Which defect causes decreases in the density of crystal?

Options

A. a)Frenkel
B. b)Schottky
C. c)Interstitial
D. d)F-centre

Answer

To solve the question of which defect causes a decrease in the density of a crystal, we can follow these steps:

### Step 1: Understand the Concept of Crystal Defects
- Crystals can have defects that affect their properties, including density. The main types of defects include vacancies, interstitials, and substitutional defects.

**Hint:** Familiarize yourself with the different types of crystal defects and their characteristics.

### Step 2: Identify the Types of Defects
- The common types of defects are:
1. **Vacancy Defect**: Missing ions in the crystal lattice.
2. **Interstitial Defect**: Extra ions occupying spaces between the regular lattice positions.
3. **Substitutional Defect**: Atoms in the lattice are replaced by different atoms.
4. **Schottky Defect**: A type of vacancy defect where an equal number of cations and anions are missing.

**Hint:** Recall the definitions and implications of each defect type.

### Step 3: Analyze the Schottky Defect
- The Schottky defect involves the creation of vacancies for both cations and anions. This means that there are fewer ions in the unit cell.

**Hint:** Consider how the removal of ions (vacancies) affects the overall mass of the unit cell.

### Step 4: Relate Mass, Volume, and Density
- Density is defined as mass per unit volume (Density = Mass/Volume). If the number of ions (and thus the mass) decreases while the volume remains constant, the density will also decrease.

**Hint:** Remember the formula for density and how changes in mass affect it.

### Step 5: Conclusion
- Since the Schottky defect leads to a decrease in the number of ions in the unit cell, it results in a decrease in mass while the volume remains unchanged. Therefore, the density of the crystal decreases.

**Final Answer:** The defect that causes a decrease in the density of the crystal is the **Schottky defect**.