Defects in Crystal Structure, Types of Defects, Semiconductor, Electrical and Magnetic Properties

Defects in Crystal Structure are imperfections in the regular geometrical arrangement of the atoms in a crystalline solid. Defects in Crystal Structure result from the deformation of the solid, rapid cooling from high temperatures, or high-energy radiation (X-rays or neutrons) striking the solid. In this article, we will learn about Defects in Crystal Structure, its Types like Point Defects, Line Defects, Vacancy Defects, Interstitial Defects, and Impurity Defects. We will also learn about semiconductors, Electrical and Magnetic Properties like Diamagnetism, Paramagnetism and the Ferromagnetism of Crystals.

Defects in Crystal Structure

Crystalline solids are formed by the regular repetition of a large number of unit cells in all directions. There is a short-range as well as a long range order in the arrangement of all constituent particles so that the crystalline solid is a perfect crystal. A perfect crystal has all constituent particles, atoms or ions arranged in a precise geometric order. Real crystals, like real gases, are never perfect. They always contain some fault or imperfection in the formation of the crystal lattice.

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Here are some key features of crystals:

• All big crystals are found to have some defects in the arrangement of constituent particles.
• The defects are more if the process of crystallisation occurs at a faster rate. The defects can be minimised by carrying out crystallisation at the slowest rate. So that starting with one single crystal, deposition crystals take place from all directions on the original crystal.
• The defects are produced by irregularities in the arrangement of constituent particles.
• The crystal defects change the original physical and chemical properties of the crystalline solids.

The crystal defects are of four types.

• Point Defect: It is further classified into Vacancy Defect, Interstitial Defect and Impurity Defect.
• Line Defect.
• Surface Defect.
• Volume Defect.

Point Defect

The defect is due to a fault produced in the arrangement of a point i.e. a constituent particle like atom, ion or molecule in a crystalline solid.

The point defects are classified into three types

1. Vacancy Defect-Sometimes during crystallisation some of the places of the constituent particles are unoccupied and the defect generated is called vacancy defect. In the case of ionic solids, cations and stoichiometric proportions remain absent from their positions to maintain electrical neutrality. The defect produced due to vacancies caused by the absence of anions and cations in the crystal lattice is called the Schottky defect.Due to the vacancies in some of the sites of ions, the observed density of the crystal is found to be lower than the expected density. The defects are observed in solids with cations and anions of almost equal size like NaCl, KCl, CsCl etc. The figure below shows the crystalline solid having a Schottky defect.

1. Interstitial Defect- When cation or anion from ionic solid leaves its regular site and moves to occupy a place between the lattice site called interstitial position, the defect is called interstitial defect or Frenkel defect. This defect is generally observed in the case of relatively smaller cations that can fit into the interstitial space and hence produce the defect. The defect is observed in AgCl solid because of Ag+ ions or ZnS solid because of Zn++ ions. The presence of this defect does not alter the density of the solid. The defect is common when the difference in ionic radii of two participating ions is large.

1. Impurity Defect- The impurity defect occurs when a regular cation of the crystal is replaced by some different cation. The different cations sometimes may occupy the interstitial sites. If the impurity cation is substituted in place of regular cation, it is called a substitution impurity defect. If the impurity is present in the interstitial positions it is called an interstitial impurity defect. The impurity defect changes, almost completely the original properties of crystalline solid. Alloys are formed by substitution defects. Brass is a substitution also formed by substituting copper metal with zinc metal in a ratio 3:1. Stainless steel is an interstitial alloy formed by introducing carbon atoms as impurities. Pure iron is soft, malleable and ductile. Stainless steel is hard, stronger, less ductile, shiny and bright in appearance.

1. Line Defect- The defect is due to irregularity in a complete line, a row of lattice points of the constituent.
2. Surface Defect- Surface defects are the boundaries or planes that separate material into regions, with each region having the same crystalline structure but different orientation. Surface defects are usually formed by surface finishing methods like embossing or by degradation caused by weathering or environmental stress cracking.
3. Volume Defect- Volume defects are Voids, i.e. the absence of a number of atoms to form internal surfaces in the crystal. They have similar properties to microcracks because of the broken bonds at the surface.

Read about Amines here.

Electrical Properties

Elemental solids are divided into three categories namely metals, non-metals and metalloids. In the modern periodic table, non-metals are placed on the right side of the table. There are seventeen non-metals. Most of the non-metals in the periodic table are separated from metals by metalloids like boron, silicon, germanium, arsenic, antimony tellurium, polonium, astatine etc. There are a total of seventeen metalloids. Metals are good conductors of heat and electricity. This characteristic property is because of the formation of metallic bonds consisting of positive ions in a sea of delocalised mobile electrons.Non-metals are poor conductors of electricity and heat due to the absence of delocalised mobile electrons. 

Metalloids are intermediate between metals and non-metals. Metalloids like boron, silicon and germanium exhibit bright metallic lustre. Metalloids are poor conductors of heat and electricity. They are not malleable and ductile but are brittle. Silicon is a better conductor of electricity than non-metals but not as efficient as metals. Hence silicon is called a semiconductor. The variation in the property of the ability to conduct electricity of metals, non-metals and semiconductors can be explained with the help of band theory. The theory assumes that the atomic orbitals of the atoms in the crystal combine to form molecular orbitals which are spread over the complete crystal structure. With the increase in the number of atoms participating in crystal formation, the number of molecular orbitals containing electrons is greatly increased.

Band Theory

A small plate of about 1 g of metal may contain a very large number of about 10 electrons. With the increase in participating atoms of the crystal, the number of molecular orbitals containing electrons increases enormously. As the number of molecular orbitals increases the energy difference between the adjacent orbitals i.e. energy levels decrease. Until finally the energy gap becomes very small and molecular discrete energy levels merge into one another to form a continuous band of molecular orbitals which extends over the entire length of the crystal.

