Stoichiometric Defects
- These ranges of point defects do not disturb the stoichiometry of
the solid. - They are also called intrinsic or thermodynamic defects.
- Non-ionic solids exhibit their defects as vacancy defects and interstitial defects.
Vacancy Defect:
A crystal containing vacant lattice sites is said to have vacancy defect. This defect can also develop due to heating up of a substance.
Interstitial Defect:
A crystal in which some constituent particles occupy an interstitial site,
are said to have interstitial defect which increases the density of the substance.
are said to have interstitial defect which increases the density of the substance.
Ionic solids exhibit their defects as Frenkel and Schottky defects.
Frenkel Defect:
- This defect is exhibited by the ionic substances due to the large difference in size of ions.
- The smaller ion (cation) is displaced from its normal site to an interstitial site which creates a vacancy defect at its original site and an interstitial defect at the new location.
- It is also called dislocation defect.
- It does not alter the density of the solid.
- For example, ZnS, AgCl, AgBr and AgI due to small size of Zn2+ and Ag+ ions.
Schottky Defect:
- This defect in ionic solids is equivalent to the vacancy defect in non-ionic solids.
- In order to maintain electrical neutrality, the number of missing cations and anions are equal
- Schottky defect also decreases the density of the substance.
- This defect is shown by ionic substances in which the cation and anion are of almost similar sizes.
- In NaCl there are approximately 106 Schottky pairs per cm3 at room temperature. In 1 cm3 there are about 1022 Thus, there is one Schottky defect per 1016 ions.
- For example, NaCl, KCl, CsCl and AgBr.
- Out of these AgBr shows both, Frenkel as well as Schottky defects.
