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# Explain the following term with suitable examples: F-centres

F-centres :
Alkali halides like NaCl and KCl show this type of defect when crystals of NaCl are heated in a atmosphere of sodium vapour the sodium atoms are deposited normal on the surface of the crystal.The $Cl^-$ ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl.As a result crystal has now excess of sodium.They impart yellow colour to the crystal of NaCl.
Examples are : LiCl,KCl etc