Locating Octahedral Voids
- Take a unit cell of ccp or fcc.
- The body center of the cube, C is not occupied but surrounded by six atoms on face centers.
- Joining the face centers creates a octahedron.
- Thus, this unit cell has one octahedral void at the body centre of the cube.
- There is one octahedral void at the centre of each
of the 12 edges.
- It is surrounded by six atoms, three belonging to the same unit cell and three belonging to
two adjacent unit cells.
- Each edge of the cube and octahedral void are shared between four adjacent unit cells.
- Only ¼ of each void belongs to a particular unit cell
In cubic close packed structure octahedral void at the body-center of the cube = 1
12 octahedral voids positioned at each edge and shared between four unit cells =12 x 1/4 =3
Total number of octahedral voids = 4
In ccp structure, each unit cell has 4 atoms. Thus, the number
of octahedral voids is equal to this number.