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

Options

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.

Explanation

To determine which statements regarding defects in solids are correct, let's analyze each statement one by one.

### Step 1: Analyze the First Statement
**Statement:** Frenkel defect is usually favored by small difference in the sizes of cation and anion.

**Analysis:** This statement is incorrect. The Frenkel defect occurs when there is a significant difference in the sizes of cations and anions. Typically, a smaller cation can leave its lattice position and occupy an interstitial site, while the larger anion remains in place. For example, in silver bromide (AgBr), silver (Ag) is a smaller cation compared to bromide (Br), which is larger. Therefore, this statement is false.

### Step 2: Analyze the Second Statement
**Statement:** Frenkel defect is a dislocation defect.

**Analysis:** This statement is correct. The Frenkel defect is indeed classified as a dislocation defect. It involves the displacement of a smaller ion from its normal lattice position to an interstitial site, creating a vacancy and an interstitial defect simultaneously. This aligns with the definition of a dislocation defect.

### Step 3: Analyze the Third Statement
**Statement:** The trapping of the electron leads to formation of F-centers.

**Analysis:** This statement is correct. F-centers are formed when electrons are trapped in the vacant lattice sites created by the absence of anions or cations. The trapped electrons give rise to color centers in the crystal, which can affect the optical properties of the solid. Thus, this statement is true.

### Step 4: Analyze the Fourth Statement
**Statement:** The Schottky defect has no effect on the physical properties of solids.

**Analysis:** This statement is incorrect. The Schottky defect involves the simultaneous absence of equal numbers of cations and anions from the crystal lattice, which leads to a decrease in the overall density of the solid. This change in density can significantly affect the physical properties of the material, such as its electrical conductivity and mechanical strength. Therefore, this statement is false.

### Conclusion
Based on the analysis:
- **First Statement:** Incorrect
- **Second Statement:** Correct
- **Third Statement:** Correct
- **Fourth Statement:** Incorrect

The correct statements regarding defects in solids are the second and third statements.

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