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My book says:

$\ce{NaCl}$ shows metal excess defect, which results into the formation of an F-center in the crystal. This F-center is responsible for the yellow color of $\ce{NaCl}$.

How does an F-center provide color to the solid?

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An F-center (or any color center) is a point defect in the material that generates localized electronic states. When these states are in the band gap, they will result in strong peaks in optical absorption. An F-center in an alkali-halide material is an electron bound to a negative ion vacancy. A brief overview may be found in Ashcroft and Mermin's Solid State Physics, Chapter 30.

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    $\begingroup$ But how does the electron impart color to the crystal? I think it is something related to it gaining energy energy from visible light. Could you please explain this in terms of absorption and release of energy by the electron? $\endgroup$ – agdhruv Jul 17 '15 at 19:40
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    $\begingroup$ This is heading far more into solid state physics than chemistry... Crystals have extended electronic states, defining bands of allowable states. In NaCl there is a fully occupied valence band, an energy gap, and an empty conduction band, somewhat like many oxides. You can see through a good quality NaCl crystal because visible light does not have enough energy to promote an electron from the valance to the conduction band, so no absorption is possible. The color center is a localized state (or states) in the gap, that allow absorption in the visible because there is now a state present. $\endgroup$ – Jon Custer Jul 17 '15 at 19:51
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f-centers leads to the formation of an anionic vacancy

this space is occupied by an electron

these electrons absorb energy from visible region and radiate yellow colour when they jump back to their original state

this is what my teacher's guide book says

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  • $\begingroup$ Absorbing light at one wavelength and re-radiating it at a different wavelength is fluorescence. It's possible, but more likely that it's just selectively absorbing shorter wavelengths and leaving longer ones alone as described in Wikipedia's article on f-center, but I don't know. Does the guide book include any references for that? $\endgroup$ – uhoh Aug 7 '17 at 1:42

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