Schottky vs Frenkel Defect: Difference and Comparison

Key Takeaways

1. Schottky defects occur in ionic crystals with a large difference in size between cations and anions. They involve pairs of cation and anion vacancies, resulting in a stoichiometric imbalance and nonstoichiometry. Schottky defects decrease the density of the crystal.
2. Frenkel defects also occur in ionic crystals with a large cation-anion size difference, but the cations are much smaller. In Frenkel defects, a cation is displaced from its lattice site and occupies an interstitial position, creating an anion vacancy. Frenkel defects do not lead to stoichiometric imbalance or significant density changes.
3. Both Schottky and Frenkel defects involve the presence of vacancies within crystal lattices, but Schottky defects create vacancy pairs, while Frenkel defects involve cation displacement and interstitial positions. The overall charge of the crystal remains neutral in both cases.

What is Schottky Defect?

A Schottky defect takes place in ionic crystals. It occurs when an equal number of cations and anions are missing from their respective lattice sites. It leads to a vacancy defect. This defect can be caused due to thermal vibrations.

Another reason for this defect can be impurities during crystal growth. It is vastly seen in the ionic crystals that share similarities with cation and anion sizes like NaCl or KCl. This defect targets the physical properties of a crystal. The density and melting point of the crystal gets compromised.

The mass of the lattice gets reduced in Schottky defect, and in the process, the ions and atoms take an exit from the lattice. However, there are no modifications in the value of the dielectric constant.

What is Frenkel Defect?

A Frenkel defect happens in crystal structures. It can occur in both ionic and covalent crystals. The most common crystal structure for this defect is the ones with small ions, for example, alkali metal halides.

Yakov Frenkel discovered this defect, and in his name, the name of this defect has been placed. This defect alters the properties of ionic crystals. It upsurges the electrical conductivity of the crystal.

Things that can cause Frenkel defects to occur for several reasons. High temperature and chemical doping are a few reasons for those. The defect starts when an ion moves from its original lattice site to occupy an interstitial site within the crystal frame. This leads to a vacancy defect at the original lattice site and an interstitial defect at the new site occupied by the ion.

Difference Between Schottky and Frenkel Defect

1. Schottky defects occur when an equal number of cations and anions are missing from their respective lattice sites. It creates vacancy defects. On the other hand, Frenkel defects take place when an ion is displaced from its lattice site and moves to an interstitial site.
2. The density of Schottky defects is lower than Frenkel defects.
3. Schottky defects leave an impact on the density and melting point of a crystal. On the other hand, Frenkel defects impact a crystal’s optical and electrical properties.
4. Schottky defects are seen in ionic compounds such as NaCl and KCl, whereas Frenkel defects affect compounds such as ZnS and AgBr.
5. Frenkel defects are seen at higher temperatures, and Schottky defects, on the contrary, take place at lower temperatures.
6. In the case of Schottky defects, the mass of the lattice is reduced, whereas, in Frenkel defects, the mass of the lattice does not face any changes.

Comparison Between Schottky and Frenkel Defect

Parameter of Comparison	Schottky Defect	Frenkel Defect
Density	Its density is lower.	It has a higher density.
Temperature	It is more common in lower temperatures.	It is more common in lower temperatures.
Diffusion	It can not lead to ion diffusion.	It can result in ion diffusion.
Occurrence	It occurs in ionic crystals.	It occurs in both ionic and covalent crystals.
Formation	Schottky defects originated because of the mutual attraction of cations and anions.	Frenkel defects are developed due to the relatively low energy the cation demands to move to the interstitial site.
Mass of the lattice	With this defect, the mass of the lattice gets reduced.	With this defect, the mass of the lattice stays as it is.
Atoms and ions position	Here, the atoms and ions leave the lattice.	Here atoms and ions leave their place but remain inside the lattice.

References

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