Question

What are non stoichiometric defects of crystals explain about the metal excess defects?

Answer 1

metal excess defect is due to anionic vacancies..
alkali halides like nacl and kcl show this type of defect. the anionic vacancies are occupied by unpaired electrons called f- centres ( from the German word farbenzenter for colour centre/point).
ex. a) they impart yellow colour to the crystals of nacl.
b) excess of lithium makes licl crystals pink.
c) excess of potassium makes kcl crystals violet ( or lilac).