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Crystal Defects: Point Defects

Introduction

To make crystalline solids, many tiny crystals are linked together. Several types of defects are identified in crystals during the crystallization process.

Point defects are compensated for when the crystallization process moves at a fast pace. A mismatch of the component particles causes the bulk of these defects. When the precise arrangement of solids in a crystalline solid is broken around a point/atom, it is called a point defect.

The four forms of defects found in crystalline solids are line faults, point defects, volume defects, and surface defects. Point defects will be discussed here.

Crystallographic flaws

Although no crystal is entirely regular, the regular atomic arrangement is an essential property of crystals. Any deviation from this ideal atomic periodicity is referred to as an imperfection, sometimes known as a lattice fault.

A lattice defect is a circumstance in which the atomic organization in a smaller piece of the material deviates from the uniformity of the structure.

Types of Point Defects

There are essentially four forms of point defects:
1. Stoichiometric Defect
2. Non-Stoichiometric Defect
3. Frenkel Defect
4. Schottky Defect

Definition of Stoichiometry

Stoichiometry is precisely what it sounds like. In a chemical process, it is the numerical relationship between the number of various products and reactants. Chemical reactions must be balanced, which means that the products must contain the same number of different atoms as the reactants. It must obey three laws- the law of conservation of mass, the law of constant composition, and the law of definite proportions.

Defects in Stoichiometry

These are point flaws that do not affect the solid's stoichiometry. These flaws are sometimes known as inherent or thermodynamic flaws.

  • Defects of vacancy
  • Defects in the interstitial space.

Vacancy Defects: A vacancy defect occurs when a few lattice sites in a crystal are unoccupied. As a consequence, the material's density decreases. When a material is heated, this fault might also appear.

Interstitial Defect: An interstitial defect occurs when specific component units, such as atoms, occupy an interstitial location in a crystal. The mass of the material is increased as a result of this flaw.

Frenkel defect is a combined vacancy and interstitial defect whereas Schottky defect is simply an example of a vacancy defect.

Non-Stoichiometric Defects
Non-stoichiometric imperfections are defects in crystals that disrupt the stoichiometry of the chemical compound. As a result, they do not obey the three laws of conservation of mass, constant composition law, and definite proportions. Non-Stoichiometric Defects are classified into two categories:

Metal Excess Defects - As the name implies, this defect occurs when metal ions are present in excess in the crystal lattice. It can be accomplished in one of two ways:

  • Anionic Vacancy- When a negative ion or anion is absent from its lattice position, a vacancy is created. To keep the existing electric charge constant, an electron fills this vacancy. It is referred to as the F - center. The chemical's color is thought to be due to its F-centre ion.
     
  • Excess Cations - When certain crystals are heated, extra cations can fit within the interstitial location. To keep the crystal's electrical neutrality, an equal number of electrons do the very same thing.

Metal Deficiency Defects - Some compounds have a more significant metal deficiency than their optimal described stoichiometric proportions. It is most common in transition elements, which have various valencies.

  • Frenkel Defect

A Frenkel defect is a type of point defect. An atom or ion occupies another unoccupied site on the same crystal while leaving its lattice site empty. It is also known as a dislocation defect, and it reveals vacancy and self-interstitial flaws.

Frenkel Defect Characteristics

The Frenkel defect has the following characteristics:

  • In these defects, ions leave the lattice but remain in the interstitial region of the crystal lattice.
  • It does not influence the density of the atom.
  • In these defects, ions forsake their lattice locations.
  • The Frenkel defect occurs in ionic crystals with large ion size fluctuations (anions and cations).
  • The Frenkel defect, also known as a dislocation defect, is seen in compounds such as NaCl, ZnS, AgI, and others.
  • Schottky Defect

The Schottky defect is another sort of point defect. A Schottky defect is a point defect or imperfection in solids generated by atoms or ions migrating out from the crystal's interior to the surface, resulting in a space in the crystal lattice.

Schottky Defect Characteristics

The following are some of the unique characteristics of the Schottky defect:

  • The difference in size between the cation and the anion is insignificant in the Schottky defect.
  • The solid crystal releases both the anion and the cation.
  • Atoms depart the crystal forever, unlike the Frenkel defect, where they remain in the crystal.
  • The density of the solid decreases.
  • Silver bromide (AgBr), potassium chloride (KCl), and potassium bromide (KBr) are examples of chemicals that exhibit the Schottky defect.

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