Open In App

Imperfections or Defects in a Solid

Last Updated : 11 Feb, 2022
Improve
Improve
Like Article
Like
Save
Share
Report

Matter can exist in broadly three states named solids, liquids, and gases. Solids are those substances that have short intermolecular forces between them that keep molecules (atoms or ions) closely packed. They have definite mass, volume, and shape. Their intermolecular forces are strong and intermolecular distances are short. They are rigid and incompressible.

Solids can be classified as crystalline or amorphous based on the nature of order present in the arrangement of their respective constituent particles. A crystalline solid is a large number of small crystals. These crystals have a definite characteristic geometrical shape. The arrangement of particles inside these crystals is ordered and is repeated on all three dimensions.

Imperfections or Defects in Solids

Though crystalline solids are arranged in short-range and long-range order, the crystals are not perfect. A solid usually consists of a large collection of small crystals. These small crystals have defects or imperfections in them.

Defects occur in crystals when the crystallization process takes place at a very fast or moderate rate. Single crystals are formed when the crystallization process takes place at an extremely slow rate. We consider the defects to be irregularities in the arrangement of constituent particles. Defects are considered to be of two types: Point defects and Line defects.  

Point defects are the deviations or irregularities from the model arrangement around a point or an atom in a crystalline substance, whereas line defects are the irregularities or deviations from the model structuring arrangement in complete rows of different lattice points.

These irregularities are called crystal defects. Point Defects are of three types:

  1. Stoichiometric Defects
  2. Impurity Defects
  3. Non-Stoichiometric Defects

Stoichiometric Defects

Stoichiometric Defects are basically the point defects that don’t disturb the stoichiometry of the given solid. Stoichiometry is the relationship between the given quantities of reactants and respective products before, during, and following the chemical reactions. Stoichiometric Defects are also called intrinsic or thermodynamic defects.

  • Vacancy Defect

In the crystals, when some lattice sites are left vacant, the crystal is said to have a vacancy defect. This causes a decrease in the density of the given substance. This defect can also form when the substance is heated. 

Vacancy Defect

  • Interstitial Defect

In the crystals, when some constituent particles occupy an interstitial site, the crystal is said to have an interstitial defect. Contrary to vacancy defect, this causes an increase in the density of the given substance.

Interstitial Defect

Note: Vacancy and Interstitial Defects are observed in non-ionic solids. Ionic solids need to maintain electric neutrality. Thus, they do not have simple Vacancy and Interstitial defects but have Frenkel and Schottky Defects.

  • Frenkel Defect

This defect is present in ionic solids. The smaller ion (mostly the cation) gets dislocated from its normal site and gets moved to an interstitial site. This shift in ion creates a vacancy defect at the original site and an interstitial defect at the new location. Due to this, the Frenkel defect is also called the dislocation defect. It doesn’t affect the density of the solid as the ion is still present in the structure. This defect is found mostly in ionic substances where there is a large difference between the size of the ions. For example, ZnS, AgCl, AgBr, and AgI show Frenkel defects as Zn2+ and Ag+ ions are small in size.

Characteristic of Frenkel Defect

  1. This defect occurs exclusively when the cations are smaller than the anions. There are no changes in chemical attributes as well.
  2. The Frenkel defect also has no effect on the density of the solid, hence both the volume and mass of the solid are conserved.
  3. Substances retain their electrical neutrality in such instances.
  4. As the like charge ions get closer together, the dielectric constant rises.
  5. Because of the presence of unoccupied lattice sites, materials with Frenkel defects exhibit conductivity and diffusion in the solid state.
  6. The Frenkel flaw reduces the lattice energy and stability of crystalline solids. This flaw has an impact on the chemical characteristics of ionic compounds.
  7. The solid’s entropy rises.

Frenkel Defect

  • Schottky Defect

The Schottky defect is like a vacancy defect in the ionic solids. The number of mission cations and anions are always constant in this defect to uphold electric neutrality. Just like a simple vacancy defect, the Schottky defect also ends up decreasing the density of the given substance. The number of defects in ionic solids is pretty significant. This defect can be observed in ionic solids having cations and anions of similar sizes. For example, KCL, NaCl, CsCl, and AgBr.

Characteristics of Schottky Defect

  1. The size difference between cation and anion is negligible.
  2. Both cation and anion depart the solid crystal.
  3. Atoms are also permanently ejected from the crystal.
  4. In most cases, two positions are created.
  5. The density of the solid decreases significantly.

Schottky Defect

Exception: AgBr shows both Frenkel and Schottky defects.

Impurity Defects

In impurity defects, foreign components or impurities replace the position of existing ions. For example, if molten NaCl is crystallized in the presence of SrCl2, some of the sites of Na+ ions can be occupied by Sr2+ ions. To maintain electric neutrality, each Sr2+ ion will replace two Na+ ions. Sr2+ ion occupies the site of one Na+ ion and the other Na+ ion site remains vacant. Thus, the number of vacancies created is also equal to the number of Sr2+ ions present.

