explain type of defect found in non stoichiometric crystal structure
Answers
Answer:
Explanation:
The defects discussed so far do not disturb the stoichiometry of the crystalline substance. However, a large number of non-stoichiometric inorganic solids are known which contain the constituent elements in non-stoichiometric ratio due to defects in their crystal structures.
These defects are of two types:
(a) Metal Excess Defect and (b) Metal Deficiency Defect.
(i) Metal Excess 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 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 the crystal now has an excess of sodium. The anionic sites occupied by unpaired electrons are called F-centres (from the German word Farbenzenter for colour centre). They impart yellow colour to the crystals of NaCl. The colour results by excitation of these electrons when they absorb energy from the visible light falling on the crystals. Similarly, excess of lithium makes LiCl crystals pink and excess of potassium makes KCl crystals violet (or lilac).
Metal excess defect due to the presence of extra cations at interstitial sites: In this case, 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 Zinc Oxide. Zinc oxide is white in colour at room temperature. On heating it loses oxygen and turns yellow.
ZnO —-> Zn2+ + ½ O2 +2e–
Now there is 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.
(b) Metal Deficiency Defects
These contain less number of positive ions than negative ions. These arise due to two ways:
(i) Cation Vacancies: In some cases, the positive ions may be missing from their lattice sites. 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. 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 into the interstitial sites.
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