Question

Write in detail about metal deficiency and non-stoichiometric defects.Write answer of the given question in detail.

Answer 1

However, a large number of non-stoichiometric inorganic solids are known which contain the constituentelements in non-stoichiometric ratio due to defects in their crystal structures. These defects are of two types: (a) MetalExcess Defect and (b) Metal Deficiency Defect.  Metal excess defect due to anionic vacancies: In this case, negative ions may be missing from their lattice sites leaving holes in which the electrons remain entrapped to maintain the electrical neutrality.
This type of defect is observed in those crystals which are likely to form Schottky Defects. For example, Alkali metal halides like NaCl and KCl

Answer 2

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*Non-stoichiometric Defects:

There are large numbers of inorganic solids found which contain the constituent particles in non-stoichiometric ratio because of defects in their crystal structure. Thus, defects because of presence of constituent particles in non-stoichiometric ratio in the crystal structure are called Non-stoichiometric Defects.



*Metal – Excess Defect: The solids with metal excess defect contain metal in excess to the stoichiometric ratio. Such defect is caused due to either of the following reasons.

✔Metal – Excess Defect due to Anionic Vacancies:

This kind of defect is shown by alkali halides such as, sodium chloride NaCl and Potassium Chloride KCl.When crystal of an alkali halide, say sodium chloride NaCl is heated in an atmosphere of alkali metal vapour.

The metal atoms get deposited on the surface of the crystal. The halide ions diffuse to the surface of the crystal and combine with alkali metal to form alkali halide.

The electron released during this reaction diffuses into the crystal and occupies the anionic (halide ion) site.

✔Metal – Excess Defect due to Presence of extra cations at he the interestial sites:

Zinc oxide is white at room temperature, but on heating it loses oxygen and turns yellow. This reaction is reversible.

The Zn2+ ions so formed enter the vacant voids (interstices) in the crystal .The released electrons get traps in its neighbouring interstitial sites .
This results into a slight excess of zinc in the crystal. Such a sample of zinc oxide can be described the formula Zn 1+x O. The non-stoichiometric ZnO shows increased conductivity due to these trapped electrons.

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