Write a note on interstitial sites or voids.

(N/A) $(i)$ Tetrahedral Voids:
In a $fcc$ or $ccp$ structure,the unit cell is divided into eight small cubes. Each small cube has four atoms at alternate corners. When these four atoms are joined,they form a regular tetrahedron,enclosing one tetrahedral void. Thus,there are eight tetrahedral voids in a $fcc$ unit cell.
$(ii)$ Octahedral Voids:
In a $fcc$ structure,the body centre is surrounded by six atoms located at the centres of the faces. Joining these atoms forms a regular octahedron,resulting in one octahedral void at the body centre.
Additionally,there is one octahedral void at the centre of each of the $12$ edges of the cube. Each edge-centre void is shared between four adjacent unit cells. Therefore,only $\left(\frac{1}{4}\right)$ of each edge-centred octahedral void belongs to a particular unit cell. The total number of octahedral voids in a $fcc$ unit cell is $1 + (12 \times \frac{1}{4}) = 4$.