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Imperfections: Stoichiometric Defects

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Imperfections: Stoichiometric Defects - Lesson Summary

If the constituent particles, such as atoms, ions or molecules, in a solid crystal are arranged in a regular fashion, then the crystal is said to be an ideal or perfect crystal. Almost all the crystals encountered in practice suffer from defects. These defects in solids are broadly classified into point defects and line defects.

Point defects:
Point defects are the irregularities or deviations from the ideal arrangement around a point or an atom in a crystalline substance. These may arise in a crystal due to one of several reasons:
  • The absence of a constituent particle
  • The presence of a foreign particle at the lattice site
  • The presence of a foreign particle at the interstitial site
  • The displacement of a particle to the interstitial site 

Point defects are classified into three types, namely
  • stoichiometric defects,
  • Impurity defects and
  • Non-stoichiometric defects.

Stoichiometric defects: 
A ‘stoichiometric defect’ is a defect by which there is no disturbance in the stoichiometry of the solid.
These are also called intrinsic or thermodynamic defects. Stoichiometric defects are further classified into vacancy defects and interstitial defects.
 
Vacancy defect:
The defect in which a constituent particle is missing from the crystal lattice is known as a vacancy defect. This defect develops when a substance is heated. The major consequence of a vacancy defect is that the density of the substance decreases.
 

 
Interstitial defect:
An interstitial defect arises when some of the constituent particles occupy the interstitial sites in a crystal lattice. As a result of this defect, the density of the substance increases.
 

 
Ionic solids show Frenkel and Schottky defects.

Frenkel defect: 
A Frenkel defect is caused if some of the ions of a lattice occupy the interstitial sites by leaving a corresponding number of normal lattice sites vacant. It does not influence the density of the substance. This type of defect occurs in compounds with ions of different sizes.
EX: Zinc sulphide, silver bromide and silver iodide etc.
 

 
Schottky defect:
A Schottky defect is caused if some of the lattice sites remain unoccupied. The number of missing positive ions is equal to the number of missing negative ions, and thus, the crystal remains electrically neutral. A Schottky defect causes a decrease in the density of a substance.
 
EX: Sodium chloride, potassium chloride and caesium chloride etc.
 

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