Which of the following compounds is likely to show both Frenkel and Schottky defects in its crystalline form?

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

Explain stoichiometric defects shown by non-ionic solids.

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

In spite of long-range order in the arrangement of particles,why are the crystals usually not perfect?

What is a point defect?