Question

What are F-centres in an ionic crystal?

Answer 1

Non-Stoichiometric Defects

• A vast variety of nonstoichiometric inorganic solids have constituent elements in non-stoichiometric ratios due to the presence of defects in their crystal structures.
• The metal excess defect is a type of non-stoichiometric effect.

Metal Excess Defect

• This type of defect can be found in alkali halides such as $\mathrm{NaCl}$ and $\mathrm{KCl}$.
• When $\mathrm{NaCl}$ crystals are heated in a sodium vapor environment, ${\mathrm{Cl}}^{–}$ ions diffuse to the crystal's surface and mix with $\mathrm{Na}$ atoms to form $\mathrm{NaCl}$.
• This occurs as a result of sodium atoms losing electrons to generate ${\mathrm{Na}}^{+}$ ions.
• The electrons that have been liberated diffuse within the crystal and occupy anionic sites.
• As a result, there is now an excess of sodium in the crystal.

F-centers

• F-centres are anionic sites occupied by unpaired electrons.
• These F-centers produce different colors, such as $\mathrm{NaCl}$ produces yellow color, $\mathrm{KCl}$ produces violet color, and $\mathrm{LiCl}$ produces pink color.