What kind of defects do most ionic crystals possess?

The correct statement regarding $F$-centre is

Due to vacancy defects,the density of a solid substance .............

The incorrect statement about the imperfections in solids is .... .

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

Missing of one monovalent cation and one monovalent anion from the crystal lattice is called