Which is the correct statement in the following ?

Options

A. The coordination number of `Na^+` ion in NaCl is 4
B. The unit cell having crystal parameters, `a=bnec,alpha=beta=90^@,gamma=120^@` is hexagonal
C. The ionic crystal of AgBr has only Schottky defect
D. In ionic compounds having Frenkel defect the ratio `(gamma_(+))/(gamma_(-))` is high

Answer

To solve the question regarding the correct statement about the properties of ionic compounds and crystal structures, we will analyze each statement step by step.

### Step-by-Step Solution:

1. **Evaluate Statement A**:
- The statement claims that the coordination number of Na⁺ in NaCl is 4.
- In the NaCl crystal structure, each Na⁺ ion is surrounded by 6 Cl⁻ ions. Therefore, the coordination number of Na⁺ is actually 6, not 4.
- **Conclusion**: Statement A is incorrect.

2. **Evaluate Statement B**:
- The statement describes a unit cell with parameters a = b ≠ c, and angles α = β = 90° and γ = 120° as hexagonal.
- This is consistent with the characteristics of a hexagonal crystal system. In a hexagonal system, the lengths of two axes (a and b) are equal, while the third axis (c) is different, and the angles between the axes are as described.
- **Conclusion**: Statement B is correct.

3. **Evaluate Statement C**:
- The statement asserts that the ionic crystal of AgBr only exhibits Schottky defects.
- However, AgBr can exhibit both Schottky and Frenkel defects. Schottky defects occur when equal numbers of cations and anions are missing from the lattice, while Frenkel defects involve a smaller cation leaving its lattice position to occupy an interstitial site.
- Additionally, the statement about the ratio of r⁺ to r⁻ being high in ionic compounds with Frenkel defects is incorrect. In such cases, the cation is usually smaller than the anion, resulting in a ratio less than 1.
- **Conclusion**: Statement C is incorrect.

4. **Final Conclusion**:
- The only correct statement among the options is Statement B, which describes the hexagonal unit cell parameters accurately.

### Summary of Correct Statement:
- The correct statement is: The unit cell having crystal parameters a = b ≠ c, α = β = 90°, and γ = 120° is hexagonal.