What is F-centre ?

Explanation

**Step-by-Step Text Solution:**

1. **Definition of F-Center**: An F-center, or Farbzentrum, is a type of point defect in a crystalline solid. It is specifically an anionic vacancy where an anion (usually a negative ion) is missing from the crystal lattice.

2. **Formation of F-Centers**: F-centers are typically formed during the process of crystal growth or due to the introduction of defects in the crystal structure. When an anion is missing from its lattice site, it creates a vacancy that can trap an electron.

3. **Color Production**: The presence of F-centers in a crystal can lead to the absorption of certain wavelengths of light, which results in the appearance of color in the material. For example, F-centers can produce colors such as yellow, violet, or blue depending on the type of crystal and the concentration of the F-centers.

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