Difference Between Schottky Defect and Frenkel Defect

Main Difference – Schottky Defect vs Frenkel Defect

A crystal lattice is the symmetrical 3D arrangement of atoms in a crystal. A point defect is a vacant point created due to the loss of an atom from the lattice. Therefore, the lattice becomes irregular and asymmetrical. Schottky defect and Frenkel defect are two types of point defects found in crystal lattices. The main difference between Schottky defect and Frenkel defect is that Schottky defect reduces the density of the crystal whereas Frenkel defect does not have any impact on the density of the crystal.

Key Areas Covered

1. What is Schottky Defect
      – Definition, Structure, Properties, Examples
2. What is Frenkel Defect
      – Definition, Structure, Properties, Examples
3. What is the Difference Between Schottky Defect and Frenkel Defect
      – Comparison of Key Differences

Key Terms:  Crystal, Frenkel Defect, Frenkel Disorder, Frenkel Pair, Lattice Structure, Point Defect, Schottky Defect

What is Schottky Defect

Schottky defect is a type of point defect which occurs due to the loss of atoms from a crystal lattice in stoichiometric units. This phenomenon was named after Walter H. Schottky. The Schottky defect can occur in ionic or non-ionic crystals. In ionic crystals, the defect is formed when a stoichiometric unit of atoms leave the lattice. In other words, the defect arises when one building block leaves the lattice. This leaving creates a vacancy in the lattice.

However, the overall charge is maintained neutral since the loss occurs in stoichiometric units. The stoichiometric units are composed of oppositely charged atoms in equal ratios. For example, in NaCl lattice, Na⁺ cation and Cl^– anion can leave, forming Schottky defect.

Figure 1: Schottky defect in NaCl lattice. The Na atom is shown in red color and Cl atom in yellow color.

The Schottky defect causes the reduction of density in the lattice. This defect is highly found in ionic compounds. When it is present in non-ionic crystals, this is called a vacancy defect. Most of the times, the Schottky defect is found in lattices having ions with nearly equal sizes. This type of defect is common in lattices like NaCl, KBr, and KCl.

What is Frenkel Defect

Frenkel defect is a type of point defect that occurs due to the loss one atom or a small ion. This creates a vacant site inside the lattice structure. This is also called Frenkel pair or Frenkel disorder. These names are created in honor of the first person to discover this defect, Yakov Frenkel.  If the leaving chemical species is an ion, it is usually a cation. This atom or ion then occupy an interstitial site. An interstitial site is a location nearby the vacant point.

Frenkel defect does not have any effect on the density of the lattice. This is because the leaving atoms or ions occupy a nearby position instead of leaving the lattice completely. Then both the mass and volume of the lattice remain unchanged.

The Frenkel defect is found mostly in ionic compounds. Unlike Schottky defect, the Frenkel defect is found in lattices composed of atoms or ions that have considerable differences between their sizes. One or more atoms or ions may undergo Frenkel defect. Therefore, the number of interstitial sites is equal to the number of defects present in the lattice.

Figure 2: Frenkel defect in AgCl lattice. The green color circles indicate Ag+ ions and ash color circles indicate Cl- ions.

The above example shows the Frenkel defect in the AgCl lattice. Here, the number of atoms present in the same volume is the same as it was before the defect. Therefore, the density is the same. Some examples for Frenkel defect are AgCl, AgI, CaF₂ and ZnS.

Difference Between Schottky Defect and Frenkel Defect

Definition

Schottky Defect: Schottky defect is a type of point defect that occurs due to loss of atoms from a crystal lattice in stoichiometric units.

Frenkel Defect: Frenkel defect is a type of point defect occurs due to loss of one atom or a small ion.

Density

Schottky Defect: Schottky defect causes the density of the lattice to be reduced.

Frenkel Defect: The Frankel defect does not have any influence on the density of the lattice.

Mass

Schottky Defect: The mass of the lattice is reduced when Schottky defect occurs.

Frenkel Defect: The mass remains constant even after the Frenkel defect has occurred.

Leaving Species

Schottky Defect: Atoms or ions leave the lattice, creating a Schottky defect.

Frenkel Defect: Atoms or ions leave their position but remain inside the lattice in Frenkel defect.

Size of Ions

Schottky Defect: Schottky defect occurs in lattices with ions in similar sizes.  

Frenkel Defect: Frenkel defect occurs in lattices with ions with large differences in their sizes.

Conclusion

Both Schottky defect or Frenkel defect create a vacant point in the lattice. The type of defect is mainly dependent on the size difference between the cations and anions in the lattice. Furthermore, the Schottky defect causes the lattice density to be decreased whereas the Frenkel defect does not have any effect on the density of the lattice. This is the main difference between Schottky defect and Frenkel defect.

References:

1.”Frenkel defect.” Wikipedia. Wikimedia Foundation, 16 July 2017. Web. Available here. 26 July 2017. 
2. “Point defects.” LinkedIn SlideShare. N.p., 17 Apr. 2015. Web. Available here. 26 July 2017.