Difference between Schottky and Frenkel Defect

A crystal's symmetrical, three-dimensional arrangement of atoms is known as a lattice. A point defect is a void that develops when one atom is removed from the lattice. As a result, the lattice develops irregularities and asymmetry. Two other point defects that may be discovered in crystal lattices are the Schottky defect and the Frenkel defect. The significant distinction between a Frenkel defect and a Schottky defect is that a Frenkel defect does not affect the density of the crystal. In contrast, a Schottky defect causes a reduction in its density.

Crystal defects are flaws that appear in the regular geometry of an atom in a crystalline solid. They develop for a variety of causes, including:

1. Internal atom impurities
2. The crystalline structure is formed under conditions of changing temperature.
3. The circumstances under which crystals are produced.

The sorts of faults in a crystalline structure are determined by the flaws and include

1. point defect
2. line defect
3. A plane defect

• In a point fault, the flaw only manifests at specific points along the lattice.
• When one atom is misplaced, absent, or replaced by another atom from its location along a line, it is said to have a line defect.
• A plane defect occurs when atoms are dispersed along a plane's line of symmetry.

There are two types of defects stoichiometric and non-stoichiometric point defects, which are further divided into several categories.

The stoichiometric defect is of two types i.e., vacancy defect and interstitial defect.

Non- Stoichiometric defect has further types i.e. metal excess defect and metal deficiency defect.

A stoichiometric defect occurs when the crystal's chemical composition does not change due to the flaw. It develops when two or more atoms move to a different location within the lattice or when an atom is moved from its position.

When the crystal altars' chemical composition is known, a non-stoichiometric defect is present. If various impurities are introduced to the crystal lattice, it results.

We must be aware that stoichiometric faults include Schottky and Frenkel defects.

What is Schottky Defect?

A stoichiometric unit loss of atoms from a crystal lattice causes a point defect known as a Schottky defect. Walter H. Schottky is honoured by the naming of this phenomenon. Crystals that are neither ionic nor metallic may have the Schottky defect. When a stoichiometric unit of atoms breaks away from the lattice, an imperfection is created in ionic crystals. In other words, when a construction brick falls out of the lattice, a defect occurs. The lattice becomes vacant as a result of this departure.

Yet, since the loss happens in stoichiometric units, the total charge is kept neutral. Equal ratios of atoms with opposed charges make up stoichiometric units. For instance, the Na+ cation and Cl- anion may separate from the NaCl lattice and create a Schottky defect.

The Schottky defect causes the fall in density in the lattice. The prevalence of this flaw is higher in ionic substances. This defect is known as a vacancy when it appears in non-ionic crystals. Most often, lattices containing roughly similar-sized ions exhibit the Schottky defect.

Characteristics of the Schottky Defect

1. When the size of the anion and the size of the cation are almost equal, Schottky defects develop in ionic crystals.
2. The Schottky defect is caused when one anion and cation depart the crystal at the exact moment. None of them ever return to the interstitial webpage.
3. Two vacancies are created as a result of one Schottky defect.
4. The whole crystal is kept electrically neutral.
5. When vacancies are produced, the density of the crystal drops for Schottky defects.

Examples of materials where Schottky defects can be found:

• Sodium Chloride (NaCl)
• Potassium Chloride (KCl)
• Potassium Bromide (KBr)
• Silver Bromide (AgBr)
• Cerium Dioxide (CeO₂)
• Thorium Dioxide (ThO₂)

What is Frenkel Defect?

When an atom or a small ion is lost, a form of point defect known as a Frenkel defect develops. The lattice construction is left with space as a result. Frenkel pair or Frenkel disorder are other names for this. These designations are given in recognition of Yaakov Frenkel, the first person to identify this flaw. It is often a cation if the exiting chemical species is an ion. Then, this atom or ion moves into an interstitial location.

The density of the lattice is unaffected by the Frenkel defect. This occurs due to the departing atoms or ions occupying a neighbouring place instead of leaving the lattice. The lattice's mass and volume then stay unaltered.

In ionic compounds, the Frenkel defect is most often seen. In contrast to the Schottky defect, the Frenkel defect appears in lattices where the sizes of the atoms or ions vary significantly. The Frenkel defect may occur in one or more atoms or ions. As a result, the number of interstitial sites and lattice defects are identical.

Characteristics of Frenkel Defects

1. In the Frenkel Defect, neither foreign nor native atom exits the solid crystal, it occupies an interstitial location. Hence, the crystal solid's total density does not change.
2. When the size of the anion is much bigger than the size of the cation in an ionic crystal, this defect is often seen. Examples include silver chloride (AgCl) and zinc sulphide (ZnS).
3. The more significant ions (anions) stay in their respective lattice positions while the smaller ions (often cation) shift from the original lattice site to the interstitial position.
4. It has no impact on the solid's chemical properties.
5. Moreover, electrical neutrality is preserved in this defect.

Examples of Frenkel Defect

• Zinc Sulphide (ZnS)
• Silver Chloride (AgCl)
• Silver Bromide (AgBr)

The Difference Between the Schottky and Frenkel defects

• Definition

Schottky Defect: The loss of atoms from a crystal lattice in stoichiometric units causes the Schottky defect, a point defect.

Frenkel Defect: A form of point defect known as the Frenkel defect results when a single atom or a small ion leaves its original location and moves to occupy an interstitial location

• Density

Schottky Defect: Due to the Schottky defect, the lattice's density is decreased.

Frenkel Defect: The density of the lattice is unaffected by the Frenkel defect.

• Mass

Schottky Defect: The lattice's bulk is lowered when this defect manifests.

Frenkel Defect: Even after the Frenkel defect has occurred, the mass does not change.

• Giving up on species

Schottky defect: A Schottky defect is produced when atoms or ions break out from the lattice.

Frenkel Defect: With a Frenkel defect, atoms or ions move from their positions but stay inside the lattice.

• Quantity of Ions

Schottky Defect: Ions in lattices of identical sizes exhibit the Schottky defect.

Frenkel Defect: Ion lattices with significant size discrepancies exhibit the Frenkel defect.

Conclusion

A gap in the lattice is produced by either the Schottky or Frenkel defects. The size difference between the cations and anions in the lattice significantly determines the defect type. Additionally, the Frenkel defect does not impact the lattice density, but the Schottky defect causes a drop in that density. The primary distinction between the Schottky defect and the Frenkel defect is this.