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:
The three types of point defects:
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.
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.
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:
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.
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.
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-
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.
There are numerous applications of various flaws, as described below.
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.
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.
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.
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.
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