Even in the closed packed structures when the spheres or the particles touch each other, octahedral certain amount of space in between the particles is left unoccupied. For example, in hcp and ccp structures 26% of the space is left unoccupied. This is because the spheres touch each other only at certain points as governed by the geometry of packing. The space enclosed in between the packed spheres or the particles is commonly called as voids or the holes or interstices.
Formation of the voids in closed packed structure can be visualized as follows:
During the three dimensional packing of spheres in the B voids of the first layer and then the C voids should remain unoccupied due to geometric considerations. Now, in the second layer so formed, there will be two types of voids:
- 1} the voids over the C voids in the first layer. These voids are made of two voids in two different layers i.e. , layer-1 and layer-2. Let us label these voids as C voids.
- 2} the voids in the second layer over the locations of the spheres in the first layer. We call these voids as B’ voids.
While the B and C voids of the first layer are both triangular in shape, in the second layer, only the B’ voids are triangular The C’ voids of the second layer are the combination of two triangular voids of the first and second layers with their vertices in the opposite directions.
A single triangular void in a crystal is surrounded by four spheres and is called as a tetrahedral void or hole.
A double triangular void like C’ is surrounded by six spheres, and is called an octahedral void.
There are two types of this voids which first is Tetrahedral Voids or Holes and second is Octahedral Voids or Holes. In tetrahedral voids it may be noted that the shape of the void is triangular and not tetrahedral. However, the arrangement of spheres around the void is tetrahedral.