Which of the following defects is observed in steel?

The pyknometric density of sodium chloride crystal is $2.165 \times 10^3 \ kg \ m^{-3}$ while its $X$-ray density is $2.178 \times 10^3 \ kg \ m^{-3}$. The fraction of unoccupied sites in sodium chloride crystal is

Schottky defect defines imperfection in the lattice structure of a

When $SrCl_2$ is added to molten $NaCl$ and crystallized,the defect formed is ........

Explain the following term with a suitable example:
$(iii)$ Interstitials

Schottky defect is generally observed in which type of compounds?