Question

Write the differences between ‘Schottky’ defect and ‘Frenkel’ defect.

Answer 1

Hint: Imperfections occuring in the regular geometry of a crystalline solid atom is called crystal defect. They are formed due to many reasons like:
1.Impurities within the atom
2.Temperature changes during the formation of the crystalline structure.
3.Conditions under which the crystals are formed.

Complete answer:
-Based on the imperfections, the types of defects in a crystalline structure are
1. point defect
2. line defect
3. plane defect
-In point defect, the defect occurs only at certain positions of the lattice.
In line defect, the defect occurs when an atom is misplaced or missing or is replaced by another atom from its position along a line.
If atoms are displaced along a line in the plane, then the defect is plane defect.
-Point defect is further classified as stoichiometric and non-stoichiometric defect.
When the chemical formula of the crystal remains the same due to the defect, it is a stoichiometric defect. It arises when an atom is displaced from its position or when two or more atoms change their position within the lattice.
 When the chemical formula of the crystal altars, then the defect is a non-stoichiometric defect. It arises if certain different impurities are added in the crystal lattice.
-We must know that both Scottky and Frenkel defects are an example of stoichiometric defects.
Now we need to see the differences between the two defects.

Schottky defect	Frenkel defect
It occurs when the atoms are totally missing from the crystal lattice.	It occurs when the atoms are just displaced from their position to some voids or interstitial sites of the crystal lattice.
It generally occurs in crystal lattice where the size ratio of cation and anion is nearly 1	It occurs generally in the lattice where the size ratio of cation and anion is large
In this, both the cation and the anion leave the lattice, thus ensuring that the overall crystal is neutral.	In this, only the smaller ion is displaced.Cation is smaller and so it gets displaced in interstitial sites.
Two atoms are removed from the lattice for 1 single defect	Number of atoms remaain same before and after the defect.
Thus,there is decrease in the density of the crystal	Thus,the density of the crystal is unaffected
Two vacancies are created when 1 pair of ions are removed	One vacancy is created and one interstitial site is occupied
Eg NaCl, KCl, KBr	Eg ZnS, AgCl, AgBr

Thus, we studied the differences between Frenkel and Schottky defects.

Note:
To check if a defect is Scottky or Frenkel, always first check if the defect is stoichiometric or non-stoichiometric. If suppose in a lattice, 2$N{{a}^{+}}$cations are present in the interstitial site of the lattice and 1 $M{{g}^{2+}}$cation is present at one of the positions of sodium cation, then the defect is not Frenkel Defect. It is a non-stoichiometric defect. So, always check in order for the defects.