Defects in Solids

Introduction

Solids are formed as a result of systematic arrangement of many smaller units, crystals. Crystals are arranged in a certain pattern allowing the solids to be in a way they are. Due to the variation in the pace of arrangement of crystals, they lead to defects. Defects disturb the physical, mechanical, and chemical properties of materials (for example, electrical conductivity, magnetic permeability, strength, density, and plasticity). Solid defects are defined as the irregularities of the constituent crystals of a metal.

 

Classification of defects in solids

The 2 major defects based on the irregular arrangement of their crystals are, point defect and line defect. Point defects refer to the irregularities or the deviations of crystals from ideal arrangement around a point or an atom in a crystalline substance. On the other hand, line defects (dislocations) are lines along which whole rows of atoms in a solid are arranged anomalously. Metals are also influenced by the level of temperature into which they are exposed and they are called thermodynamic defects. Based on this, they are classified into stoichiometric defects, non-stoichiometric defects and impurity defects. Let us discuss the different types of defects in the solids.

 

Stoichiometric Defects

These  are the type of thermodynamic defects  further classified into Vacancy Defect and interstitial defects. Vacancy defect occur as a result of vacant sites in the lattice  and the interstitial Defect  occurs due to the movement of constituents particles to the interstitial site of the lattice. Interstitial sites are the free spaces around the crystals.  Both of these are applicable only for the non-ionic solids. For ionic solids which maintain the neutrality of the crystal, the 2 main defects are Schottky defect and the Frenkel defects.

 

Schottky defects

The Schottky defect is the vacancy defect seen in ionic solids that leads to problem in its neutrality. The electrical neutrality of the solid depends both on the number of cations and anions. The ionic solid is said to be electrically neutral if and only if the number of cations is equal to the number of anions. Generally, the Schottky defects lead to a decrease in the density of the ionic solids. Schottky defects are always described by ionic solids in which the positively charged ions and the negatively charged ions have the same size.

 

Frenkel defect

In Frenkel defect, the smaller ion, usually the cation is dislocated from the normal site to an interstitial site. This leads to a vacancy defect at its existing site and an interstitial defect at the new location. Remember, this does not change the density of the solid.

 


Non-stoichiometric defects
Non-stoichiometric defects are majorly shown by solids that combine their elements in a ratio different from that required by the stoichiometric formulae. They are sub-divided into two categories.

Metal Excess defect

Metal excess defects are as a result of vacancies created by negatively charged ions.  In this case, the anions may be missing from the lattice site thus leaving an electron(s) behind to balance the remaining charges. The f-center also was known as the Farbenzcenter is the one that is responsible for the colour of the compound in the lattice.

 

Metal deficiency defect

Metal deficiency defect occurs when the metal displays variable valency. For instance, iron oxide occurs as iron(II) and iron (III) ions in nature. In crystals of iron oxide, some Fe2+ ions are missing, and loss of positive charge is likely made up by the presence of the required number of iron (III) ions.

 

Impurity Defects

In the molten state, NaCl contains a small amount of SrClin the form of impurity in the crystal. Because of the presence of SrClsome sites of Na+ ion is occupied by Sr2+ which lead to defective solid. For maintaining the neutrality of the crystal one Sr2+ replaces two ions of Na+ and remaining one site of Na+ will be vacant. Example: CdCl2 and AgCl.

 

  1.   

Solid state

2.

States of matter                                         

3.

Liquid state and viscosity

4.

Nature of matter

5.

Ionic compounds

 

Check your understanding

  1. Classify the defects in solids.

  2. What is a non-stoichiometric defect?

  3. What type of defect is the Schottky defect?

  4. Why do solids have defects?

  5. Differentiate between a point defects and line defect.

 



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