The cubic close-packed structure

Below we reproduce the FCC structure that was shown above.

You will notice that the B-layer atoms form a hexagon, but this is a cubic structure. How can this be? The answer is that the FCC stack is inclined with respect to the faces of the cube, and is in fact coincident with one of the three-fold axes that passes through opposite corners. It requires a bit of study to see the relationship, and we have provided two views to help you. The one on the left shows the cube in the normal isometric projection; the one on the right looks down upon the top of the cube at a slightly inclined angle.

Both the CCP and HCP structures fill 74 percent of the available space when the atoms have the same size. You should see that the two shaded planes cutting along diagonals within the interior of the cube contain atoms of different colors, meaning that they belong to different layers of the CCP stack. Each plane contains three atoms from the B layer and three from the C layer, thus reducing the symmetry to C₃, which a cubic lattice must have.

مواضيع ذات صلة

Crystal Defects

Properties of Solids

Molecular Solids

Covalent Network Solids

Metallic Solids

Ionic Solids

The Fundamental Types of Crystalline Solids

Interstitial Void Spaces

The FCC unit cell

The Hexagonal closed-packed structure

Two choices for the third layer lead to two different close-packed lattice types

Close-packed lattices in three dimensions