In fluorite structure of `AB_2`

Options

A. On subjecting to strong heating it changes to antifluorite structure
B. Interchanging the position of positive and negative ion changes to antifluorite structure
C. Coordination number of A=8 and B=4
D. Coordination number of A=4 and B=8

Answer

To solve the question regarding the fluorite structure of `AB2`, we will analyze the structure, the coordination numbers, and the conditions under which it transforms into an anti-fluoride structure.

### Step-by-Step Solution:

1. **Understanding the Fluorite Structure**:
- The fluorite structure is represented by the formula `AB2`, where `A` is a cation and `B` is an anion. A common example is calcium fluoride (CaF2).
- In this structure, the `A` ions (Ca²⁺) occupy the face-centered cubic (FCC) lattice points, and the `B` ions (F⁻) occupy the tetrahedral voids.

**Hint**: Recall that in a fluorite structure, the cation occupies the lattice points while the anions occupy the voids.

2. **Coordination Numbers**:
- In the fluorite structure, the coordination number of the `A` ion (Ca²⁺) is 8 because each `A` ion is surrounded by 8 `B` ions.
- The coordination number of the `B` ion (F⁻) is 4 because each `B` ion is surrounded by 4 `A` ions.

**Hint**: Coordination number refers to the number of nearest neighboring ions surrounding a particular ion.

3. **Calculating the Number of Ions**:
- In an FCC structure, the number of `A` ions can be calculated as follows:
- There are 8 corner atoms, each contributing 1/8, and 6 face-centered atoms, each contributing 1/2.
- Thus, the total number of `A` ions = \(8 \times \frac{1}{8} + 6 \times \frac{1}{2} = 1 + 3 = 4\).
- The number of `B` ions (F⁻) can be calculated from the tetrahedral voids:
- The number of tetrahedral voids in an FCC structure is \(2 \times z\), where \(z = 4\) for FCC.
- Thus, the total number of tetrahedral voids = \(2 \times 4 = 8\).

**Hint**: Use the formula for calculating the number of ions based on their positions in the unit cell.

4. **Transformation to Anti-Fluoride Structure**:
- The anti-fluoride structure occurs when the positions of the cations and anions are interchanged. In this case, the `B` ions become the cations and the `A` ions become the anions.
- The anti-fluoride structure can be represented by a formula like `Na2O`, where the anions are larger than the cations.

**Hint**: Understand how the positions of ions affect the structure type.

5. **Evaluating Statements**:
- **Statement 1**: "On subjecting to strong heating, it changes to anti-fluoride structure." - This statement is **incorrect**.
- **Statement 2**: "Interchanging the position of positive and negative ion changes to anti-fluoride structure." - This statement is **correct**.
- **Statement 3**: "Coordination number of A is 8 and B is 4." - This statement is **correct**.
- **Statement 4**: "Coordination number of A is 4 and B is 8." - This statement is **incorrect**.

**Hint**: Carefully analyze each statement based on the definitions and properties of the structures.

### Final Conclusion:
- The correct statements regarding the fluorite and anti-fluoride structures are:
- Interchanging the positions of positive and negative ions changes to anti-fluoride structure.
- Coordination number of A is 8 and B is 4.