This article provides a comprehensive explanation of point defects, which refer to the
flaws found in solid structures
, as well as the various types of point defects. Crystalline solids are created by combining many tiny crystals. Once the process of crystallization is complete, different kinds of defects can be identified within the crystals.
Point defects are usually observed when the process of crystallization happens at a rapid pace. These defects typically occur due to irregularities in the arrangement of the constituent particles. In a
crystalline solid
, a point defect is a distortion in the ideal arrangement of solids around a specific point or atom.
The flaws or imperfections in a crystalline solid can be classified into four primary groups, namely
line defects, point defects, volume defects, and surface defects.
Historically, point defects were first identified in ionic crystals, which are more complex than metal crystals.
Point defects can be categorized into three main types:
This type of point defect does not disturb the ratio of positive to negative ions (stoichiometric) or the electrical neutrality of a solid. It is sometimes also referred to as intrinsic or thermodynamic defects.
They are fundamentally of two types:
Vacancy defect:
This defect arises when an atom is absent from its lattice site, leaving the site vacant and creating a vacancy defect. This causes a decrease in the density of the substance.
Interstitial defect:
This defect is characterized by an atom or molecule occupying the space between molecules in crystals. This defect increases the density of the substance.
Non-ionic compounds mainly exhibit vacancy and interstitial defects. An
ionic compound
exhibits these defects in the form of Frenkel and Schottky defects.
2. Frenkel Defect:
In ionic solids, the smaller ion (cation) often moves from its original position to occupy an intermolecular space. This results in a vacancy defect at its original position and an interstitial defect at its new location.
It is also referred to as a dislocation defect.
The density of the substance remains the same.
It occurs when there is a significant size difference between anions and cations.
This type of vacancy defect is found in Ionic Solids. However, in ionic compounds, it is necessary to maintain the electrical neutrality of the compound, which means an equal number of anions and cations will be absent from the compound.
It decreases the density of the substance.
In this type of defect, the sizes of the cations and anions are almost the same.
Impurity Defect:
For instance, if molten KCl is crystallized with
BaCl
2
compound, the Ba
2+
ions replace two K
+
ions and occupy the place of one K
+
ion. This leaves the lattice site of one K
+
ion vacant, creating an impurity defect.
Non-Stoichiometric Defect:
This defect occurs when the
ratio of cations to anions
is disturbed either due to the addition or removal of ions.
Types of Non-Stoichiometric Defect:
Metal deficiency defect:
This defect is characterized by the solids having fewer metals compared to the defined Stoichiometric proportion.
Metal excess defect:
There are two types of metal excess defects:
Metal excess defect due to anionic vacancies
:
This defect occurs when anions are absent from their original lattice site in crystals. As a result, electrons occupy their position instead of anions.
Metal excess defect due to the presence of extra cations at interstitial sites
:
Here, upon heating the compound, it releases extra cations. These cations occupy the interstitial sites in crystals, and the same number of electrons moves to nearby interstitial sites.
There is a vast amount of information to learn about the structure of crystalline solids and the defects they may contain. This article merely provides an overview of point defects in solids.
Point defects are zero-dimensional lattice defects, meaning they have no lattice structure in any dimension. Impurity atoms in a pure metal, vacancies, and self-interstitials are examples of point defects.
What are point defects in solids?
Defects that occur only at or near a single lattice point are known as point defects. They are not physically extended in any way. In most cases, there are no clear constraints on how small a point defect can be. These flaws, on the other hand, usually only entail a few extra or missing atoms.
Which is a line defect?
Line defects, also known as dislocations, are lines in a solid along which entire rows of atoms are arranged abnormally. The ensuing spacing irregularity is most apparent along a line known as the line of dislocation. Solids can be weakened or strengthened by line faults.
What is point defect in crystal?
The simplest defects that can be found in any crystal phase are point defects. They are focused on single crystal structure sites, which may be consistently inhabited by chemical species or otherwise unoccupied sites of the vacant interstitial sublattice.
Is dislocation a point defect?
The interaction between a dislocation and a point defect is modelled as follows: a dislocation, which is considered to be straight, is located at a glide plane projected distance from a point defect, which is located at a height z above the glide plane.