This is a multiple-choice question with potentially more than one correct answer. Let's analyze each statement regarding defects in solids.

Step 1: Analyze Statement A

Statement: Frenkel defect is usually favoured by a very small difference in the sizes of cation and anion.

• Definition of Frenkel Defect: A Frenkel defect is a point defect in a crystal lattice where an atom or ion leaves its own lattice site, creating a vacancy, and occupies a nearby interstitial site.
• Condition for Frenkel Defect: This defect is more common in ionic solids where the size of the cation is significantly smaller than the size of the anion. The small cation can be accommodated in the interstitial voids of the lattice. Examples include AgCl, AgBr, AgI, and ZnS.
• Conclusion: The statement claims it's favored by a small difference in sizes, which is incorrect. A large difference in the sizes of the cation and anion is required. Therefore, statement A is incorrect.

Step 2: Analyze Statement B

Statement: Frenkel defect is a dislocation defect.

• Definition of Dislocation Defect: A dislocation is a crystallographic defect, or irregularity, within a crystal structure. The presence of dislocations strongly influences many of the properties of materials.
• Nature of Frenkel Defect: In a Frenkel defect, an ion is dislocated from its normal lattice position to an interstitial position. This movement and displacement fit the description of a dislocation defect.
• Conclusion: The Frenkel defect involves the dislocation of an ion, so it can be classified as a dislocation defect. Therefore, statement B is correct.

Step 3: Analyze Statement C

Statement: Trapping of an electron in the lattice leads to the formation of F-centre.

• Definition of F-centre: An F-centre (from the German Farbenzenter, meaning color center) is a type of crystallographic defect in which an anionic vacancy in a crystal is occupied by one or more unpaired electrons. These trapped electrons absorb light in the visible spectrum, causing the crystal to become colored.
• Formation of F-centre: This typically occurs in alkali halides. For example, when NaCl is heated in an atmosphere of sodium vapor, Cl⁻ ions diffuse to the surface, leaving behind anionic vacancies. The sodium atoms on the surface ionize to Na⁺, releasing electrons. These electrons diffuse into the crystal and get trapped in the anionic vacancies.
• Conclusion: The statement accurately describes the formation of an F-centre. It is an electron trapped in a specific lattice site (an anionic vacancy). Therefore, statement C is correct.

Step 4: Analyze Statement D

Statement: Schottky defects have no effect on the physical properties of solids.

• Definition of Schottky Defect: A Schottky defect is a point defect in ionic crystals where an equal number of cations and anions are missing from their lattice sites, creating a pair of vacancies. This maintains the overall electrical neutrality of the crystal.
• Effect on Physical Properties:
  1. Density: Since atoms/ions are missing from the crystal while the volume remains essentially unchanged, the mass of the crystal decreases. Consequently, the density of the solid decreases. This is a significant change in a physical property.
  2. Electrical Conductivity: The presence of vacancies allows for ionic conduction, as ions can hop into the vacant sites. This increases the electrical conductivity of the solid.
  3. Stability: The presence of defects decreases the lattice energy and thus the stability of the crystal.
• Conclusion: The statement that Schottky defects have no effect is false. They clearly affect physical properties like density and electrical conductivity. Therefore, statement D is incorrect.

Final Summary

• Statement A is incorrect.
• Statement B is correct.
• Statement C is correct.
• Statement D is incorrect.

The correct statements are B and C.