select the correct statement (s)

Options

A. The ionic crstal of AgBr may have Schottky defect
B. The unit cell having crystal parameters, `a=b ne c,alpha=beta=90^(@), gamma =120^(@)` is hexagonal
C. In ionic compounds having Frenkel defect the ratio `r^(+)//r^(-)` is high
D. The co-ordination number of `Na^(+)` ion in NaCl is 6

Answer

To solve the question, we need to analyze each statement provided and determine whether it is correct or incorrect based on our knowledge of solid-state chemistry.

### Step-by-Step Solution:

1. **Analyzing Statement A**:
- **Statement**: "The ionic crystal of AgBr may have Schottky defect."
- **Explanation**: A Schottky defect occurs when equal numbers of cations and anions are missing from their lattice sites. For AgBr, the sizes of Ag⁺ and Br⁻ ions are comparable, which allows for the possibility of Schottky defects.
- **Conclusion**: This statement is **correct**.

2. **Analyzing Statement B**:
- **Statement**: "Unit cell having these crystal parameters is hexagonal."
- **Explanation**: The parameters given (A = B, C ≠ A, α = β = 90°, γ = 120°) are characteristic of a hexagonal unit cell.
- **Conclusion**: This statement is **correct**.

3. **Analyzing Statement C**:
- **Statement**: "In ionic compounds having Frenkel defect the ratio R⁺ to R⁻ is high."
- **Explanation**: A Frenkel defect occurs when a cation dislocates from its lattice site to an interstitial site. For this to happen, the size difference between cations and anions must be significant (R⁺ to R⁻ ratio should be low). Therefore, the statement that the ratio is high is incorrect.
- **Conclusion**: This statement is **incorrect**.

4. **Analyzing Statement D**:
- **Statement**: "Coordination number of sodium ion in NaCl is 6."
- **Explanation**: In NaCl, each Na⁺ ion is surrounded by six Cl⁻ ions in an octahedral arrangement, confirming that the coordination number of Na⁺ is indeed 6.
- **Conclusion**: This statement is **correct**.

### Final Conclusion:
The correct statements are A, B, and D.