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So while studying defects, I found that:

  1. Interstitial defect is when constituent particle is present in the interstitial site. My question is where this constituent particle is coming from? As it leads to increase in density so It must come from outside?

  2. In metal excess defect (due to anion vacancy) : In case of $\ce{NaCl}$ it says, when heated with sodium vapours, chloride ion leaves the crystal and combines with $\ce{Na}$ atom outside to release an electron. Why it is breaking the bonds with $\ce{Na}$ ion to again join with $\ce{Na}$ atom?

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  1. If the defect is thermodynamic, i.e. consists of the same material as the bulk and is in equillibrium, then it is already "included" in the regular, measureable density of the material. If not, well, then it comes from "outside".

  2. Equillibrium. If the concentation of Na on the outside is high enough, then some wandering defect chloride from inside will get stuck. The Na atom outside will happily throw away an electron to bond with the chloride. The separation of charges already happened before, when the chloride left its place in the lattice to leave behind a positively charged hole.

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