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

Due to $Schottky$ defect:

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

Analysis shows that nickel oxide has the formula $Ni_{0.98}O_{1.00}$. The fractions of $Ni^{2+}$ and $Ni^{3+}$ ions in the crystal are ......

If molten $NaCl$ contains $SrCl_2$ as impurity,crystallization can generate

Select the incorrect statement.