Which of the following statements for crystals having frenkel defects is not correct?

Options

A. The density of crystals having Frenkel defect is less than that of a pure percfect crystal
B. IFrenkel defects asre observed where the differnce in size of cation and anion is large
C. In an ionic crystal having Frenkel defect may also contian Schottky defrect
D. Usually alkali halides do not have Frenkel defect

Answer

To determine which statement regarding crystals with Frenkel defects is not correct, we need to analyze the characteristics of Frenkel defects and evaluate the provided statements. Here’s a step-by-step solution:

### Step 1: Understand Frenkel Defect
- **Definition**: A Frenkel defect occurs when a cation (which is typically smaller) leaves its lattice position and occupies an interstitial site within the crystal structure. This results in a vacancy where the cation was originally located.

### Step 2: Analyze the Density of Crystals
- **Density Consideration**: Since the cation moves from a lattice site to an interstitial site without any loss of mass, the overall density of the crystal remains unchanged. Therefore, the density of crystals having Frenkel defects is the same as that of perfect crystals.

### Step 3: Evaluate the Statements
1. **Statement**: The density of crystals having Frenkel defect is less than that of a perfect crystal.
- **Analysis**: This statement is incorrect because the density remains the same.

2. **Statement**: Frenkel defects are observed when the difference in size of cation and anion is large.
- **Analysis**: This statement is correct. A large size difference allows the smaller cation to move to an interstitial site.

3. **Statement**: An ionic crystal having a Frenkel defect may also contain Schottky defect.
- **Analysis**: This statement is correct. Some ionic crystals can exhibit both types of defects.

4. **Statement**: Usually, alkali halides do not have Frenkel defects.
- **Analysis**: This statement is correct. Alkali halides typically do not show Frenkel defects due to their ionic size and structure.

### Conclusion
The statement that is not correct regarding crystals with Frenkel defects is:
- **The density of crystals having Frenkel defect is less than that of a perfect crystal.**