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Imperfections in Crystals-Types of Defects

Imperfections in Crystals


Here is the complete details of imperfection in crystals.In a sound crystal (or an ideal crystal), the atoms are arranged regularly and periodically in three dimensions. But developed crystals [or real crystals] may have flaws or defects. These defects are mainly divided into point, line, surface and volume defects. Imperfections in crystals are discussed below :

(1) Point defects: 

As the name indicates, these defects are found in some points of the glass. So these are also called zero dimension defects. The specific defects are divided into three categories: (a) defects of the website; This includes Schottky's defect and interstitials [ Frenkel's defect], (b) structural defects; is
This includes institutional impurities and interstitial impurities and (c) electronic defects.These defects are discussed below.

(a) Lattice site defects: In this some atoms may not exist in their regular atomic sites.They are:

Lattice site defects

(i) Vacancies: As shown in the given image in a network point, one or two or three atoms are lost, and they are called single, double or triple vacancies, respectively. Vacancies occur due to incomplete packaging during crystallization or due to thermal vibration at high temperatures.

Schottky defect

In ionic crystals, if there is a cation vacancy, there is also an ion vacancy in a very close space. That is the ions and the cation pair are generally transported to the surface of the crystal to charge neutrality.
The vacancy area is shown according to the given figure.This is called the Schottky defect. Crystals like NaCl, KCl, KBr, etc. denote Schottky's defect.

(ii) Interstitial defect: 

Frenkel defect: In ionic crystals, if a cation [positive ion] moves to an interstitial space, a blank is formed in its atomic position. Here, charge neutrality is maintained in the defective region as shown in the given figure. This type of defect is known as Frenkel defect. Crystals such as CF2, AgBr, AgI etc. denote the Frenkel defect.

( B ) Compositional defect: Compositional defect are caused by the presence of impurity atoms in the crystal. Regularity atoms are present at the locations of regular parent atoms or in interstitial locations. These defects are described below.

(i) Substanal defects: As shown in the given figure. During crystallization some foreign atoms occupy the space containing regular parent atoms. For example, in external semiconductors either the third or the phyth group occupies the sites of atoms, silicon or germanium atoms.

Substitutional defect and interstitial defect

(ii) Interstitial impurity defects: The spaces between parental atoms in the crystal are known as interstitial spaces. Small-sized [low atomic numbers] atoms, such as hydrogen, etc. can fit in these interstitial locations. These atoms are known as interstitial atoms and the defect due to the presence of differential atoms is Known as interstitial defects.
Three-dimensional view of perfect crystal; front view of perfect crystal

(c) Electronic defect: 

Non-homogeneity of charge or energy distribution in a crystal is called an electronic defect. The presence of impurity atoms, such as sensory and interstitial atoms and vacancies, can cause uniform distribution of electronic charge in the crystal. So, the presence of these defects also leads to electronic defects. In semiconductors, temperature variation charges the concentration, so temperature variation [i.e., thermal energy] leads to electronic defects.

(2) Line defect: 

If a crystal plane ends up somewhere in the crystal, it creates a defect in the crystal line at the edge of that incomplete plane.

There are two types of fault lines: they are
(i) edge dislocation and (ii) screw dislocation.
These are described below.
positive and negative edge dislocations

(i) Edge dislocation: In the given diagram. One shows a three-dimensional view of the ideal crystal and a front face. The vertical crystal planes are parallel to the side faces of a crystal as shown in the figure.
(ii) Screw dislocation: Crystal planes spiral about a line in a crystal, called a dislocation line. The screw dislocation is shown in the figure below.Due to the spiraling of crystal planes, atoms at one end of the plane are displaced by an atomic distance in relation to the other end of the plane in a direction perpendicular to the plane.

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