Question

The correct statement regarding defects in solids is

• 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-centre
• D. Schottky defects have no effect on the physical properties of solids

Answer 1

Answer: (B), (C)

Frenkel defect is a defect which 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.
Frenkel defect is exhibited in ionic compounds in which the radius ratio is low. The cations and anions differ much in their sizes and the ions have low co-ordination numbers. Examples are $ZnS,AgBr,AgI,AgCl.$
The Schottky defect is a defect which is produced when one cation and anion are missing from their respective positions leaving behind a pair of holes. The crystal as a whole remains neutral because the number of missing positive ions (cations) and negative ions (anions) is the same.
Consequences of Schottky defect:
1. The density of the crystal decreases.
2. The crystal begins to conduct electricity to a small extent by the ionic mechanism.
3. The presence of too many voids lowers lattice energy or the stability of the crystal.
4. F-center: A colour centre consisting of an electron trapped by a negative ion vacancy in an ionic crystal such as an alkali halide or an alkaline-earth fluoride or oxide.