Other examples of impurity defects are CdCl2 and AgCl.

Impurity Defect

Non-Stoichiometric Defects

A large number of nonstoichiometric inorganic solids are known which contain the constituent elements in a non-stoichiometric ratio due to defects in their crystal structures. These defects are of two types: 

  1. Metal excess defect
  2. Metal deficiency defect
  • Metal Excess Defect

Metal excess defect due to anionic vacancies – this defect is observed in alkali halides like NaCl and KCl. In this defect, crystals have an excess of cation because the anion present combines with an external ion to form another compound. For example, when NaCl crystals are heated in presence of sodium vapors, the Cl ions diffuse and form NaCl with the sodium vapor. Thus, the crystals now have an extra Na+ ion present. This anionic site is occupied by an unpaired electron. We call this site an F-center. This word is derived from the German word Farbenzenter for color center). This gives off a yellow color to the NaCl crystals. The color appears from the excitation of these electrons when they absorb energy from the visible light falling on the crystals. Just like the above cases, excess Lithium makes LiCl crystals pink and excess Potassium makes KCl crystals violet (or lilac).

Consequences of metal excess defect

The existence of free electrons in crystals with metal excess flaws causes them to be coloured. Because of the presence of free electrons, crystals with metal excess flaws conduct electricity and are semiconductors. Because the electric transport is mostly accomplished by “excess” electrons, these are referred to as n-type (n for negative) semiconductors. Because of the existence of unpaired electrons at lattice sites, crystals with metal excess defects are often paramagnetic. When the crystal is exposed to white light, the trapped electron absorbs some of it in order to be stimulated from the ground state to the excited state. Color is created as a result of this. These are known as F-centres. Positive ion vacancies accompany such surplus ions. These vacancies serve the same purpose as anion vacancies in trapping electrons. V-centres are the colour centres that result from this process.

F-center in a crystal

The metal excess defect is caused by the presence of extra cations at the interstitial sites – at room temperature, Zinc oxide is white. When it gets heated, it loses oxygen and turns from white to yellow.

ZnO ⇢ Zn2+ + ½O2 + 2e (in presence of heat)

As you can see in the above equation, there is now an excess of Zinc in the crystal and its formula becomes Zn1+xO. The extra Zn2+ ions get shifted to interstitial sites and the respective electrons to neighboring interstitial sites.

  • Metal Deficiency Defect

As we know, it is tough to prepare several solids in stoichiometric composition. These solids have less amount of metal as compared to stoichiometric proportion.

A perfect example of this type is FeO which is mostly found with a composition of Fe0.95O. It may range from Fe0.93O to Fe0.96O. In the crystals of FeO, there is a loss of positive charge due to the missing Fe2+ cations. The loss of charge is covered by the presence of the Fe3+ ions to make the substance electrically stable.

Sample Problems

Question 1: Which defects decrease the density of the substance? Explain those defects. 

Answer:

Density of substance decreases in Vacancy defect and Schottky Defect. 

In the crystals, when some lattice sites are left vacant, the crystal is said to have a vacancy defect. This causes a decrease in the density of the given substance. This defect can also form when the substance is heated.

Schottky defect is basically a vacancy defect in given ionic solids. The number of mission cations and anions are always constant in this defect to uphold electric neutrality. Just like a simple vacancy defect, the Schottky defect also ends up decreasing the density of the given substance. The number of defects in ionic solids is pretty significant. This defect can be observed in ionic solids having cations and anions of similar sizes. For example, KCL, NaCl, CsCl and AgBr.

Question 2: What is the dislocation defect? Give examples. 

Answer:

Frenkel defect is present in ionic solids. The smaller ion (mostly the cation) gets dislocated from its normal site and gets moved to an interstitial site. This shift in ion creates a vacancy defect at the original site and an interstitial defect at the new location. Due to this, the Frenkel defect is also called the dislocation defect. It doesn’t affect the density of the solid as the ion is still present in the structure. This defect is found mostly in ionic substances where there is a large difference between the size of the ions. For example, ZnS, AgCl, AgBr, and AgI show Frenkel defects as Zn2+ and Ag+ ions are small in size.

Question 3: Which compound has both Frenkel and Schottky defects?

Answer:

Silver Bromide (AgBr) is the compound that shows both Frenkel and Schottky defects.

Question 4: Explain how defects are different in non-ionic and ionic solids. 

Answer:

Vacancy Defect – In the crystals, when some lattice sites are left vacant, the crystal is said to have a vacancy defect. This causes a decrease in the density of the given substance. This defect can also form when the substance is heated.

Interstitial Defect – In the crystals, when some constituent particles occupy an interstitial site, the crystal is said to have an interstitial defect. Contrary to vacancy defect, this causes an increase in the density of the given substance.

