What type of crystal defect is indicated in the diagram below?
${\text{Na}}^{+}{\text{Cl}}^{-}{\text{Na}}^{+}{\text{Cl}}^{-}{\text{Na}}^{+}{\text{Cl}}^{-}$
${\text{Cl}}^{-}\square {\text{Cl}}^{-}{\text{Na}}^{+}{\text{Cl}}^{-}{\text{Na}}^{+}$
${\text{Na}}^{+}{\text{Cl}}^{-}{\text{Na}}^{+}\square {\text{Na}}^{+}{\text{Cl}}^{-}$
${\text{Cl}}^{-}{\text{Na}}^{+}{\text{Cl}}^{-}{\text{Na}}^{+}{\text{Cl}}^{-}{\text{Na}}^{+}$

The diagram shows missing pairs of ions (${\text{Na}}^{+}$ and ${\text{Cl}}^{-}$) from their normal lattice sites. This type of point defect, where equal numbers of cations and anions are missing from the crystal lattice to maintain electrical neutrality, is called a Schottky defect.