Chemistry Imperfections in Solids
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### Topics to be covered :

This lecture covers following topics :

● Imperfections in Solids
● Types of Point Defects
● Stoichiometric Defects
● Frenkel and Schottky Defects
● Impurity Defects
● Non-Stoichiometric Defects

### Imperfections in Solids :

Defects : These are basically irregularities in the arrangement of constituent particles. Defects are of two types which are point defects and line defects.

Note : Defects in crystals are due to the fast or moderate speed of crystallisation.

Point Defects : This is the deviation from ideal structure around a point or atom in a solid.

Line Defects : This is the deviation from ideal structure in entire rows or lattice points. These irregularities are called crystal defects.

### Types of Point Defects :

There are three types of defects :

(i) Stoichiometric Defects

(ii) Impurity Defects

(iii) Non-Stoichiometric Defects

### Stoichiometric Defects :

In this, Stoichiometry of the solid does not change. They are called intrinsic or thermodynamic defects.

### Types of Stoichiometric Defects :

(i) Vacancy Defect : This is due to the vacancy present in the crystal. Due to this, density of crystal decreases. This defect can develop when a substance is heated.

(ii) Interstitial Defect : In this, some constituent particles occupy an interstitial site. Due to this defect, density of the substance increases.

Note : (i) Both these defects are shown by non-ionic solids.

(ii) Ionic crystals always maintain electrical neutrality.

(iii) Vacancy and interstitial defects are shown as frankel and schottky defects.

### Frenkel Defect :

(i) Shown by ionic compounds.

(ii) In this, smaller ion (usually cation) is dislocated from its normal site to an interstitial site.

(iii) It creats a vacancy defects at its original site and an interstitial defect at its new location.

(iv) Also called dislocation defect.

(v) Density of the solid remains same.

(vi) Shown by ionic compounds which have large difference in the size of ions.

(vi) e.g. ZnS, AgCl, AgBr and AgI

### Schottky Defect :

(i) It is a vacancy defect.

(ii) Shown by ionic compounds.

(iii) In order to maintain electrical neutrality,

The no. of missing cations = no. of missing anion

(iv) Density of the solid decreases.

(v) No. of defects in solid is quite significant. e.g. there are 10^6 schottky pairs per cm^3 at room temperature and in 1cm^3 there are 10^(22) ions. So, there is one schottky defect per 10^(16) ions.

(vi) Shown by ionic compounds in which cations and anions are of similar size.

(vii) e.g. NaCl, KCl, CsCl and AgBr

Note : AgBr shows both Frenkel and Schottky defects.

### Impurity Defects :

If molten NaCl containing a little amount of SrCl_2 is crystallised, some of the sites of Na^(+) ions are occupied by Sr^(2+) (Fig.1.27). Each Sr^(2+) replaces two Na^(+) ions. It occupies the site of one ion and the other site remains vacant. The cationic vacancies thus produced are equal in number to that of Sr^(2+) ions. Another similar example is the solid solution of CdCl_2 and AgCl.

### Non-Stoichiometric Defects :

These defects are of two types :

(i) Metal Excess Defect

(ii) Metal Deficiency Defect

(i) Metal Excess Defect :

(a) Metal Excess Defect due to Anionic Vacancies : Alkali halides like NaCl and KCl show this type of defect. When crystals of NaCl are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The Cl^(-) ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl. Sodium atoms to lose electron to form Na^(+) ions. The released electrons diffuse into the crystal and occupy anionic sites (Fig. 1.28). As a result, the crystal has an excess of sodium. The anionic sites occupied by unpaired electrons are called F-centres (from the German word Farbenzenter for colour centre). They impart yellow colour to the crystals of NaCl. The colour is due to the excitation of these electrons when they absorb energy from the visible light falling on the crystals. Similarly, excess of lithium makes LiCl crystals pink and excess of potassium makes KCl crystals violet (or lilac).

(b) Metal Excess Defect due to the presence of Extra Cations at Interstitial Sites : Zinc oxide is white in colour at room temperature. On heating it loses oxygen and turns yellow

ZnO oversettext(heating)→ Zn^(2+) +1/2 O_2 +2 e^(-)

Due to this, there is excess of zinc in the crystal and its formula becomes Zn_(1+x)O. The excess Zn^(2+) ions move to interstitial sites and the electrons to neighbouring interstitial sites.

(ii) Metal Deficiency Defect : Many solids are difficult to prepare in the stoichiometric composition and contain less amount of the metal as compared to the stoichiometric proportion. Example is FeO which is mostly found with a composition of Fe_(0.95) O. It may range from Fe_(0.93) O to Fe_(0.96)O. In crystals of FeO, loss of positive charge (Fe^(2+)) is made up by the presence of required number of Fe^(3+) ions.