Atomic Imperfections (Atomic Defects)

September 30, 2020

The atomic imperfections (defects) are caused by some disorder in the arrangement of the constituent particles in any crystal.
There are two kinds of atomic defects. They are

i) Point defects

ii) Lattice defects

Point defects

The defects caused by a departure from the periodic arrangement of particles in the vicinity of an atom or group of atoms, are called point defects.
These defects involve only a few lattice points.
The point defects occur in many ways.
As a result, there are more than one kind of point defects. These are:

a) Schottky defect

If an atom or ion is missing from its lattice site, the vacancy created is termed as a Schottky defect.
In ionic solids, vacancy at a cation site is accompanied by another vacancy at a nearby anion site.
Such paired vacancies called Schottky defects, preserve the electrical neutrality of the crystal.
In sodium chloride, there are approximately 10⁶ Schottky pairs per cm³ at room temperature.
There are about 10²² ions in one cm³ of sodium chloride.
Therefore, in sodium chloride at room temperature there is one Schottky defect per 10¹⁶ ions.
The presence of Schottky defects in a crystal lowers its density appreciably

b) Interstitial defect

In crystals, a part of the empty space is distributed throughout in the form of voids.
These voids are also called interstices, or interstitial sites.
Sometimes, certain small atoms or ions occupy these voids.
The atoms or ions, which occupy the vacant voids in a crystal, are called interstitials.
This defect increases the density of the crystal slightly.

c) Frenkel defect

These are commonly found in ionic solids.
Whenever an ion leaves its lattice site and occupies an interstitial site, such hybrid defect (combination of Schottky and interstitial) is called frenkel defect.
In pure alkali halides, Frenkel defects are not observed, because the ions are bigger than the void size.
Frenkel defects are found in silver halides (due to smaller size of Ag⁺).
Frenkel defects do not affect the density of the crystal.

d) F- centre

Sometimes an anion is missing from its lattice site in a crystal.
Such single anion vacancy in an ionic crystal creates a local excess positive charge at that lattice site.
An electron can migrate to this site and get trapped in the anion vacancy. Such a defect is called F-centre.
If the concentration of F-centres in any crystal is high, then the otherwise colourless crystals develop some colour.
For example, F-centres impart violet colour to KCl, and pink colour to LiCl crystals.
In alkali halides, F-centres are created by heating alkali halide crystals in an atmosphere of alkali metal vapours.
The metal atoms deposited on the surface of the crystal react with halide ion diffusing into the surface to give the halide.
The electron released during this reaction diffuses into the crystal and combines with the anion the vacancy to give a F-centre.

Lattice Defects

If the deviations from the periodic arrangement extend over microscopic regions, the defects are called lattice imperfections or lattice defects.

Line defects: When a lattice defect extends along lines e.g., displacement of a row of lattice points, it is called line defect.
Plane defects: When the irregularities extend along surfaces i.e., along the lattice plane, the defects is known as plane defect.

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