Defects in Solids: When we talk about solids, we say that the constituent particles are held together by strong forces of attraction, and because of this force, the particles of solid are packed in proper order or symmetry. However, there are certain defects in solids that give them specific characteristic features and we will talk about these in the coming sections.

There are different types of defects we will talk about like Stoichiometric defect, Crystal defect and Schottky defect. In this article, we will explore more about these defects, how they affect the solid-state, the consequences of the Schottky defect with reasons, explain the metal deficiency defect with examples and more.

What are Defects in Solids?

Any irregularity in the pattern of crystal arrangement in solids is called imperfection in solids. The occurrence of defects takes place when the process of formation of crystals happens. It may occur at a very fast or at an intermediate rate. It happens because particles do not get enough time to arrange themselves in a regular pattern.

Crystal Defect

No crystals are absolutely perfect. Only at the unattainable absolute \(0\,{\rm{K}}\) temperature will a crystal be perfect. A departure from the complex order and deviation from the periodic arrangement of the particles in a crystal lattice is called a crystal defect.

Causes of Crystal Defect

Defects occur in crystals due to the following factors:
1. Vacancies in the lattice
2. Dislocation of a particle in the lattice
3. Nonstoichiometric proportions of the ions
4. Impurities in the lattice

Types of Defects in Solids

Crystal defects occur as points, along lines or in the form of a surface, and they have been classified as point defects and line defects.

Point defect- The imperfection in the crystal lattice due to the dislocation of a particle from one position to another position in the lattice.

Line defect- The imperfection extending over a microscopic region along a row of the crystal lattice is known as a line defect.

Types of Point Defects

Stoichiometric Defects

This type of defect is also called intrinsic defect or thermodynamic defect. In stoichiometric defects, the proportion of the elements in the solid is not disturbed. This defect arises when there is some vacancy in the crystal lattice or when some constituent particles move from their original sites and occupy interstitial sites. Stoichiometric defects have been observed in both the non-ionic and ionic solids.

Stoichiometric Defect in Non-ionic Solids

There are two types of defects in non-ionic solids-

I. Vacancy defect

1. It arises when some sites in the crystal lattice are vacant; that is, there is no particle of the solid at some sites.
2. This type of defect is observed in metals and non-ionic solid substances.
3. This defect is caused when the crystal is heated abruptly.
4. This defect causes a decrease in the density of the crystal.

II. Interstitial Defect

1. This defect arises when some additional constituent particles of the crystal occupy the interstitial site. However, there is no vacant site in the lattice.
2. This type of defect is observed in metals and non-ionic solid substances.
3. This defect causes an increase in the density of the crystal.

Stoichiometric Defect in Ionic Solids

There are two types of such defects in ionic solids-

I. Schottky Defect

The idea of this defect was given by German scientist Schottky in \(1930\). Schottky defect in crystals is observed when:

This type of defect is also called point defect. It is observed when an equal number of cations and anions are missing from the lattice. The presence of the Schottky defect does not change the stoichiometry of the crystal. However, the presence of this defect lowers the density of the solid.

The presence of the Schottky defect provides a facile mechanism by which atoms can move within the lattice.

An example of this defect can be seen in \({\rm{ NaCl, KCl,}}\) and other such ionic compounds.

Write the Consequences of Schottky Defect with Reasons

1. This defect lowers the density of the solid.
2. This defect provides a facile mechanism by which atoms or ions can move within the crystal lattice.
3. This defect does not change the stoichiometry of the lattice.

II. Frenkel Defect

The idea of this defect was given by Russian scientist Frenkel in \(1926.\)

In a crystal, when an ion leaves its regular site and occupies an interstitial site, the defect is termed as Frenkel defect. This defect is most likely to occur in crystals in which the cation and anion differ in size. This defect does not alter the density of the crystal.

Frenkel defects provide a facile mechanism by which the ions can move within the crystal lattice.

An example of this defect can be seen in \({\rm{AgCl}}\) and \({\rm{AgBr}}.\)

Consequences of Frenkel Defect

1. This defect does not affect the density of the solid.
2. This defect provides a facile mechanism by which atoms or ions can move within the crystal lattice.

Non Stoichiometric Defect

Nonstoichiometric defects arise because of the following two reasons-

1. The solid substance contains more proportion of cations than anions in the lattice. This defect is also called metal excess defect.
2. The solid substance contains a lesser proportion of cations than anions in the lattice. This defect is also called a metal deficiency defect.

