Question

Which of the following statements regarding defects in solids is/are correct?

• A. Frenkel defect is usually favoured by a very small difference in the sizes of cation and anion.
• B. Frenkel defect is a dislocation defect.
• C. Trapping of an electron in the lattice leads to the formation of F-center.
• D. Schottky defects have no effect on the physical properties of solids.

Answer 1

Answer: (B), (C)

The correct options are
B Frenkel defect is a dislocation defect.
C Trapping of an electron in the lattice leads to the formation of F-center.

Frenkel defect is a defect is created when an ion leaves its appropriate site in the lattice and occupies an interstitial site. A hole or vacancy is thus produced in the lattice.
The electroneutrality of the crystal is maintained since the number of positive and negative ions is the same. Since positive ions are small in size, they usually leave their positions in the lattice and occupy interstitial positions.
Consequences of Frenkel defect:
1. The closeness of like charges tends to increase the dielectric constant of the crystal.
2. The crystal showing a Frenkel defect conducts electricity to a small extent by ionic mechanism.
3. The density of the crystal remains the same.
In metal excess defects caused by anion vacancies, one or more anions are missing from the lattice points which are occupied by electrons.
The overall electrical neutrality is maintained. When NaCl crystal is heated with Na vapours, this kind of defect is observed.
The electrons impart colour to the crystal (yellow colour to NaCl). These are known as F-centers (Farbe's centres).