Tetrahedral sites in face-centred cubic lattices

Each sphere in a close-packed lattice and the three spheres into which it nestles in the close-packed layer below surround the second type of space in close-packed lattices.

Roll your mouse back and forth over the image to see that

• in the face-centred unit cell, the blue sphere at the corner sits in the hollow created by the three green spheres in the close-packed layer below.
• the line passing through the center of the triangle of green spheres also passes through the center of the blue sphere.

Similarly, the blue sphere at the back of the diagram fits into a hollow created by the three gold spheres in the layer above.

The space between the blue sphere at the left and the three green spheres is referred to as a tetrahedral site because it is surrounded by four spheres.

The three gold spheres and the blue sphere at the back also surround a tetrahedral site.