Dear Devang,

Metal excess defect occurs due to :-
1) Anionic vacancies.
2) Presence of extra cations.
 
1) Metal excess defect due to anionic vacancies- When crystals of NaCl(example) are heated in an atmosphere of Na vapour,Na atoms are deposited on the surface of the crystals and Cl- ions diffuse to the surface of the crystals to combine with Na to form NaCl.This happens by loss of electrons by Na atoms to form Na+ .These realesed electrons diffuse into the crystals and occupy anionic sites.As the result,crystal now has an excess Na.
 
2) Metal excess defect due to presence of extra cations - On heating ZnO(example),it loses oxygen.Now there is an excess Zn+2 ion.

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