The Solid State

Question 21

What type of stoichiometric defect is shown by
(i) ZnS, (ii) AgBr (iii) KCl


(i) Frenkel defect.
(ii) Schottky defect and Frenkel defect.
(iii) Schottky defect.

Question 22

Explain how vacancies are introduced in an ionic solid when a cation of higher valence is added as an impurity in it.

If we mix a small amount of molten strontium chloride (SrCl2) with molten sodium chloride (NaCl) and the resulting solution is cooled, in the crystals of NaCl some Na+ ions will get replaced by Sr2+ ions. In order to maintain electron neutrality too Na+ ions are to leave their respective lattice sites. However, one out of them will be occupied by the Sr2+ion while the other will remain unoccupied or it will be vacant. Thus vacancies are introduced in an ionic solid.
Question 23

Ionic solids, which have anionic vacancies due to metal excess defect, develop colour. Explain with the help of a suitale example.

When crystals of NaCl are heated in an atmosphre of sodium vapour, the sodium atoms are deposited on the surface of the crystal. The CI ions diffuse to the surface of the crystal and combine with Na atoms to give NaCl. This happens by loss electron by sodium atom to form Na+ ions. The released electrons diffuse into the crystals and occupy anionic sites. As a result the crystal now has an excess of sodium. They impart yellow colour to the crystals of NaCl.
Question 24

A group 14 element is to be converted into N-type semi-conductor by doping it with a suitable impurity. To which group should this impurity belong?

This impurity should belong to group 15.

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