Question

explain non-stoichiometric defect.​

Answer 1

Non-Stoichiometric Defects

Nonstoichiometric inorganic solids contain the constituent elements in a non-stoichiometric ratio due to defects in their crystal structures.

These defects are of two types: (i) metal excess defect and (ii) metal deficiency defect.

(i) Metal Excess DefectMetal excess defect due to anionic vacancies:

This type of defect is exhibited by alkali halides like NaCl and KCl.

Application of heat to NaCl in an atmosphere of sodium vapour results in deposition of the sodium atoms on the surface of the crystal.

The sodium atoms lose electron to form Na+

The Cl– ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl.The released electrons diffuse into the crystal and occupy anionic sites.

The anionic sites occupied by unpaired electrons are called F-centres that imparts yellow colour to the crystals of NaCl due to the excitation of the electrons on absorption of energy from the visible light falling on the crystals.

Excess of lithium makes LiCl crystals pink and excess of potassium makes KCl crystals violet.

Metal excess defect due to the presence of extra cations at interstitial sites:

Heating Zinc oxide that exists in white colour at room temperature loses oxygen and turns yellow.

This results in excess of zinc in the crystal and its formula becomes Zn1+xO.

The excess Zn2+ ions move to interstitial sites

and the electrons to neighbouring interstitial sites.

Metal Deficiency Defect:

Many solids possess less amount of the metal as compared to the stoichiometric proportion.

For example, FeO is mostly found with a composition ranging from Fe 93O to Fe 0.96O.

In crystals of FeO some Fe2+ cations are missing and the loss of positive charge is made up by the presence of required number of Fe3+

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Answer 2

The non-stoichiometric defect affects the stoichiometry of the compounds.

Explanation:

• Non-stoichiometric defects are flaws that interfere with the stoichiometry of the compounds. These defects are produced by either an excess or a deficiency of metal ions.
• There are two types of metal defects

              a) metal excess defect

               b) metal deficiency defect.

• The Excess metal defect produced by anionic vacancies is as follows: This type of flaw may be seen in alkali halides including NaCl and KCl. Whenever sodium atoms were deposited mostly on surface of NaCl throughout the presence of sodium vapour, heat is applied.