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

Answer

To solve the question regarding the Schottky defect in crystals, we can break down the information step by step:

### Step 1: Understand Schottky Defect
A Schottky defect is a type of point defect in a crystal lattice. It occurs when there are vacancies in the lattice structure.

### Step 2: Identify Characteristics of Schottky Defect
In a Schottky defect, both cations (positively charged ions) and anions (negatively charged ions) are missing from their lattice sites. This results in a decrease in the density of the crystal.

### Step 3: Analyze the Options
The question provides several options regarding the conditions under which a Schottky defect occurs:
1. Cations move to interstitial sites.
2. Equal number of cations and anions are missing from the lattice.
3. Some lattice sites are occupied by electrons.
4. Some impurity is present in the lattice.

### Step 4: Identify the Correct Option
From our understanding of Schottky defects, the defining characteristic is that equal numbers of cations and anions are missing from their lattice sites. Therefore, the correct answer is:

**Equal number of cations and anions are missing from the lattice.**

### Final Answer
Schottky defect is observed in crystals when **equal number of cations and anions are missing from the lattice.**

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