Ion pairs are missing in which of the following stochiometric defects?

Schottky defect

Schottky Defect: It is basically a vacancy defect in ionic solids. In order to maintain electrical neutrality, the number of missing cations and anions are equal. Schottky defect also decreases the density of the substance. Number of such defects in ionic solids is quite significant.

For example, in NaCl there are approximately 10⁶ Schottky pairs per cm³ at room temperature. In 1 cm³ there are about 10²² ions. Thus, there is one Schottky defect per 10¹⁶ ions. Schottky defect is shown by ionic substances in which the cation and anion are of almost similar sizes. For example, NaCl, KCl, CsCl and AgBr. It may be noted that AgBr shows both, Frenkel as well as Schottky defects.