* Crystal defect:
The real, naturally occurring crystalline substances do not have perfect crystal structures. Crystal defect is the irregularities in the arrangement of constituent particles of solid crystal. Crystal defect is created during crystallization. It can be minimized at a slower rate. Crystal with no defect forms at absolute temperature.
There are three types of defect: point deffect, line defects and plain defects. We are going to discuss about point defect
* Point defect:
These defects are irregularities produced in the arrangement of basic at lattice points in crystalline solids.
There are three major classes of point defect: stoichometric point defect, impurity defect and nonstoichiometric point defect.
a. Stoichometric point defect:
Chemical formula of a compound shows fixed ratio of atom or number of cation or anion
There are four types of stiochiometric point defects: vacancy defect, self interstitial defect, Schottky defect and Frenkel defect.
1.Vacancy defect:
During crystallization a partical missing from its regular site in the crystal lattice. Missing particle creats vacancy. This crystal have vacancy defect. This defect is also developed when substance is heated. Density of substance decreases.
Interstitial sites in a crystal are the space or voids in between the particles at lattice points. When some particles interstitial sites in the crystal structure, it is called self interstitial defect.
This defect occurs in the following two ways:
Firstly, an extra particle occupies an empty interstitial space in the crystal structure. The extra particle is same as those already present at the lattice points. The extra partucle increases the total mass of the substance without increasing volume. Hence its density increases.
Secondly, in an elemental solif a particle gets shifted from its original lattice point and occupies an interstitial space in the crystal.
3. Schottky defect:
In an ionic solid , equal number of cations and anions are missing from their regular position in crystal lattice creating vacancies. A paired cation- anion vacancy defect is called Schottky drfect.
Conditions for the formation of Schottky defect:
1. High degree of ionic character.
2. High coordination number of anion.
3. Small difference between size of cation and anion. The ratio rcation÷ ranion is not far below unity.
Consequences of Schottky defect:
1. As the number of ions decreases, mass decreases. However, volume remains unchanged. Hence, the density of a substance decreases.
2. The number of missing cations and anions is equal, the electrical neutrality of the compound is preserved.
This defect is found in ionic crystals such as NaCl, AgBr and KCl.
4. Frenkel defect:
Frenkel defect arises when an ion compound is missing from its regular lattice site and occupies interstitial position between lattice points. The cations are usually smaller than anions. It is, therefore, more common to find the cations occupying interstitial sites. It is easier for small cations to accomodate the interstitial space.
Conditions for the formation of Frenkel defect:
1. Frenkel defect occurs in ionic compounds with large difference between sizes of cation and anion.
2. The ions of ionic compounds must be having low coordination number.
Consequences of Frenkel defect:
1. As no ions are missing from the crystal lattice as a whole, the density of solid and its chemical properties remain unchanged.
2. The crystal as a whole remains electrically neutral because the equal number of cations and anions are present.
This defect is found in ionic crystals like ZnS, AgCl, AgI, AgBr, CaF2.
b. Impurity defect:
Impurity defect arises when foreign atoms that is, atoms different from the host atoms, are present in the crystal lattice. There are two kinds of impurity defects: Substitutional and interstitial impurity defects.
1. Substitutional impurity defect:
In this defect, the foreign atoms are found at the lattice sites in place of host atoms. The regular atoms are displaced from their lattice sites by impurity atoms.
2. Interstitial impurity defect:
In this deffect, the impurity atoms occupy interstitial space of lattice structure. For example in steel, Fe atoms occupy normal lattice sites. The carbon atoms are present at interstitial space.
Nonstoichiometric defect arises when the ratio of number of atoms of one kind to that of other kind or the ratio of number of cations to anions becomes different from that indivcate by its chemical fotmula. In short, stoichiometry of the compound is changed.
It is important to note that the change in stoichiometry does not cause any change in the crystal structure.
There are two types of nonstoichiometric defects
1.Metal deficiency defect :
This defect is possible only in co.pounds of metals that show variable oxidation state. In some crystals, positive metal ions are missing from their original lattice sites. The extra negative charge is balanced by the presence of cation of the same metal with higher oxidation state than that of missing cation.
There are two types of metal excess defects.
● A neutral atom or an extra positive ion occupies interstitial position:
ZnO present two of metal excess defect. In tbe first case in ZnO lattice one neutral Zn atom is present in the interstitial space.
● By anion vacancies:
This type of defect imparts colour to colourless crystal. Foe example, when NaCl crystal is heated in the atmosphere of sodium vapour, sodium atoms are deposited on the crystal surface.
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