Imperfections/Defects in solids

Imperfections in Crystals:

The defects in the crystalline solid can be defined as the irregularities in the arrangement of constituent particles in the crystalline solid.

• There are major two types of defects:

• These irregularities or deviations occur in the crystal lattice are known as crystal defects.

Types of Point Defects:

(a)  Stoichiometric Defects: 

These point defects generally do not disturb the stoichiometry of the solids. They are also called as intrinsic or thermodynamic defects. The stoichiometric defects are classified into two types,

1)    Vacancy defect:

When some of the lattice sites remain vacant in the crystalline substance, then the crystal is said to have vacancy defect.

1.  This defect arises due to absorption of heat from the surroundings. It causes a decrease in density of the substance.
2.  This effect is shown by non-ionic solids.

2)    Interstitial defect:

When extra constituent particles are present in the interstitial sites then the crystal is said to have the interstitial defect. 

1. This defect causes an increase in the density of the substance.
2. This effect is shown by non-ionic solids.

3)    Schottky defect:

 In an ionic crystal of the type A⁺B^-, an equal number of cations and anions are missing from their lattice sites so that the electrical neutrality is maintained. This defect is called a Schottky defect. The Schottky defect contains one pair of holes due to missing of one cation and one anion. Hence, generally, it is a vacancy defect.

4)    Frenkel defect:

If an ion is missing from its lattice site causing hole and it occupies the interstitial site, electrical neutrality, as well as the stoichiometry of the compound, are maintained. This type of effect is called Frenkel defect.

1. This defect is the combination of vacancy defect and interstitial defect.
2. This defect is found in silver halides like AgCl, AgBr and AgI because of the small size of Ag⁺ ion.

(a)   Non-stoichiometric Defects:

Due to the imperfections in the crystal, if the ratio of the cations to the anions is different from that of ideal chemical formula, the defects are called non-stoichiometric defects.

This defect gives two types:

 1)    Metal Excess:

 This occurs in two ways, 

a)   By anion vacancies: 

1. In this defect, a negative ion is missing from its lattice site, leaving behind a hole which is occupied by an electron and maintain the electrical balance.
2. The sites which contain the electrons are trapped in the anion vacancies. They are known as F-centres because they are responsible for giving colour to the crystal.
3. When NaCl is heated in an atmosphere of Na vapours, the excess of Na atoms gets deposited on the surface of the crystal.
4.  At the same time, Cl^- ions diffuse to the surface and combine with Na atoms which become ionized by losing electrons.
5.  These electrons diffuse back to the crystal and occupy the vacant sites left by the Cl^- ions.
6.  These electrons absorb white light and give a yellow colour to the NaCl.
7.   Also the excess of Li in LiCl gives pink colour and excess of K in KCl gives violet colour to crystal. 

 b)   By the presence of extra cations in the interstitial sites: 

1. The metal excess defect may also cause by extra cations which occupy the interstitial sites.
2. Electrical neutrality is maintained by an electron present in another interstitial site.
3. This defect is found in the crystals which show Frenkel defect.
4. When ZnO is heated, it loses oxygen and turns yellow.
5.   The excess of Zn²⁺ ions are trapped into the vacant interstitial sites and electrons are trapped into the neighbouring interstitial sites.
6. The crystals with this type of effect act as a semiconductor as they contain some free electrons.

 2)    Metal Deficiency: 

1.  This defect mainly occurs in the transition metals which show variable valency.
2.  This defect caused by missing of cation from its lattice site and presence of cation with the higher charge in the adjacent lattice site.
3.   FeS, FeO and NiO show this defect. For example, it is very difficult to prepare ferrous oxide with the ideal composition, FeO as x = 0.95 in Fe_xO.

(b)  Impurity Defects:

• These defects arise when foreign atoms are present at the lattice sites instead of host atoms or at the vacant interstitial sites.
• For example, steel is the carbide of iron. In steel, carbon atoms are present at the interstitial sites of Fe atoms.

• These defects arise when foreign atoms are present at the lattice sites instead of host atoms or at the vacant interstitial sites.
• For example, steel is the carbide of iron. In steel, carbon atoms are present at the interstitial sites of Fe atoms.
• When foreign atoms are present at the lattice sites, we get substitutional solid solutions. Formation of this depends upon the electronic structure of impurity.
• When foreign atoms are present at the vacant interstitial sites, we get interstitial solid solutions. Formation of this depends upon the size of impurity.
• Addition of impurities changes the properties of crystal.