Difference between Schottky and Frenkel Defects

Difference between Schottky and Frenkel Defects

S. NoSchottkyFrenkel
1These defects arise in a crystal when the same number of cations and anions are missing from their normal lattice site and as a result, a pair of holes are formed.These defects arise in a crystal due to the presence of a hole in the cationic lattice site and cation occupies an interstitial position.
2This defect lowers the density of the crystal.This defect does not change the density of the solid.
3These defects are generally present in highly ionic solids that have high coordination number and in which cations and anions are almost of similar sizes. E.g., NaCl, CsCl.These defects are generally present in ionic solids which have low coordination number and whose anions are much larger in size than cations. E.g., AgCl, AgBr, ZnS.

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Shottky Effect
Frenkel Effect

Non-Stoichiometric Defects

Non-Stoichiometric compounds are those in which the number of positive and negative ions are not in the same ratio as indicated by their chemical formulae.

Examples areFe0.95O, Cu1.97S and VOx, where x ranges from 0.6-1.3.

The point defects that disturb the stoichiometry of the compound are called Non-Stoichiometry defects.

Metal excess Defects

Extra cations occupying the interstitial sites. When a cation and an electron of some non-metal atom occupy interstitial positions in the lattice with complete cationic and anionic lattices, the defect is called a Metal Excess defect due to interstitial cations.

Difference between Schottky and Frenkel Defects

When a negative ion is missing from its lattice site and in its vacancy, an electron associated previously with it is left, the defect is called Metal Excess Defect due to anion vacancies.

Difference between Schottky and Frenkel Defects

Metal Deficiency Defects

When a cation is missing from its lattice site and adjacent metal ion acquires higher oxidation state, the defect is called Metal Deficiency Defect due to cation vacancies.

Metal Deficiency Defects

Cation vacancies in some crystals make them good catalysts. This is because the positive charge due to missing cation is balanced by an adjacent ion having charge which leads to.

Difference Between Metal Excess Defect and Frenkel Defect

S.No.Metal Excess DefectFrenkel Defect
1It has no holes in the crystal lattice.It has holes in crystal lattice.
2It has interstitial electrons.It has no interstitial electrons.

Difference Between Metal Excess Defect and Schottky Defect

S.No.Metal Excess DefectSchottky Defect
1In this type of defect, there are holes in the crystal lattice, due to missing a negative ion from the lattice site.Crystal lattice has a pair of holes due to missing of a cation and anion from their normal lattice site.
2Holes are occupied by free electrons.Holes are not occupied by free electrons.

Impurity Defect

If a foreign atom or ion as impurity occupies a lattice site, the defect is called impurity defect. If the foreign atom or ion occupies voids in the structure, the defect is called interstitial impurity defect.

Impurity defects in ionic crystals can be introduced by adding impurity of ions. If the impurity ions are in a different valence state from that of the host ions, vacancies are created.

For Example-
Addition of CdCl2 to AgCl yields solid solutions in which the divalent Cd occupies the Ag sites.
These cation vacancies result in higher conductivity of the solid.

Introduction of defects by incorporating a small amount of foreign impurity in the host crystal is called DOPING.

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