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Imperfections: Impurity And Non-Stoichiometric Defects

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Imperfections: Impurity And Non-Stoichiometric Defects - Lesson Summary

Impurity defects:
If a foreign atom or ion or molecule is present in a crystal, then it has an impurity defect.

EX: solid solution of cadmium chloride and silver chloride.

This defect does not disturb the stoichiometry of crystalline substances.

Non-stoichiometric defect:
The defect that causes the ratio of the number of cations to anions to be different from that indicated by the ideal chemical formula is known as a non-stoichiometric defect.

Non-stoichiometric defects are of two types - metal excess defects and metal deficiency defects.

Metal excess defects:
A metal excess defect may arise in crystals due to anionic vacancies or the presence of extra cations at the interstitial sites.

The anionic sites occupied by unpaired electrons are called F-centres. Here, F stands for farbenzenter.

Metal excess defect also arises due to the presence of extra cations at the interstitial sites.

Metal deficiency defect:

This defect may arise due to cationic vacancies.

It occurs when positive ions are missing from its crystal lattice.

Line defects:

Line defects are the irregularities or deviations from the ideal arrangement in entire rows of lattice points. These arise mostly due to,

o The misalignment of particles or the presence of vacancies along a line.


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