Question

An example of Frenkel defect is:
A. NaBr
B. TiBr
C. AgBr
D. CuBr

Answer 1

Frenkel defect occurs when an atom occupies the place of the voids. It occurs in compounds where the ions are of very small size such that they can enter the voids. So it is governed by the size of the ions and so mostly occurs when cations occupy the voids as they are smaller in size.

Complete step by step answer:
- Cations are smaller in size than anions. This is because there are less electrons in a cation compared to neutral atoms and more electrons in anion compared to neutral atoms. Due to less electrons, the attraction on the electrons by the nucleus is much more compared to that in anion. So cations are smaller in size.
- 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.
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.
- Frenkel defect occurs when the atoms are just displaced from their position to some voids or interstitial sites of the crystal lattice. It occurs generally in the lattice where the size ratio of cation and anion is large. Cation is smaller and so it gets displaced in interstitial sites. Number of atoms remain the same before and after the defect. Thus, the density of the crystal is unaffected. One vacancy is created and one interstitial site is occupied.
- The type of void present between the atoms of the lattice depends on the ratio of the size of cation and anion and different voids can be shown as

Cation to anion ratio (R)	Type of void occupied
R < 0.155	Linear
0.155 < R < 0.225	Triangular
0.225 < R < 0.414	Tetrahedral
0.414 < R < 0.732	Octahedral
0.732 < R < 1	cubical

Examples of the crystal lattice where frenkel defect occurs are ZnS, AgCl, AgBr
Thus the correct option is C. AgBr.

Note: Frenkel defect is not to be confused by schottky defect. Both are stoichiometric point defects only but are completely different from 1 another. Schottky defect results in change of density of the lattice and can result from the displacement of both cation and anion.