Crystalline defects

No crystal is perfect !
The real crystal has a surface sets an upper limit on its perfection, and actually macroscopic samples of most solids include many crystallites, randomly oriented, with grain boundaries separating one crystal from the next.
6.1 Point defects
Point defects are where an atom is missing. Point defects are defects that occur only at or around a single lattice point. They are not extended in space in any dimension. Point defects include vacancies, self-interstitial atoms, interstitial impurity atoms, and substitutional atoms.
Vacancy defect
A vacancy is produced when an atom is missing from a normal site see Fig.23. It is created when the solid is formed.
At R.T, very few vacancies are present, but this number increases exponentially as we increase the temperature (see Eq. 1).
Interstitial defect
An interstitial defect is formed when an extra atom is inserted into the lattice structure in a place between atoms (see Fig.23). The number of this defects is independent of temperature
Substitutional