Voids

Whatever may be perfection of packing because of being spherical, atoms will leave some empty space between them. These 3-D interstitial gaps (empty sites) are known as voids.

Types of voids (on the basis of number of surrounding atoms)

1. Trigonal void (2D-void) surrounded by three spheres in same plane whose centers are at corner of triangle.

2. Tetrahedral void: Void whose surrounding spheres are located at the corners of a regular tetrahedron.

3. Octahedral void: Void resulted from overlaping of two trigonal void of adjacent layer and whose surrounding spheres are located at corners of octahedron.

4. Cubic voids: Void surrounded by eight spheres located at corners of a cube.

Location of voids in unit cell

(i) Octahedral voids (0-voids)

The octahedral voids are located at the middle of the cell edges and at the centre of cubic unit cell of FCC type.

Total no of octahedral voids are = 
(ii) Tetrahedral voids (T-voids)

The tetrahedral voids are located at the body diagonals (in a FCC unit cell), two in each body diagonal at one-fourth of the distance from each end.

Total no. of tetrahedral voids per unit cell = 8

Thus in CCP, the total number of voids per unit cell = 

Note: The number of T-voids in HCP unit cell are 12 and 0-voids are 6 in HCP unit cell.