What is Frenkel defect

A pair of one cation and one anion can be missing from an ionic crystal as shown in the following figure. Such a pair of vacant sites is called Schottky defects. Also, in ionic crystals, an ion displaced from a regular site to an interstitial site is called Frenkel defect. As cations are generally the smaller in size, it is possible for them to get displaced into void space. A Frenkel defect does not change the overall electrical neutrality of the crystal. These types of defect are dominant in alkali halides.