F-Centres are:

Options

A. fcc center
B. Metal deficiency defect
C. Anion vacancy defect
D. Face center

Answer

### Step-by-Step Solution:

1. **Definition of F-Centres**:
F-centres are a type of point defect in ionic crystals. They are specifically known as "anion vacancy defects" or "metal excess defects."

2. **Formation of F-Centres**:
F-centres occur when an anion (negatively charged ion) is missing from its regular lattice site in the crystal structure. This absence creates a vacancy.

3. **Electron Occupation**:
To maintain electrical neutrality in the crystal, electrons occupy these vacant sites. The presence of these electrons in the anion vacancies is what characterizes F-centres.

4. **Example of F-Centres**:
A common example of F-centres can be found in sodium chloride (NaCl). When NaCl is subjected to certain conditions, such as heating or exposure to radiation, some chloride ions (Cl⁻) may be missing, leading to the formation of F-centres.

5. **Color Imparted by F-Centres**:
The presence of F-centres in NaCl imparts a yellow color to the crystal. This is due to the electronic transitions of the electrons in the F-centres when they absorb light.

6. **Clarification of Misconceptions**:
It is important to note that F-centres are not related to face-centered cubic (FCC) structures or metal deficiency defects. They are specifically related to anion vacancies.

### Summary:
F-centres are defects in ionic solids caused by the absence of anions, which are compensated by electrons, leading to color changes in materials like NaCl.

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