Select the incorrect statement `:`

Options

A. Stiochiometery of crystal remains uneffected dure to Schottky defect
B. Frenkel defect is usually shown by ionic compounds having low coordinaiton number
C. F-centres generation is responsible factor for imparting the colour to the crystal
D. Density of crystal always increases due to substitutional impurity defect.

Answer

To solve the question of selecting the incorrect statement regarding defects in crystals, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement Analysis**:
- **Statement 1**: "Stoichiometry of the crystal remains unaffected due to Schottky defect."
- **Explanation**: In a Schottky defect, equal numbers of cations and anions are missing from the lattice. This maintains the overall electrical neutrality and does not alter the stoichiometry of the crystal. Therefore, this statement is **correct**.

2. **Statement 2**: "Frenkel defect is shown by ionic compounds having low coordination number."
- **Explanation**: A Frenkel defect occurs when a cation moves from its lattice site to an interstitial site, typically in ionic compounds where there is a significant size difference between cations and anions. This often occurs in compounds with low coordination numbers. Thus, this statement is also **correct**.

3. **Statement 3**: "F center generation is responsible factor for imparting the color to the crystal."
- **Explanation**: F centers (or Frenkel centers) are created when an anion is missing from the lattice and an electron occupies the vacant site. These electrons can absorb light and become excited, which can impart color to the crystal. Hence, this statement is **correct**.

4. **Statement 4**: "Density of crystal always increases due to substitutional impurity defect."
- **Explanation**: In a substitutional impurity defect, a smaller ion may be replaced by a larger ion. For example, if Ag+ ions are replaced by Cd2+ ions, the density does not necessarily increase; it could remain the same or even decrease because two smaller ions are replaced by one larger ion. Therefore, this statement is **incorrect**.

### Conclusion:
The incorrect statement is: "Density of crystal always increases due to substitutional impurity defect."

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