(i) The above defect is stoichiometric because equal number of cations and anions are missing from lattice sites.
(ii) The term used for this type of defect is Schottky defect. These defects are vacancy defects shown by ionic solids in which the anion and cation are nearly of the same size.
Examples are NaCl are KCl show Schottky defect.
(iii) The presence of the Schottky defects in a crystals lattice
lowers its density appreciably.