What is Metal Excess Defect?

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.