Vacancy and Interstitial Defects are observed in non-ionic solids. Ionic solids need to maintain electric neutrality. Thus, they do not have simple Vacancy and Interstitial defects but have Frenkel and Schottky Defects.

Question 5: Why does NaCl acquire yellow color when subjected to excess sodium?

Answer:

When NaCl crystals are heated in presence of sodium vapours, the Cl ions diffuse and form NaCl with the sodium vapour. Thus, the crystals now have an extra Na+ ion present. This anionic site is occupied by an unpaired electron. We call this site an F-center. This word is derived from the German word Farbenzenter for colour centre). This gives off a yellow colour to the NaCl crystals. The colour appears from the excitation of these electrons when they absorb energy from the visible light falling on the crystals. Just like the above cases, excess Lithium makes LiCl crystals pink and excess Potassium makes KCl crystals violet (or lilac).

Question 6: What are line defects?

Answer:

Line defects are the irregularities or deviations from the model structuring arrangement in complete rows of different lattice points.

Question 7: What are thermodynamic defects? Mention its types. 

Answer:

Stoichiometric Defects are basically the point defects that don’t disturb the stoichiometry of the given solid. Stoichiometry is the relationship between the given quantities of reactants and respective products before, during and following the chemical reactions. Stoichiometric Defects are also called intrinsic or thermodynamic defects.



Previous Article
Next Article

Similar Reads

Point Defects
Solids are made up of a lot of small crystals. However, the defect in solids occurs during the crystallization process due to the fast or moderate rate of crystal formation. Defects are described as irregularities in the arrangement of constituent particles in general. The defect could be a point or a line defect based on the irregular arrangement.
9 min read
Class 8 NCERT Solutions - Chapter 10 Visualising Solid Shapes - Exercise 10.1
Question 1: For each of the given solid, the two views are given. Match for each solid the corresponding top and front views. The first one is done for you. Answer: Please find the matches in the below image. Question 2: For each of the given solid, the three views are given. Identify for each solid the corresponding top, front and side views. Answ
1 min read
Class 8 NCERT Solutions - Chapter 10 Visualising Solid Shapes - Exercise 10.3
Question 1: Can a polyhedron have for its faces(i) 3 triangles? (ii) 4 triangles? (iii) a square and four triangles? Solution: (i) 3 triangles: No, because polyhedron must have minimum 4 faces i.e all edges should meet at vertices. (ii) 4 triangles: Yes, as all the edges are meeting at the vertices and has four triangular faces. (iii) a square and
4 min read
Class 8 NCERT Solutions- Chapter 10 Visualising Solid Shapes - Exercise 10.2
Question 1. Look at the given map of a cityAnswer the following.(a) Colour the map as follows: Blue-water, red-fire station, orange-library, yellow- schools, Green - park, Pink - College, Purple - Hospital, Brown - Cemetery.(b) Mark a green ‘X’ at the intersection of Road ‘C’ and Nehru Road, Green ‘Y’ at the intersection of Gandhi Road and Road A.(
1 min read
Mapping Space Around Us - Visualizing Solid Shapes | Class 8 Maths
In Geometry, 3D shapes are known as three-dimensional shapes or solids, or solid shapes. 3D shapes or solid shapes have three different measures such as length, width, and height as its dimensions. A polygon is a 2D shape with straight sides. A regular shape has all sides the same length and all interior angles the same size. An irregular shape has
7 min read
How to Separate a Mixture of a Solid and a Liquid?
In our daily life, we use different products. And, always keep in search of pure products as mostly also found written on the packaging of items. But scientifically the term pure is used for those matters which are made up of particles that have the same chemical nature or formation. Hence, the majority of substances surrounding us are mixtures of
10 min read
Solid Wastes Management
Solid waste management​ refers to the systematic collection, transportation, treatment, analysis, and disposal of solid waste​s. It involves various processes and strategies to minimize environmental and health impacts, including waste reduction, recycling, composting, landfilling, and waste-to-energy technologies. Solid waste management is essenti
7 min read
Why Sound Travels Faster in Solid?
Answer: Sound travels faster in solids because the particles in solids are closely packed, leading to a higher elasticity and greater intermolecular forces, facilitating the transmission of mechanical waves at a quicker pace compared to liquids and gases.The speed of sound in a material is determined by its density, elasticity, and the intermolecul
1 min read
Amorphous Solid
Amorphous Solid is a type of solid matter characterized by the absence of a well-defined crystalline structure. The word amorphous means "having no definite form." Unlike crystalline solids, which exhibit a highly ordered and repeating three-dimensional lattice arrangement of atoms or molecules, amorphous solids lack long-range order. In this artic
9 min read
Solid State Chemistry
Solid State is a state of matter in which the shape and volume of matter remain unchanged irrespective of the container they are kept in. Solid state materials are of huge importance due to their wide ranging applications in daily lives. This versatility of solid state lead to formation of a separate field under the domain of chemistry named as Sol
15 min read