Charged point defect is a defect which is ready to be ionized and provides a complimentary electronic charged defect. Various such combinations are possible such as
Cation and anion vacancies (VM
and VO)
Vacancies and interstitial ion of same kind i.e. VOand Oi or
VMand Mi
Vacancies and misplaced atoms for the same kind of atom (VM + MO)
Interstitial and misplaced atoms i.e.,Oi and MO
Interstitial atoms i.e. Mi and Oi
Among all of these, the first two are most important as these are regularly seen in many important oxides. The first is called Schottky disorder while the second is called as Frenkel disorder.
2.4.1Schottky Disorder
This defect normally forms at the outer or inner surfaces or dislocations. It eventually diffuses into the crystal unit as equilibrium is reached.
Figure 2.1 Schottky Disorder
The defect reaction is written as
0 (or Null) VM''+ V0••
This defect is preferred when cations and anions are of comparable sizes.
Examples are rocksalt structured compounds such as NaCl, MgO, Corundum, Rutite etc..
2.4.2 Frenkel Disorder
:
Figure 2.2 Frenkel Defect
This defect can form inside the crystal.
It forms where cations are appreciably smaller then anions.
Defect reaction is written as
0 VM'' + Mi••
In cases where anions form the disorder, then it is called as Anti-Frenkel. The corresponding defect reaction in that case would be
0 V0••+ Oi''
Examples of compounds showing this defect are AgBr type compounds such as AgBr, AgI etc.
2.4.3 Intrinsic Ionization
Thermal creation of electron hole pair and is depicted by
VM''+ V0••
