Which of the following is not a function of an impurity present in a crystal?

Which type of crystal defect is observed in the diagram given below?
$Na^{+} Cl^{-} Na^{+} Cl^{-} Na^{+} Cl^{-}$
$Cl^{-} \square Cl^{-} Na^{+} \square Na^{+}$
$Na^{+} Cl^{-} \square Cl^{-} Na^{+} Cl^{-}$
$Cl^{-} Na^{+} Cl^{-} Na^{+} \square Na^{+}$

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

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

Which is not correct about the Schottky defects?

Point defects are present in