Frenkel Defect

Introduction

The Frenkel Defect is a flaw in the production of lattice crystals in which an ion or perhaps atom occupies a normally unoccupied position. It is called after Russian physicist Yakov Frenkel. The vacancy in the crystal is generated by the voluntary spacing out of 1 atom. This defect is sometimes known as a dislocation defect since it exhibits both valencies as well as self-interstitial faults. Small cations in the crystal lattice are displaced from their original places, resulting in an empty site in the crystal lattice.

Formation of Frenkel Defect

The following is how the mentioned defect forms in the crystal lattice −

• A cation exits the lattice as well as becomes interstitial.

• A void is produced in the lattice.

• The uprooted cation settles in a nearby position among the other cations as well as anions.

Occurrence of the Imperfections in Solid Crystal

The precise alignment of ions is 1 of the most essential properties of crystals that distinguishes them from amorphous substances. However, no material is completely aligned, & therefore this flaw known as lattice defects are detected.

The definition of defect is flaws or imperfection. Solid crystals have flaws, which are explored in solid-state chemistry. Some crystal units may have 1 or more atoms than other perfect crystal units. These crystal flaws are referred to as crystal defects. As a result, crystallographic defects are disruptions in regular patterns in crystalline substances. There are several forms of crystallographic defects, including planar defects, line defects, as well as point defects & so on. Frenkel defects are single-point flaws.

Frenkel Defect Definition

This defect is a shortcoming in the crystal forming process. Many units in solid crystals have fewer atoms than ideal crystal units. This is known as a defect. A crystallographic fault is a disruption in a regular pattern in crystalline materials. Defects come in a variety of forms, each indicating a major hiatus in the internal development of crystals.

The following are the many sorts of defects −

• Impurity Defects

• Stoichiometric Defect,

• Frenkel Defect

• Schottky Defect

Frenkel Defect Example

Some examples of Frenkel Defect are as follows −

• ZnS (Zinc sulfides)

• NaCl (Sodium Chloride)

• AgCl (Silver Chloride)

• AgBr (Silver Bromide)

• AgI (Silver Iodide)

Reasons for the Frenkel Defect

Defects are prevalent in solid-state formations because the locations of molecules or atoms in crystals are dictated by unit cell characteristics at repeated set distances. These defects are created largely by particle irradiation. A crystal structure is typically flawed as well as changeable. The enthalpy during the time of creation is larger than at any other point in particle irradiation, hence the equilibrium does not reach the detection limit. These defects can also form naturally in materials with displaced cations.

Calculation of Number of Frenkel Defects

The following are the key variables for calculating the no. of mentioned defects in a crystal −

• The defect's enthalpy release

• Positions usually taken

• The temperature of the location

• No. of the available position

The Frenkel defect may be calculated using the following formula −

$$\mathrm{\sqrt{NN^\prime} e^{-\Delta H/2RT}}$$

Where,

N denotes normally occupied positions

N_' denotes no. of available positions

ΔH denotes 1 Frenkel Defect's enthalpy formation

R denotes the gas constant

T denotes temperature

Difference between Schottky and Frenkel Defect

Defection processes in solid-state crystals are found in their development as well as confinement. The Schottky Defect describes a flaw in crystalline solids, whereas the Frenkel Defect describes the creation of crystalline ionic complexes.

Schottky Defect	Frenkel Defect
It is commonly referred to as the valency fault.	It is sometimes referred to as the dislocation defect.
The lattice's density is lowered.	It does not affect the lattice or atom density.
The mass of the lattice decreases.	The lattice's atom or mass is unchanged.
Ions escape from the crystal lattice in stoichiometric units.	Ions escape their lattice positions but stay in the interstitial area.
The defective ions entirely exit the crystal lattice.	The defective ions depart their lattice position.
The ions are all the same size.	Ions vary in size as well as are irregular.
It occurs in CsCl,KBr, as well KCl complexes.	It is present in AgCl,ZnS, as well as NaCl complexes.

Conclusion

When an ion or atom fills an empty spot in a crystal lattice, it causes a Frenkel defect. When a cation leaves its site, a vacancy is generated & the cation goes to a nearby position. This defect is caused by particle irradiation. The chemical characteristics do not change as a result of the Frenkel defect. It does not give density or electrical neutrality to the crystal. The quantity of Frenkel defect is determined by the no. of occupied as well as open locations, plus the temperature. Frenkel defect is also known as dislocation defect because of the displacement of ions.

FAQs

1. Why do alkali metal halides lack the Frenkel defect?

The mentioned defect in an atom demands low coordination no., as well as the crystal lattices, being open for the molecule. Because cations, as well as anions, have almost identical sizes & cations cannot be accommodated in interstitial locations, thus the defect does not exist in alkali metal halides.

2. What causes AgBr to exhibit both Schottky as well as Frenkel defects?

Because AgBr has an intermediate radius ratio, it exhibits both Frenkel & Schottky defects. When both anions, as well as cations, are missing from the crystal lattice, AgBr exhibits Schottky defects. Ag^+ ions are quite mobile & tend to migrate about inside the lattice. As a result, they also display the Frenkel defect.

3. What are the characteristics of the Frenkel Defect?

The Frenkel Defect has the following characteristics −

• It produces relatively open lattices.

• When the size of cations is smaller than that of anions, this defect develops.

• In this defect, the coordination no. of a solid-state is low

• It does not affect a substance's chemical characteristics.

• Because the mentioned defect does not affect the density of the structure, the solid's mass, as well as volume, are preserved.

• Substances preserve their electrical neutrality during the Frenkel Defect experiment.

4. What is the Effect of the Frenkel Defect on Density?

The Frenkel defect does not affect the crystal's density or stability. Although the defect has a direct influence on ion migration, the volume, as well as density of the solid-state structure, do not change. The interstitial atoms in close-packed formations cause tensions that cause the lattice to expand. This expansion outweighs the shrinking of the lattice owing to vacancy.

5. What are the Effects of the Frenkel Defect?

The mentioned defect produces ion displacement. This defect is also referred to as the dislocation defect because the smaller ion, which is a cation, is moved from its initial position as well as put in an interstitial site. Even though the density stays constant, the complexes have low coordination no. as well as a wide range of anions & cations in size.