Schottky defect is observed in crystals when

Options

A. some cations move from their lattice site to interstitial sites
B. equal number of cations and anions are missing from the lattice
C. some lattice sites are occupied by electrons
D. some impurity is present in the lattice

Explanation

To explain the Schottky defect in crystals, we can break down the explanation into a few clear steps:

### Step-by-Step Solution:

1. **Definition of Schottky Defect**:
- A Schottky defect is a type of point defect in ionic crystals where an equal number of cations and anions leave their lattice positions, creating vacancies.

2. **Ionic Structure Example**:
- Consider an ionic solid like sodium chloride (NaCl), which consists of Na⁺ (sodium cations) and Cl⁻ (chloride anions) arranged in a regular lattice structure.

3. **Formation of the Defect**:
- In the case of a Schottky defect, one Na⁺ ion and one Cl⁻ ion will leave their respective lattice sites. This results in vacancies where these ions used to be.

4. **Visual Representation**:
- If we visualize the NaCl lattice, it would look like this:
```
Na⁺ Cl⁻ Na⁺ Cl⁻
Cl⁻ Na⁺ Cl⁻ Na⁺
```
- When a Schottky defect occurs, it may look like this:
```
Na⁺ Cl⁻ Na⁺ Cl⁻
Cl⁻ Na⁺ Cl⁻ Na⁺
```
- Here, the empty spaces represent the missing Na⁺ and Cl⁻ ions.

5. **Characteristics of Schottky Defect**:
- The key characteristic of a Schottky defect is that it maintains charge neutrality. For every cation that leaves, an anion must also leave, so the overall charge of the crystal remains balanced.

6. **No Interstitial Occupation**:
- Unlike other types of defects, such as Frenkel defects, in Schottky defects, the missing ions do not occupy interstitial sites within the lattice; they simply create vacancies.

### Summary:
In summary, a Schottky defect occurs in ionic crystals when an equal number of cations and anions leave their lattice sites, resulting in vacancies while maintaining the overall charge neutrality of the crystal structure.