In the $fcc$ structure, the number of tetrahedral holes per sphere is

$A$ metal forms an $fcc$ structure. Calculate the volume of the $fcc$ unit cell in $\text{cm}^3$ if the void volume is $1.66 \times 10^{-23} \text{ cm}^3$.

The spinel structure consists of ............

Calculate the void volume of a $bcc$ unit cell if the volume of the unit cell is $8.0 \times 10^{-23} \text{ cm}^3$.

Write a note on interstitial sites or voids.

Hexagonal close packing $(HCP)$ is found in the crystal lattice of: