Frenkel Defect

We know that every object will have a definite shape and that they gain this shape from the close arrangements of atoms and molecules. When we take crystal lattices into consideration, they arrange their ions in a strict layout, but sometimes one ion may go missing or shift its spot. There are many reasons for this phenomenon, and one of them is defects in the crystal lattice. Imperfections or defects in crystalline compound may be widely categorised into 4 groups

• point defect 
• line defect
• surface defect 
• volume defect

The Frenkel Defect is a Stoichiometric Defect which comes under the Point defect.  Zero dimensional Defects are known as point  defects because here  a point is considered dimensionless and unit less quantity. Here point Defect happens when one or more than one atom of an ionic compound leaves their original lattice position or another atom binds in that space of the crystal. Point defects are further divided into Stoichiometric Defect and Non Stoichiometric Defect. In Stoichiometric Defect the Stoichiometry of the solid is not distributed, which means the electric neutrality(ratio between negative ions  and positive ions) is not altered.Stoichiometric Defect is further divided into Schottky Defect and Frenkel Defect.  In this article we will deeply discuss the Frenkel Defect.

Classification of Point Defect 

Point defect 

• Stoichiometric Defect 

       * Schottky Defect

       * Frenkel Defect 

•  Non-Stoichiometric Defect

       *Metal excess Defect

       *Metal Deficiency Defect

Definition –  Frenkel Defect

Soviet physicist Yakov Frenkel discovered the Frenkel Defect. As said before, a Frenkel Defect is a type of Point Defect. When a smaller ion (generally cation) or an atom moves away from its original lattice position in the solid crystal a vacancy is created when they occupy another interstitial position. 

This is called Frenkel Defect.Some examples of compounds which show Frenkel Defect are Silver Bromide, Zinc Sulphide, Silver Chloride, Silver Iodide. Here in these compounds the size of anions when compared with the size of cations will be larger. Frenkel Defect is also called a dislocation Defect. 

In a Frenkel Defect the size of anions will be large when compared with the size of cations(Cations will be small).Ions occupy interstitial positions in the lattice because of this size difference. Neutral nature is shown by Ionic crystals having Frenkel defects , because here the number of cations and anions remain equal in the compound. The ionic solids with large size differences between anion and cation show the Frenkel Defect.

Characteristic

• In a Frenkel Defect the size of anions will be large when compared with the size of cations(Cations will be small). 
• Frenkel defect doesn’t change the density of the compound
• Mass or volume of a crystal is not affected by the Frenkel Defect because migration of ions is happening within the crystal.
• Ions always maintain electrical neutrality in the compound.
• Increase in the dielectric constant is seen when like charge  ions come closer.
• Due to the presence of vacant lattice sites The compounds which display Frenkel Defect show conductivity and diffusion.
• The increase in Entropy of the solid is seen.
• The decrease in the stability and lattice energy of the solid happens due to the Frenkel defect.

Schottky Defect – Frenkel defect

The Frenkel defect and Schottky Defect are Stoichiometric Defects which come under Point Defect. Schottky Defect is a type of point defect shown by ionic solids where there is a vacancy or a missing number of anions and cations from their lattice positions in stoichiometric units. Schottky Defect affects the density of the compound by decreasing the density of the substance. The Schottky defect is mostly seen in highly ionic compounds like CaCl, NaCl, etc., which will have anions and cations of the same size. When heat is applied to highly ionic compound crystals, Schottky defects occur. This is because the applied heat will increase the temperature that leads to thermal vibration in the crystal. Now this thermal vibration will create gaps in the pattern of crystals.Based on the availability of ions in the compounds, voids are created.

Comparison of the Frenkel defect and Schottky Defect.

CONCLUSION: 

In this article we discussed Frenkel Defect, characteristics of Frenkel defects and Comparison of Frenkel defect and Schottky Defect.Frenkel Defect is Stoichiometric Defect which comes under the Point defect.When a smaller ion (generally cation) or an atom moves away from its original lattice position in the solid crystal a vacancy is created when they occupy another interstitial position. This is called Frenkel Defect.Some examples of compounds which show Frenkel Defect are Silver Bromide,Zinc Sulphide,Silver Chloride,Silver Iodide.Here in these compounds the size of anions when compared with the size of cations will be larger. Density is unchanged in the Frenkel Defect.