Defects which do not disturb the stoichiometry are called stochiometric defect. At the time of crystal formation an ion may not occupy its expected position leaving a void or hole at that site. As a metter of fact one positive and one negative ion are missing from their respective positions leaving behind a pair of holes. This vacancy is a Schottky defect. Due to this defect density decreases. Defect arises in which certain atoms or ions have migrated to interstitial position some distance removed from the holes which they vacated. In this case there is no change in the density of the crystal.
Mark the false statement :

Options

A. CsCl crystal shows Schottky defect
B. Crystal having F - centres are coloured and paramagnetic
C. Photosensitivity of AgBr is due to the presence of Frenkel defect in this crystal.
D. None of the above

Answer

To solve the question regarding the false statement about defects in solids, we will analyze each statement provided.

### Step-by-Step Solution:

1. **Understanding Stoichiometric Defects**:
- Stoichiometric defects do not disturb the stoichiometry of the compound. They occur when an ion is missing from its lattice position, resulting in a vacancy.

2. **Identifying Schottky Defect**:
- A Schottky defect occurs when an equal number of cations and anions are missing from the lattice, creating vacancies. This defect leads to a decrease in density since there are fewer ions in the crystal structure.

3. **Analyzing the Statements**:
- **Statement 1**: "Cesium chloride crystal shows Schottky defect."
- **Analysis**: Cesium chloride (CsCl) does exhibit Schottky defects because both Cs⁺ and Cl⁻ ions are relatively large and can create vacancies without disrupting the overall stoichiometry. This statement is **correct**.

- **Statement 2**: "Crystals having F center are colored and paramagnetic."
- **Analysis**: F centers are created when an anion vacancy is filled with an electron, which gives rise to color and paramagnetism due to the presence of unpaired electrons. This statement is **correct**.

- **Statement 3**: "Photosensitivity of AgBr is due to the presence of Frenkel defect in this crystal."
- **Analysis**: AgBr typically exhibits Frenkel defects, where silver ions are displaced to interstitial sites. This defect does contribute to the photosensitivity of AgBr, as it can dissociate under light. This statement is **correct**.

- **Statement 4**: "None of the above statements is incorrect."
- **Analysis**: Since all the previous statements have been found to be correct, this statement is also **correct**.

4. **Conclusion**:
- Since all statements provided are correct, the answer to the question is that there is no false statement among the options given. Therefore, the correct conclusion is that **none of the above statements is incorrect**.

### Final Answer:
None of the above statements is incorrect.