Crystal Imperfections

CRYSTAL IMPERFECTIONS

In an ideal crystal (perfect crystal), the atomic arrangement is perfectly regular and continuous throughout.

But in real crystals due to some reasons the regular orientation of atoms may be disturbed at a point, along a line or in a region.

Definition

The disturbance occurred in the regular orientation of atoms is called crystal defect or imperfection.

The imperfections or defects are always present in the actual crystal and their effects are often very important in understanding the properties of crystals.

Some properties of crystal defects are structure sensitive i.e., properties such as mechanical strength, ductility, crystal growth, magnetic hysteresis, dielectric strength are greatly affected by the relatively minor changes in crystal structure caused by the imperfections.

Some other properties of crystals are structure-insensitive i.e., properties such as stiffness and density are not affected by the presence of imperfections.

Classification of crystal imperfections (or Defects)

Crystalline imperfections are classified on the basis of their geometry as follows:

1. Point Defects

(a) Vacancies

(b) Intersititials

(c) Impurities

2. Line Defects

(a) Edge dislocation

(b) Screw dislocation

3. Surface Defects

(a) Grain boundaries

(b) Tilt boundaries

(c) Twin boundaries

(d) Stacking faults

4. Volume Defects 

Cracks

Point Defects

Point defects are crystalline irregularities of atomic dimensions. They are imperfect points like regions in the crystal. One or two atomic diametre is the typical size of a point imperfection.

• Point defects take place due to imperfect packing of atoms during crystallisation.

• They produce distortion inside the crystal structures.

• They produce strain only in its surroundings but they do not affect the regularity in other parts of the crystal.

Types of point defects

The different types of point defects are

(a) Vacancies

(b) Interstitial

(c) Impurities

(a) Vacancies

A vacancy is the simplest point defect in a crystal. It refers to a missing atom or vacant atomic site.

Whenever one or more atoms are missing from a normally occupied position as shown in fig, the defect caused is known as vacancy.

Vacancies may be single as shown in figure or two or more of them.

• These defects may arise due to imperfect packing during original crystallisation and thermal vibrations of the atoms at high temperatures.

The atoms surrounding the vacancies are displaced inwards thereby distorting the regularity of arrangement. There are different kinds of vacancies like Frenkel defect, Schottky defect, Colour center, etc.

Schottky defect

It refers to the missing of a pair of positive and negative ions in an ionic crystal.

Here, two oppositely charged ions are missing from an ionic crystal, therefore a cation-anion divacancy is created, (fig. 1.30) This is known as schottky defect or schottky imperfection or iron pair vacancies. Since a pair is missing, electrical neutrality is maintained.

Frenkel Defect

A vacancy associated with interstitial impurity is called Frenkel defect.

Here a missing atom (responsible for vacancy) occupies interstitial position (responsible for interstitial defect) ( fig 1.31.)

This defect always occurs in ionic crystal. If a positive ion moves into an interstitial site in an ionic crystal, a cation vacancy is created in normal ion site, this vacancy-interstitial pair is known as Frenkel defect.

• Frenkel defect does not change the overall electrical neutrality of the crystal.

• The presence of these defects in ionic crystals causes an increase in electrical conductivity.

(b) Interstitial defect

When an extra atom occupies interstitial space (i.e., voids) within the crystal structure without removing parent atom, the defect is called interstitial defect.

An atom can enter into interstitial space or void only if it is smaller than the parent atom otherwise it will produce atomic distortion or strain because interstitial atom tends to push the surrounding atoms further apart.

Types of interstitial defect

Interstitial defect has two types

(i) Self interstitial

(ii) Foreign interstitial

(i) Self intertitial

If an atom from same crystal occupies intertitial site, then it is called self interstitial. (Fig. 1.32 (a))

(ii) Foreign intertitial

If an impurity atom (foreign atom) occupies intertitial site, then it is called foreign interstitial. (Fig. 1.32(b))

(c) Impurities

When the foreign atoms (impurities) are added to crystal lattices, they are known as impurities. The defect is called impurity defect.

The impurity atom may fit in the structure in two ways giving rise to two kinds of impurity defects. They are

(i) Substitutional impurity defect

(ii) Interstitial impurity defect

(a) Substitutional impurity defect

A substitutional impurity refers to a foreign atom that replaces a parent atom in the lattice (figure 1.33.)

• Substitutional impurities change the electrical properties enormously.

Example

1. n-type and p-type semiconductors have substitutional impurities from Vth group and IIIrd group elements.

A controlled addition of impurity to a very pure semiconductor is the basis of producing many electronic devices like diode and transistors.

2. During the production of brass alloy, zinc atoms are doped in copper lattice. Here, zinc atoms are called as substitutional impurities.

(ii) Interstitial impurity

An interstitial impurity is a small size atom occupying the empty space (interstitial) in the parent crystal, without dislodging any of the parent atoms from their sites (Fig. 1.40)

An atom can enter into interstitial or empty space only when it is substantially smaller than parent atom.

Example. In FCC iron, the atomic radius of iron atom is 0.225 nm. The carbon atoms with atomic radius 0.078 nm can occupy empty spaces in FCC lattice as interstitial impurities.