Which defect leads to an increase in the density of a solid?

While writing the formula of ferrous oxide,it is written as $(FeO)$,because it is

Analysis of the density of an electrically neutral $NaCl$ crystal containing vacancies yielded the following results:
$(i)$ Density of $NaCl$ crystal with all sites occupied $= 2.178 \times 10^3 \ kg \ m^{-3}$
$(ii)$ Density of $NaCl$ crystal with vacancies $= 2.165 \times 10^3 \ kg \ m^{-3}$
Then the percentage of vacancies in the $NaCl$ crystal is .....

Explain the following term with a suitable example:
$(i)$ Schottky defect

Schottky defect in a crystal arises due to

Write a short note on Frenkel defect and Schottky defect.