualize, but if one of the top layer atoms is one corner of our cube and that corner is pointing out then we obtain the cube shown in the figure below.

3 layers of circles so the structure has a hexagonal nature
Complete three layer hexagonal structure
Credit: Callister & Rethwisch 5e

This crystal structure is known as face-centered cubic and has atoms at each corner of the cube and six atoms at each face of the cube. It is shown in the figure below. This structure, as well as the next structure we are going to discuss, has the atoms packed as tightly as theoretically possible. Metals that possess face-centered cubic structure include copper, aluminum, silver, and gold.

cube made up of hexagonal nature created from three layered atoms
Face centered cubic (fcc) structure
Credit: Callister & Rethwisch 5e

In the next section, we will discuss our fourth and last crystal structure.