Imperfection in Solids: When we talk about solids, we mean that strong forces of attraction hold the constituent particles together and that the particles of the solid are packed in proper order or symmetry as a result of this force. Solids, on the other hand, also have particular imperfections that give them distinct characteristics. We’ll learn more about these imperfections and how they affect the solid-state in this post.

Defects in the Crystal

There are no crystals that are perfectly formed. A crystal can only be perfect at absolute zero \({\rm{K}}\), which is impossible to achieve.

Study Various Defects in Solids Here

A crystal defect is defined as a divergence from the complicated order and periodic arrangement of the particles in a crystal lattice.

Crystal Defects and Their Causes

The list of factors that cause crystal defects are explained below:

1. The lattice’s vacancies
2. A particle dislocates in the lattice.
3. Ion proportions that aren’t stoichiometric
4. In the lattice, there are impurities.

Types of Defects in Solids

Crystal defects are characterized as point defects, line defects, or surface defects because they appear as points, lines, or a surface.

A point defect is an imperfection in the crystal lattice caused by a particle dislocating from one place in the lattice to another.

If the flaw extends over a tiny region along a row of the crystal lattice, it is known as a line defect.

Types of Point Defects

The various types of point defects are explained below:

Stoichiometric Defects

This type of defect is also known as an inherent defect or a thermodynamic defect. The relative proportion of the elements in the solid is not altered in a stoichiometric defect. When there is a vacancy in the crystal lattice, or when constituent particles shift from their original positions to occupy interstitial sites, this defect occurs. In both non-ionic and ionic systems, stoichiometric defects have been observed.

Stoichiometric defect in non-ionic solids

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

I. Vacancy defect

I. It occurs when some crystal lattice sites are unoccupied, i.e., there are no solid particles at those locations.
II. This form of imperfection can be seen in metals as well as non-ionic solids.
III. When the crystal is heated rapidly, this defect occurs.
IV. The density of the crystal is reduced as a result of this imperfection.

II. Interstitial defect

i. When some more crystal constituent particles occupy the interstitial site, but there is no vacant site in the lattice, this defect occurs.
ii. This form of flaw can be seen in metals as well as non-ionic solids.
iii. As a result of this imperfection, the crystal’s density rises.

Stoichiometric Defect in Ionic Solids

There are two types of such defects in ionic solids-

I. Schottky defect

Schottky, a German scientist, first proposed this defect in \(1930\).

This type of imperfection is also known as a point defect. It occurs when from the lattice, an equal number of cations and anions are absent. The presence of the Schottky defect has no effect on the crystal’s stoichiometry. However, the presence of this flaw reduces the solid’s density. The Schottky defect provides a simple mechanism for atoms to move about within the lattice. This imperfection can be observed in ionic chemicals such as \({\rm{NaCl}},{\rm{KCl,}}\) and others.

Consequences of Schottky defect

i. This imperfection reduces the solid’s density.
ii. This imperfection creates a simple method for atoms or ions to travel through the crystal lattice.
iii. This imperfection has no effect on the lattice’s stoichiometry.

II. Frenkel Defect

In \(1926\), Russian scientist Frenkel proposed the concept of this imperfection.

The Frenkel defect occurs when an ion departs from its regular position and occupies an interstitial site in a crystal. This defect is particularly common in crystals where the cation and anion have large differences in size. The density of the crystal is unaffected by this imperfection.

Consequences of Frenkel Defect

i. The solid’s density is unaffected by this imperfection.
ii. This imperfection creates a simple method for atoms or ions to travel through the crystal lattice.

Nonstoichiometric Defects

Nonstoichiometric defects occur for two reasons:

1. The solid substance’s lattice contains a higher proportion of cations than anions. This imperfection is also known as a metal excess imperfection.
2. In the lattice, the solid substance has a lower fraction of cations than anions. A metal deficiency defect is another name for this defect.

I. Metal Excess Defect

When the crystal lattice contains a higher proportion of metal cations than anions, this phenomenon occurs.

There are two kinds of imperfections:

i. Anionic vacancy- This sort of defect occurs when an anion is missing from its place, and an electron fills the void to keep the solid electrically neutral. The F-centre refers to an electron trapped in an anion vacancy in a crystal lattice. Color centers are the voids in the crystal lattice that contain electrons.

