Write a short note on non-stoichiometric defects.

(N/A) When the stoichiometry of a crystal is disturbed by the presence of defects,these are called non-stoichiometric defects.
These defects arise due to the presence of either extra cations or extra anions,which disturbs the ratio of cations and anions in the crystal.
$A$ large number of inorganic compounds are known to have constituents in non-stoichiometric ratios due to these defects in the crystal structure.
In compounds with non-stoichiometric defects,electrical neutrality is maintained either by the presence of extra electrons or by the presence of extra positive charges.
There are two types of non-stoichiometric defects:
$(i)$ Metal excess defects.
$(ii)$ Metal deficiency defects.