Each atomic orbital of the crystal corresponds to one energy level in the band which contains two electrons. The electrons in the higher energy level of a band are free and mobile and are responsible for electrical conductivity.

A better understanding of band theory may be obtained by considering the example of magnesium metal with atomic number 12, with electronic configuration 1s22s22p63s2. Each magnesium atom has two valence electrons in the outermost orbital 3s. One gram of magnesium strip contains 6.022×102324=2.5×1022 closely packed atoms and there are 1022 valency shells.

The interaction between these large numbers of orbitals leads to the formation of bonding and antibonding molecular orbitals. All the molecular orbitals are very close to each other and can not be distinguished from one another therefore, these orbitals are collectively called a band. The upper parts of the energy levels are formed by overlaps of filled 3s orbitals and empty bands. The lower part of the energy levels is formed by the overlap of filled 1s, 2s and 2p orbitals of adjacent atoms.

In metallic crystals, the valence bands and conduction bands are very close to each other and very little energy is required to excite electrons from the valence band into the conduction band. In the conduction band, the electrons are delocalised and are free to move from one end to the other end of the metal piece. This migration of electrons makes the metal a good conductor of heat and electricity. The substances like glass, rubber, polythene, plastic, wood and diamond are bad conductors of heat and electricity because the spacing between the valence band and conduction band is relatively more so that more energy is required to promote electrons from the valence band to the conduction band. This relatively more amount of energy is not available. Hence electrons remain in the valence band and thus cannot move freely, do not conduct heat and electricity and act as insulators.

Learn about Oxidation Numbers here.

What are Semiconductors?

A semiconductor is a poor conductor of electricity. A substance containing a completely filled band with electrons and a completely empty band behaves as a semiconductor. If the spacing between the valence band and conduction band is very small then the electrons from the valence band can be excited to the conduction band on slight heating. On excitation, empty conduction bands contain electrons to conduct electricity.

The number of electrons available for conducting electricity is relatively less in semiconductors as compared to the number of electrons in any metal. Silicon, with electronic configuration, 

1s22s22p63s23p64s03d0 is a semiconductor. Outermost 4s and 3d orbitals are empty. Similarly, germanium is also a semiconductor. However, electrons can be added to the empty conduction band by adding impurities like arsenic with extra electrons to silicon crystal. Arsenic has electronic configuration 1s22s22p63s23p64s23d10. The five electrons from 4s and 4p orbitals go to the empty conduction band in the silicon and become a conductor of electricity. The valence bands, and the empty conduction bands of metal, semiconductor and insulator are shown in the figure.

If an impurity of arsenic is added to silicon, some of the sites of silicon in the crystal are occupied by arsenic atoms each with one extra electron in the conduction band and it will be available for movement, for transport of electricity. Such type of semiconductor with an impurity having extra negative charge due to an extra electron of impurity atom is called an n-type semiconductor. If Boron (with electronic configuration 1s22s22p1 with one electron deficient as compared to valence electrons silicon atom) is added to silicon then some atoms of boron will occupy some of the sites of silicon atoms. At all sites of boron atoms, one valence electron will be shorter as compared to silicon atoms and there will be a positive hole in the lattice. Hence, electrons from neighbouring silicon atoms jump into the electron hole and the process continues till the electron hole is transferred to the edge of the crystal lattice and movement of electrons takes place. This type of semiconductor is called a p-type semiconductor. A solar cell that converts solar energy into electrical energy is constructed by connecting n-type and p-type semiconductors.

Magnetic Properties

While electrons are revolving around the nucleus in various orbits, they are also spinning about their own axis. A spinning charge generates a magnetic field. Hence spinning electrons act like tiny magnets. Any electron orbital can accommodate a maximum of two electrons.

If an orbital contains only one electron then it may spin either clockwise or anticlockwise. The unbalanced spin exhibits magnetism. However, if the orbital contains two electrons then one electron spins in the clockwise direction and the other in the anticlockwise direction so that spins are balanced the magnetic property will not be observed. The figure shows the spinning motions of electrons clockwise and anticlockwise and their action like tiny magnets.

Diamagnetism

If an atom or a molecule contains all electronic orbitals completely filled i.e. there are two electrons in each orbital, their spins are +1/2 and -1/2 i.e. the two electrons spinning in opposite direction one clockwise and the other anticlockwise. The spins are paired and the magnetic field will be repelled. This phenomenon is called diamagnetism. Water, sodium chloride, and benzene are some diamagnetic substances.

Paramagnetism

If an atom or a molecule contains more unpaired electrons revolving in their orbitals, magnetic moments do not cancel each other. There is a net magnetic moment associated with the species and the species experiences a net force of attraction when placed in a magnetic field. This property of a substance due to the presence of unpaired electrons, due to which the substance experiences pull in the magnetic field, is called paramagnetism. The extent of paramagnetism present in the substance depends upon the number of unpaired electrons present in the substance. The more the number of unpaired electrons in the substance the stronger is the pull due to paramagnetism of the substance. Oxygen, Cu2+,Fe3+,Cr3+ ions are some examples of paramagnetic substances.

Ferromagnetism

Substances like iron, cobalt, nickel, gadolinium, CrO2, etc. exhibit very strong magnetic properties. These substances can be permanently magnetised. They contain a large number of unpaired electrons. e.g. iron with electronic configuration [Ar]3d64s2 i.e.,

There are four unpaired electrons. Hence, iron is strongly ferromagnetic. I hope this article on Defects in Crystal Structure was informative. Get some practice of the same on our free Testbook App. Download Now!