Meaning of defect is shortcomings or imperfections. Solid crystals also show some kinds of defects which we study in solid state chemistry. Some units of the crystals may have one or more atoms lesser than other ideal units of crystals. These imperfections of crystals are called defects in crystals. In other words, the interruptions in regular patterns in crystalline solids is called crystallographic defects. There are many types of crystallographic defects such as point defects, line defects, planar defects etc. Frenkel defects are point defects. We will discuss the Frenkel defect in this article in detail.
When an atom or smaller ion (generally a cation) leaves its place in the lattice, creating a vacancy and becomes an interstitial by lodging in a nearby location. Thus, a vacancy is created in the lattice. This type of defect is called Frenkel defect. It was discovered by soviet physicist Yakov Frenkel. It is a type of point defect which is also known as dislocation defect.
Following are examples of Frenkel Defect –
NaCl (Sodium Chloride) (Frenkel Defect is shown in NaCl in the image above)
Silver (I) Chloride
Silver (I) Bromide
Silver (I) Iodide
When the ionic compound size of anions is much larger than cations then Frenkel Defect occurs. As due to size difference in ions, ion occupies interstitial position in lattice. Ionic crystals having Frenkel defect also remain neutral in nature. As the number of cations and anions remain equal. Thus, it can be said that Frenkel defects are shown by those ionic solids which have large size differences between the cation and anion.
Frenkel defect doesn’t have any impact on density, mass or volume of the crystal as in this defect migration of ions takes place within the crystal. So, density, mass and volume of the ionic crystal remain the same even after having a Frenkel defect.
Number of Frenkel Defects can be calculated by following formula –
Where N = normally occupied positions
N’ = number of available positions
∆H = enthalpy released by 1 Frenkel defect
R= gas constant
T = temperature
Like the Frenkel defect, Schottky defect is also a point defect in crystalline solids. Even few crystalline ionic compounds such as silver bromide exhibit both Schottky and Frenkel defects as well.
In these defects ions leave from a crystal lattice in stoichiometric units.
In these defects ions leave its place in lattice but remain in the interstitial space of crystal lattice.
It reduces the density of the lattice.
It doesn’t affect the density of the atom or lattice.
Due to this effect mass of the lattice is reduced.
Due to this effect mass of the lattice or atom remain unaffected.
In these defects ions leave crystal lattice.
In these defects ions just leave their position in the lattice.
Schottky defect is occurred in those crystals in which sizes of ions is almost similar.
Frenkel defect is occurred in those ionic crystals in which sizes of ions (anions and cations) show large differences.
Compounds such as KCl, KBr, CsCl etc. show Schottky defect.
Compounds such as NaCl, ZnS, AgI etc. show Frenkel defect.
It is also known as valency defect.
It also known as dislocation defect.
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