The appearance of colour in solid alkali metal halides is generally due to

What is stoichiometric defect? State its types.

What causes the formation of a $Schottky$ defect?

In $Schottky$ defect,.....

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

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