The given figure below describes this defect-

This type of defect can be found in alkali metal halides such as \({\rm{NaCl}}.{\rm{KCl}},\) and \({\rm{LiCl}}\).

Alkali halides are usually colorless solids. They can, however, have distinct colors due to the existence of the F-centre.

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

ii. Extra cations- When an extra cation occupies an interstitial space in the crystal lattice, this sort of defect occurs. A suitable amount of electrons occupying other interstitial sites of the crystal lattice balances the excess positive charge.

The figure given below describes this defect-

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

II. Metal deficiency defect

It occurs when the proportion of metal cations in the crystal lattice is lower than the proportion of anions.

When cations in different oxidation states coexist in a compound, this defect occurs. For instance, in the crystals of iron and copper complexes.

Impurity Defects in Ionic solids

Impurities can be introduced into ionic solids by adding a chemical impurity that comprises ions with distinct valence states.

For example- When a small amount of \({\rm{SrC}}{{\rm{l}}_2}\) is introduced to \({\rm{NaCl}}\) as an impurity, a solid solution is formed in which the \({\rm{S}}{{\rm{r}}^{2 + }}\) ions occupy some of the lattice sites of the \({\rm{N}}{{\rm{a}}^ + }\) Ion. To maintain electrical neutrality, two ions of \({\rm{N}}{{\rm{a}}^ + }\) must be removed for every \({\rm{S}}{{\rm{r}}^{2 + }}\) ion introduction. In the lattice, this results in a cation vacancy.

The below-given figure shows this defect-

Summary

A deviation point defect occurs in an ideal crystal when there are any anomalies in the arrangement around any point or atom in the crystal. Similarly, a line defect occurs when there is a variation in the arrangement of the entire row of lattice points in crystals.

In this post, we looked at the defects or imperfections that can occur in solids. Using figures and pictures, we examined point flaws in-depth, including their types. The causes of anomalies and the features that come from them are now well understood.

FAQs on Imperfections in Solids

Q.1. What are the imperfections or defects in solids?
Ans: There are no crystals that are totally perfect. An imperfection in solids is a departure from the complex order and deviation from the periodic arrangement of the particles in a crystal lattice.
Defects in solids occur as a result of the following factors:
1. In the lattice, there are a few vacancies.
2. A particle dislocates in the lattice.
3. Ion proportions that aren’t stoichiometric.
4. Impurities in the lattice.

Q.2. How many types of solid defects are there?
Ans: There are four main types of defects or imperfections in solids that are as follows-
1. Line defects
2. Point defects
3. Volume defects
4. Surface defects

Q.3. What are point defects in solid-state?
Ans: Point defects in the solid-state are imperfections in the crystal lattice caused by the displacement of a particle from one position to another in the lattice. They are not spatially extended in any way.
Point defects can be classified into three categories.
1. Stoichiometric defects
2. Nonstoichiometric defects
3. Impurity defects

Q.4. What are the impurity defects in solids?
Ans: Impurities can be introduced into ionic solids by adding a chemical impurity that comprises ions with distinct valence states. This is known as impurity defects in solids.
For example- When a small amount of \({\rm{SrC}}{{\rm{l}}_2}\) is introduced to \({\rm{NaCl}}\) as an impurity, a solid solution is formed in which the \({\rm{S}}{{\rm{r}}^{2 + }}\) ions occupy some of the lattice sites of the \({\rm{N}}{{\rm{a}}^ + }\) Ion. To maintain electrical neutrality, two ions of \({\rm{N}}{{\rm{a}}^ + }\) must be removed for every \({\rm{S}}{{\rm{r}}^{2 + }}\) ion introduction. In the lattice, this results in a cation vacancy.

Q.5. What types of defects arise in solids?
Ans: Four main types of defects or imperfections arise in solids which are as follows-
1. Line defects
2. Point defects
3. Volume defects
4. Surface defects

Q.6. What are Schottky defects in solids?
Ans: It occurs when an equal number of cations and anions are absent from the lattice. The presence of the Schottky defect has no effect on the crystal’s stoichiometry. However, the presence of this flaw reduces the solid’s density. The Schottky defect provides a simple mechanism for atoms to move about within the lattice.
This imperfection can be observed in ionic chemicals such as \({\rm{NaCl}},{\rm{KCl}},\) and others.