FRENKEL DEFECT It is discovered by Russian scientist Frenkel 1926. This type of defect is observed in ab type crystal when some ions( usually cations) leave their normal lattice sites and occupy interstitial sites. Holes or vacancy are created at original lattice sites. This defect is also known as dislocation defect. Since cations are usually smaller in size then anions, so they are easily accommodated at interstitial sites.
Condition To Observe Frenkel Defect
(i) Ionic compound with low coordination number.
(ii) Ionic compound in which size of anions are bigger than size of cations.
e.g. AgCl, AgBr, AgI, ZnS etc.
Ag+ is smaller in size so easily accommodated at interstitial sites.
Important note AgBr shows both Schottky and Frenkel defect.
Consequences Of Frenkel Defect
(i) Electrical conductivity increases as holes are conductive in nature.
(ii) The stability of crystal decreases due to the large number of holes, which lowers that lattice energy
(iii) There is no change in the density of the compound.
(iv) There is increase in the dielectric constant as like charge ion(cations) come closer.
(v) Entropy of the compound increases
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