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The released electrons diffuse into the crystal and occupy anionic sites.
The anionic sites occupied by unpaired electrons are called F-centres that imparts yellow colour to the crystals of NaCl due to the excitation of the electrons on absorption of energy from the visible light falling on the crystals.
Excess of lithium makes LiCl crystals pink and excess of potassium makes KCl crystals violet.
Metal excess defect due to the presence of extra cations at interstitial sites:
Heating Zinc oxide that exists in white colour at room temperature loses oxygen and turns yellow.
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This results in excess of zinc in the crystal and its formula becomes Zn1 + xO.
The excess Zn 2 + ions move to interstitial sites and the electrons to neighbouring interstitial sites.
o Metal Deficiency Defect:
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Many solids possess less amount of the metal as compared to the stoichiometric proportion.
For example, FeO is mostly found with a composition ranging from Fe 93O to Fe 0.96O.
In crystals of FeO some Fe 2 + cations are missing and the loss of positive charge is made up by the presence of required number of Fe 3 +