Name: Voids in Crystal Lattices
Uploaded: 2019-07-19
Duration: 5 min 25 s
Description: None of the close packing techniques has 100% efficiency. There is always some space in the lattice that is not occupied by any particle. These empty spaces...

Question

Explain the crystal defects and their origin in the crystal.

Accepted Answer

Crystal defects, in general, are defined as those in which there will be irregularities in the arrangements of constituent particles. On the basis of irregular arrangement, the defect may be a point or line defect.

Types of defects in solids:

(i) Stoichiometric defects:
The defect in which stoichiometry of the compound remains the same as shown by their chemical formula. It is also called as intrinsic or thermodynamic defects. For ionic solids which maintain the neutrality of the crystal are shown by Frenkel and Schottky defects.

(a) Schottky defect:This defect occurs due to the missing of the same number of cations and anions from the lattice site. These type of defect is shown by this substance which having high coordination number in which the size of the cation and anion are nearly equal. As the equal number of cations and anions are missing, the crystal as a whole is neutral. The density of such type of crystal decreases. Due to the presence of ions this crystal can conduct electricity at a small extent. Example: $NaCl, CsCl, AgBr$ , etc.

(b) Frenkel defect: In this type of defect, some cations are missing from their lattice site and occupy the interstitial site of the lattice. This type of defect is shown by those substances which have low coordination number in which the size of cation and anion differ very much. The closeness of like charges tends to increase the dielectric constant of crystal. The density of these types of crystals remains the same because there is no movement of ion outside the crystal. Example: $ZnS, AgBr, AgCl$ , etc.

(ii) Non- stoichiometric defects:This defect is shown by compound of $d$ -block elements of the periodic table. These defects are classified as metal excess defects and metal deficiency defect.

(a) Metal excess defect due to anionic vacancy: When a compound having excess metal ion if an anion is absent from the lattice site there it creates a void while is there occupied by an electron. This type of defect is shown by alkali metal halides like $NaCl, KCl, LiCl$ .

When alkali metal halides are heated in an atmosphere of vapour of the alkali metal, anion vacancies are created. This anion is then diffuse to the surface of the crystal and combine with newly generated metal cations. The electron is lost by the metal atom is then diffuses the crystal and occupy the anionic vacancy site and form F-centres inside the crystal. These F-centres give different colours like $NaCl$ gives a yellow colour. $KCl$ gives a violet colour and $LiCl$ gives pink colour.

(b) Metal excess defect due to the presence of interstitial cation: When an excess positive ion is located in the interstitial site. This type of defect is shown by $ZnO$ . When $ZnO$ is heated it loses oxygen reversibly. The excess ${Zn}^{2 +}$ ions are occupied in the interstitial sites for maintaining the neutrality, electrons are enclosed in the neighbouring interstitial sites. On heating, $ZnO$ turns yellow colour by losing oxygen.

(c) Metal deficiency defect: In this defect, some cations are missing from the lattice site and for maintaining its electrical neutrality another remaining cation increase their valency. Example: In the crystal of $FeO$ , some ${Fe}^{2 +}$ cations are missing and the loss of positive charge is made up by the presence of the required number of ${Fe}^{3 +}$ ions.

Explain the crystal defects and their origin in the crystal.

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