Question

what is 'Metal excess defect'?

Answer 1

Metal Excess Defect:--These defects are either due to anionic vacancy or extra cations at interstitial sites.
(i)Metal excess defect due to anionic vacancies:- In this case, negative ions get missed from their lattice sites leaving holes in which the electrons remain entrapped to maintain the electrical neutrality and observed in those crystals which are likely to form Schottky Defects.For ex-alkali metal halides like NaCl and KCl show this type of defect. When crystals of NaCl are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The Cl–ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl. This happens by loss of electron by sodium atoms to form Na+ ions. The released electrons diffuse into the crystal and occupy anionic sites. As a result of which the crystal now has an excess of sodium. The anionic sites occupied by unpaired electrons are called as F-centres.They impart yellow colour to the crystals of NaCl.

Answer 2

Answer:

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