Point Defects

Volume defects, point defects, surface defects, and line defects are the four types of faults or imperfections seen in crystalline solids. Point defects are flaws/faults in solid crystals. Crystal point flaws were initially discovered in ionic crystals; simpler metal crystals seldom exhibit faults.

Many tiny crystals join together to form crystalline solids in a process called crystallisation. Sometimes crystals may display many forms of point defects on completion of the crystallisation process.

Point defects usually occur when the crystallisation process happens quickly, disrupting the placement of crystals around an atom. A misalignment of the particles of constituents is often the cause of such flaws. 

Point defects occur when: 

• One or more crystal atoms are absent from their respective lattice position.
• The atom(s) moves from its appropriate lattice location to the crystal’s interstitial position.
• Foreign atoms occupy the interstitial site in the crystal lattice.
• An alien atom replaces one of the crystal’s native atoms. 

Types of Point Defects

The three types of point defects:

Schottky Defect

Ionic solids have this type of vacancy defect. However, with ionic compounds, it is necessary to balance the electrical neutrality of the complex by removing an equal amount of anions and cations. The density of the material decreases in this defect.

Sodium chloride is the best example of the Schottky defect. 

One can further classify this defect into two types.

• Nonstoichiometric Defect: It occurs when the addition or removal of ions disturbs the ratio of cations to anions.
• Impurity Defect: An example can best explain this kind. When molten NaCl crystallises with the SrCl2 molecule, the Sr2+ ions replace two Na+ ions and take their position. As a result, one Na+’s lattice position becomes unoccupied, resulting in an impurity.

Frenkel Defect

In most ionic materials, the smaller ion (cation) displaces the larger ion (anions) and takes up intermolecular space. In this situation, a vacancy defect occurs in the original location, while an interstitial fault occurs in the new site.

• It is also called a dislocation defect.
• A substance’s density remains constant.
• It occurs when anions and cations have a significant size difference.
• ZnS and AgCl are two examples.

Stoichiometric Defect

The stoichiometric ratio is between negative and positive ions, and this type of point defect does not alter the neutrality of a solid. Inherent or thermodynamic flaws are other names for this defect.

They have two sub-categories:

• Vacancy defect: If an atom is absent from its lattice position, that lattice site becomes vacant, resulting in a vacancy defect. The density of a material lowers as a result of this defect.
• Interstitial defect: The density of the material rises due to this flaw.

The significant flaws in a non-ionic compound are interstitial faults and vacancy. In Schottky and Frenkel defects, an ionic compound exhibits the same behaviour.

Types of Non Stoichiometric Defect

Suppose the cations to anions ratio differentiates from that indicated by the ideal chemical formula due to the imperfections in a crystal. In that case, the defects are called nonstoichiometric defects.

Metal excess defects:

A negative ion may be absent from its lattice location, leaving a hole that an electron fills, preserving electrical neutrality. The F-centres are the interstitial sites hosting the electron, consequently trapped in the anion vacancies. They are in charge of giving the crystals their colour. 

Example: When NaCl melts in the presence of Na vapours, the surplus Na atoms deposit on the surface of the crystal. It causes Cl- ions to diffuse to the surface and interact with Na+ ions, resulting in the loss of electrons. 

These electrons are dispersed back into the crystal and occupy the empty site formed by Cl- ions, giving the NaCl crystal its yellow hue.

Extra cation present at interstitial sites:

An additional cation occupying the interstitial site might also create a metal overload. For example, when ZnO is heated, it loses oxygen and becomes yellow due to the extra interstitial sites and electrons in nearby interstitial places in ZnO. 

Zn+2 + (1/2)O2 + 2e-

Metal deficiency defect: 

When a metal’s valency changes, it is called a metal deficiency defect. The chemicals produced are nonstoichiometric due to a lack of metal. Because Ferrum occurs as both Fe+2 and Fe+3 ions, it isn’t easy to make ferrous oxide with a perfect composition. As a result, we get Fe0.95O or FexO with x = 0.93 to 0.96.

Applications of Point Defects

There are numerous applications of various flaws, as described below.

Application of Interstitial Defect

Even a small quantity of an interstitial impurity can stop the layers from sliding past one another if it forms a polar covalent link with the host atoms.

Since iron forms polar covalent connections with carbon, the strongest steel, for example, needs just around 1% carbon by mass to significantly boost its strength.

Application of Deformation Defect

Flexon is a nickel and titanium-based alloy that is both flexible and fatigue-resistant. Metallurgists were the first to find it when developing titanium-based alloys for use in missile heat shields. 

Flexon is now available as a corrosion-resistant and durable frame for glasses, among other applications.

Application of Substitutional Defect

Substitutional impurities can be seen in molecular crystals if the host’s impurity structure is comparable. They have the most significant impact on crystal characteristics. 

Pure anthracene, for example, is an electrical conductor. Despite their greater structural similarities, the presence of even small traces of tetracene in anthracene crystals delays electron transport across a molecule.

Conclusion

The article focuses on describing all the critical aspects of point defects. It discusses the different types, including stoichiometric and nonstoichiometric point defects. Further, it explores the applications of the point defects.