Schottky Defect vs Frenkel Defect: Difference and Comparison

Key Takeaways

1. Defect Composition: Schottky and Frenkel defects are types of point defects in crystals. A Schottky defect occurs when a pair of oppositely charged ions leave their lattice sites, creating a vacancy. A Frenkel defect, also known as a dislocation defect, happens when an ion leaves its correct lattice site and moves to an interstitial site, creating a vacancy and an interstitial.
2. Effect on Density: A Schottky defect involves the removal of an ion pair from the lattice, thus decreasing the overall density of the crystal. A Frenkel defect, however, doesn’t involve removal of ions, just a rearrangement within the crystal. Therefore, it doesn’t alter the density.
3. Common in Specific Crystals: Schottky defects are most commonly found in crystals with a high coordination number and similar sized cations and anions, such as alkali halides. Frenkel defects are more common in crystals where the difference in ionic sizes is significant, as this allows smaller ions to easily occupy interstitial sites. This is seen in silver halides.

What is Schottky Defect?

A schottky defect is also known as a stoichiometric defect. This is because the overall chemical composition of the crystal is the same irrespective of vacancies. It is one of the point defects formed when an equal number of anions and cations are removed from the crystal lattice. These missing anions and cations form empty spaces or vacancies at their positions.

These are common in highly ionic compounds such as metal and alkali halides. The factor affecting or influencing this defect is the high temperature. NaCl, CsCl, and KBr are a few examples of schottky defects.

What is Frenkel Defect?

A frenkel defect is also known as a non-stochiometric defect. This is because the overall chemical composition of the crystal slightly deviates from its actual composition but maintains the charge neutrality of the compound. It is also one of the point defects that can be commonly found in ionic crystal or semiconductor materials. Also, they are common in materials having high differences in sizes of both anions and cations.

It is formed when a cation migrates from an interstitial site, and a cation interstitial is formed. It happens because of high temperature, which results in increased mobility of atoms of the crystal lattice. AgBr, Pbl2, and ZnS are some examples of frenkel defects.

Difference Between Schottky Defect and Frenkel Defect

1. The nature of the schottky defect is also referred to as a stoichiometric defect, while, on the other hand, the nature of the Frenkel defect is also referred to as a non-stoichiometric defect. 
2. In schottky defect, the density of the defects is said to be low as the vacancies and interstitial sites are formed in a balanced manner. In contrast to this, in the Frenkel defect, the density of the defect is high as the vacancies and interstitial sites are formed in an unbalanced manner.      
3. The schottky defect is formed when an equal number of anion and cation vacancies are created. At the same time, the Frenkel defect is formed when a cation migrates from an interstitial site, and a cation interstitial is formed.
4. The schottky defect is common in highly ionic compounds such as metal and alkali halides. Whereas on the other hand, the Frenkel defect is common in compounds that have high differences in the sizes of anions and cations.
5. The effect of density on schottky defect is that it lowers its crystal density. In comparison, there is no such effect of density in Frenkel defect.
6. The schottky defect may affect the change in mechanical properties, such as it causes brittleness. At the same time, in the Frenkel defect, the extent of defect formation decides the change in mechanical properties.
7. Schottky defect influences electrical conductivity in some ionic compounds. Whereas on the other hand, the Frenkel defect influences or affects the electrical properties. 
8. Schottky defects are less stable, whereas, on the other hand, Frenkel defects are more stable. 
9. Example of schottky defects is NaCl, CsCl, and KBr. While on the other hand, examples of Frenkel defects are AgBr, Pbl₂, and ZnS.

Comparison Between Schottky Defect and Frenkel Defect

Parameter of Comparison	Schottky Defect	Frenkel Defect
Nature	Stoichiometric defect	Non-stoichiometric defect
Density	Low density of defects	High density of defects
Defect Formation	It is formed when an equal number of anion and cation vacancies are created	It is formed when a cation migrates from an interstitial site, and a cation interstitial is formed
Common In	They are most common in highly ionic compounds such as – metal and alkali halides	They are common in compounds that have high differences in the sizes of anions and cations
Effect on Density	Decrease	No effect
Mechanical Properties	Brittleness	Depending on the extent of defect formation
Electrical Properties	Influences electrical conductivity	Affects electrical properties
Stability	Less stable	More stable
Examples	NaCl, CsCl, KBr	AgBr, Pbl2, ZnS

References

1. https://pubs.rsc.org/en/content/articlehtml/2022/ta/d1ta10072f
2. https://iopscience.iop.org/article/10.1088/2515-7655/acbb29/meta

Last Updated : 21 August, 2023