What is the brief definition of these metal excess detects and metal deficient defects? Can't understand the way it is explained...

What is the brief definition of these metal excess detects and metal deficient defects?
Can't understand the way it is explained...


1 Answers

Aarti Gupta
askIITians Faculty 300 Points
8 years ago
When the ratio of number of cation to anion becomes different from that indicated by the ideal chemical formula, then the defects which arises are called as non – stoichiometric defects.These defects are of two types:
(a) Metal Excess Defect and
(b) Metal Deficiency Defect.

Metal Excess Defect:--These defects are either due to anionic vacancy or extra cations at interstitial sites.
(i)Metal excess defect due to anionic vacancies:- In this case, negative ions get missed from their lattice sites leaving holes in which the electrons remain entrapped to maintain the electrical neutrality and observed in those crystals which are likely to form Schottky Defects.For ex-alkali metal halides like NaCl and KCl show this type of defect. When crystals of NaCl are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The Cl–ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl. This happens by loss of electron by sodium atoms to form Na+ ions. The released electrons diffuse into the crystal and occupy anionic sites. As a result of which the crystal now has an excess of sodium. The anionic sites occupied by unpaired electrons are called as F-centres.They impart yellow colour to the crystals of NaCl.
(ii)Metal excess defect due to the presence of extra cations at interstitial sites:-In such defect,there are extra positive ions occupying interstitial sites and the electrons in another interstitial sites to maintain electrical neutrality.The defect may be visualised as the loss of non-metal atoms which leave their electrons behind.The excess metal ions occupy interstitial positions.The common example is ZnO which is white in colour at room temperature but on heating it loses oxygen and turns yellow.
(b) Metal Deficiency Defects:- In such defects it is found that there areless number of positive ions than negative ions and arises due to two ways:-
(i) Cation Vacancies:- In such cases, the positive ions may be missing from their lattice sites and the extra negative charge may be balanced by some nearby metal ion acquiring two positive charges instead of one.This type of defect is possible in metals which show variable oxidation states.The common examples of compounds having this defect are ferrous oxide, ferrous sulphide, nickel oxide etc.
(ii) Extra anions occupying interstitial sites:- In the case,the extra anions may be occupying interstitial positions and thus the extra negative charge is balanced by the extra charges on the adjacent metal ions.Such type of defect is not common because the negative ions usually very large and they cannot easily fit intothe interstitial sites.

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