It is a type of vacancy defect. In ionic compounds, some of the ions (usually smaller in size) get dislocated from their original site and create defect. This defect is known as frenkel defect. Since this defect arises because of dislocation of ions, thus it is also known as dislocation defect. As there are a number of cations and anions (which remain equal even because of defect); the density of the substance does nnot increase or decrease. Ionic compounds; having large difference in the size between their cations and anions; show Frenkel Defect, such as ZnS, AgCl, AgBr, AgI, etc. these compounds have similar size of cations compared to anions.