Metal Excess Defect

It arises when the crystal lattice contains more proportions of metal cations as compared to anions. There are two types of defects-

i. Anionic Vacancy

This type of defect arises when an anion is missing from its position, and an electron occupies that space to maintain the electrical neutrality of the solid. The electron trapped in an anion vacancy of a crystal lattice is referred to as the \({\rm{F}}\)-centre. The cavities containing electrons in the crystal lattice are colour centres.

The given figure below describes this defect-

Alkali metal halides show this type of defect like \({\rm{NaCl}},{\rm{KCl}},{\rm{LiCl}}\).

Normally alkali metal halides are colourless solids. However, they exhibit characteristic colours because of the presence of the F-centre.

Alkali halide	Colour of the crystal
Sodium Chloride	Yellow
Potassium Chloride	Violet
Lithium Chloride	Pink

ii. Extra Cations

The defect arises when an extra cation occupies an interstitial site in the crystal lattice. The extra positive charge is balanced by an appropriate number of electrons occupying other interstitial sites of the crystal lattice.

The figure given below describes this defect-

An example of this defect can be seen when zinc oxide is heated and loses electrons.

Explain Metal Deficiency Defect with Example

It arises when the crystal lattice contains a lesser proportion of the metal cations as compared to anions.

This defect has been observed when a compound has cations existing in different oxidation states. For example, in crystals of iron compounds and copper compounds.

Impurity Defects in Ionic solids

In ionic solids, the impurities can be introduced by adding a chemical impurity that contains ions in different oxidation states.

For example, when a small quantity of \({\rm{SrC}}{{\rm{l}}_2}\) is added as an impurity to \({\rm{ NaCl}}\) a solid solution is obtained in which \({\rm{S}}{{\rm{r}}^{2 + }}\) ions occupy some of the lattice points of \({\rm{N}}{{\rm{a}}^ + }\) ion. Introduction of every \({\rm{S}}{{\rm{r}}^{2 + }}\) ion leads to the removal of \({\rm{N}}{{\rm{a}}^ + }\) ion, thus maintaining electrical neutrality. The defect produces a cation vacancy in the lattice.

The below-given figure shows this defect-

How to Study Defects in Solids?

The defects were studied microscopically by crystallographers and mineralogists a long time ago.

The defects in solids can be studied by diffraction and microscopy and photographed by microscopic techniques.

The single crystal is never made of one piece of single crystals but constitutes of domains that are slightly oriented from each other forming mosaic crystals (an idealized model of an imperfect crystal, imagined consisting of numerous small perfect crystals that are to some extent randomly misoriented). The mosaic crystals can be studied by X-ray diffraction.

Consequences of Defects in Solids

Defects may be advantageous or disadvantageous as they affect material properties.
i. The interstitial defect increases the density of the substance.
ii. The closeness of the charge brought about by the Frenkel defect tends to increase the dielectric constant of the crystal. Compounds with such defects conduct electricity across the crystal.
iii. The presence of a large number of Schottky defects lowers the density of the crystal.

Summary

In this article, we studied the defects or imperfections that occur in solids. We studied in detail the point defects- their types through figures and examples. Now we know the causes of the defects and the characteristic features due to these defects.

FAQs on Defects in Solids

Frequently asked questions related to defects in solids are listed as follows:

Q.1: How many types of solid defects are there?
Ans: There are four main types of defects or imperfections in solids that are as follows-
Line defects
Point defects
Volume defects
Surface defects

Q.2: What are the imperfections or defects in solids?
Ans: No crystals are perfect. A departure from the complex order and deviation from the periodic arrangement of the particles in a crystal lattice is called an imperfection in solids.

Q.3: What are the impurity defects in solids?
Ans: In ionic solids, the impurities can be introduced by adding a chemical impurity that contains ions in different valence states. Such a defect is known as impurity defects in solids.

Q.4: What types of defects arise in solids?
Ans: Four main types of defects or imperfections arise in solids which are as follows-
Line defects
Point defects
Volume defects
Surface defects

Q.5: What are Schottky defects in solids?
Ans: The Schottky defect is observed when an equal number of cations and anions are missing from the lattice. The presence of this defect does not change the stoichiometry of the crystal. However, it lowers the density of the solid.
The presence of the Schottky defect provides a facile mechanism by which atoms can move within the lattice.
An example of this defect can be seen in \({\rm{ NaCl, KCl }}\), and other such ionic compounds.

Q.6: What are point defects in solid-state?
Ans: There are three types of Point defects-
Stoichiometric defects
Nonstoichiometric defects
Impurity defects

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