In normal spinel structure there is a closed packed array of `O^(2-)` ions. The trivalent cations are present in

Options

A. 75 % of octahedral voids
B. 50 % of octachedral voids
C. 12.5 % of tetrahedral voids
D. 25 % of octahedral voids

Answer

To solve the question regarding the presence of trivalent cations in a normal spinel structure, we can break down the information step by step.

### Step-by-Step Solution:

1. **Understanding the Spinel Structure**:
- A normal spinel structure consists of a close-packed arrangement of oxide ions (O²⁻).
- In this structure, there are two types of cations: divalent (A²⁺) and trivalent (B³⁺).

2. **Cation Positions**:
- In a normal spinel, the divalent cations (A²⁺) occupy tetrahedral sites, while the trivalent cations (B³⁺) occupy octahedral sites.
- Specifically, there is one A²⁺ ion in the tetrahedral site and two B³⁺ ions in the octahedral sites.

3. **Calculating the Number of Ions**:
- The formula for normal spinel can be represented as AB₂O₄, where A is the divalent cation and B is the trivalent cation.
- In the unit cell of the spinel structure, there are 4 oxide ions (O²⁻) due to the face-centered cubic (FCC) arrangement.

4. **Determining the Number of Tetrahedral and Octahedral Sites**:
- In a face-centered cubic structure, there are 8 tetrahedral sites and 4 octahedral sites.
- However, in normal spinel, only half of the octahedral sites are occupied by B³⁺ ions.

5. **Calculating the Occupancy**:
- Since there are 4 octahedral sites in total, and only half are occupied by B³⁺ ions, there will be 2 B³⁺ ions present in the octahedral sites.
- The tetrahedral sites are fully occupied by A²⁺ ions, which means there is 1 A²⁺ ion.

6. **Conclusion**:
- Therefore, in a normal spinel structure, the trivalent cations (B³⁺) are present in the octahedral sites, with a total of 2 B³⁺ ions in the unit cell.

### Final Answer:
In a normal spinel structure, the trivalent cations (B³⁺) are present in the octahedral sites.