Imperfections in Solids
A crystal in which the constituent particles have a definite repeating arrangement is called an ideal crystal. However, when some constituent particles do not occupy theoretical positions, the crystal structure shows some deviations called defects.
In short- Imperfection or defect in a crystal is the departure of the constituent particles of the crystal from its regular position in the lattice.
Type of defects in solids
Point defects
Point defects arise due to irregularities or deviations from ideal arrangement around a point or an atom.
Line defects
Line defects are the deviations from ideal arrangement in entire rows of lattice points. However, we shall confine our discussion to point defects only.
Types of Point Defects
- Stoichiometric defects
- Non-stoichiometric defects
- Impurity defects
Stoichiometric Defects
Stoichiometric compounds are those compounds in which the number of positive and negative ions are in the same ratio as indicated by their chemical formula.
These defects are also called Intrinsic or Thermodynamic defects because the number of defects depend upon the temperature. At absolute zero, the ions are arranged in a regular fashion in the crystal. However, on increasing temperature, the chance for an ion to escape from the lattice site and thus to create a vacant site or a defect increase.
Stoichiometric defects are mainly of two types-
Vacancy Defect
This type of defect arises when some constituent particles are missing from their regular lattice positions. The unoccupied positions are called Vacancies. Due to this type of defect, the density of the substance decreases. This defect can also develop on heating a substance.
Interstitial Defect
This type of defect arises when some constituent particles (atoms or molecules) occupy an Interstitial site. Density of the substance increases due to Interstitial defect.
The two types of defects described above can be shown by non-ionic solids. In case of ionic solids, electrical neutrality is also to be maintained.
Therefore, rather than simple vacancy or interstitial defects, they show these defects as Schottky and Frenkel defects.
Schottky Defects
This defect discovered by a German Scientist Schottky in 1930 is basically a vacancy defect. Schottky defects in ionic solids arise in a crystal when the same number of cations and anions are missing from their normal lattice sites and a result, pairs of holes are formed.
Conditions causing Schottky defects
This type of defect is generally observed in case of strongly ionic compounds:
- With high coordination number
- Having positive and negative ions of same size
Examples of compounds having this type of defect are:
NaCl, KCl, KBr, CsCl and AgBr.
Number of ions having one Schottky defect in NaCl ionic crystals
It has been observed that there are about 106 Schottky pairs per cm3 at room temperature (one cm3 has about 1022 ions) or about one Schottky defect per 1016 ions in NaCl.
Frenkel Defects
This defect was discovered by a Russian scientist Frenkel in 1926. This is a hybrid type of defect arising from the combination of Schottky defect and interstitial defect.
This type of defect arises due to the presence of a hole in the cationic lattice site and the cation occupying an interstitial position.
For example, in the crystal lattice of ZnS and AgBr, the Zn2+ and Ag+ ions occupy interstitial sites and leave behind a hole. Frenkel defects creates a vacancy defect at its original site and an interstitial defect at its new location. Frenkel defect is also called Dislocation defect.
Condition causing Frenkel defects
This defect generally occurs in compounds:
- Having low coordination number
- In which anions are much larger than cations
It may be noted that- AgBr shows Frenkel as well as Schottky defect.
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