Which defect cause decrease in the density of crystal?

Options

A. Frenkle
B. Interstitial
C. schottky
D. F-centre

Explanation

To determine which defect causes a decrease in the density of a crystal, we need to analyze the types of defects present in crystalline solids. Here’s a step-by-step solution:

### Step 1: Understand the Concept of Crystal Defects
Crystals can have various types of defects, which are irregularities in the orderly arrangement of atoms. These defects can be classified into three main types: point defects, line defects, and plane defects. Point defects are further divided into vacancies, interstitials, and substitutional defects.

**Hint:** Remember that point defects are the most common and can significantly affect the properties of the crystal.

### Step 2: Identify the Types of Point Defects
The main types of point defects include:
- **Vacancy Defects:** Missing atoms in the lattice.
- **Interstitial Defects:** Extra atoms positioned in the spaces between the regular lattice positions.
- **Frenkel Defects:** A combination of vacancy and interstitial defects, where an atom leaves its position and occupies an interstitial site.
- **Schottky Defects:** Pairs of cations and anions are missing from the lattice, maintaining charge neutrality.

**Hint:** Focus on how each defect affects the overall structure and density of the crystal.

### Step 3: Analyze the Schottky Defect
In a Schottky defect, equal numbers of cations and anions are missing from the lattice. This results in a decrease in the mass of the unit cell without changing its volume, leading to a lower density.

**Hint:** Consider how the removal of ions impacts the mass and volume of the crystal.

### Step 4: Compare with Other Defects
- **Frenkel Defect:** This defect does not change the density because while an atom is missing from its lattice site, it occupies an interstitial site, keeping the overall mass and volume constant.
- **Interstitial Defect:** Adding extra atoms increases the mass of the unit cell, which may increase density.
- **F-Center Defect:** This defect involves the presence of unoccupied lattice sites filled with electrons, which also does not significantly change the density.

**Hint:** Evaluate how the presence or absence of atoms affects the density in each case.

### Step 5: Conclusion
Since the Schottky defect involves the loss of equal numbers of cations and anions, it leads to a decrease in the density of the crystal. Therefore, the defect that causes a decrease in the density of a crystal is the **Schottky defect**.

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