Question

What type of stoichiometric defect is shown by ZnS?
(a)Schottky defect
(b)Frenkel defect
(c)Both Frenkel and Schottky defect
(d)Non-stoichiometric defect

Answer 1

Hint: Stoichiometric defects are the defects in a solid which does not disturb the stoichiometry of the solid. Zn and sulphur have different atomic sizes, this can cause a stoichiometric defect.

Complete step by step solution:
Some crystal lattices of solids have the same number of positive ions and the same number of negative ions. Their ratios are exactly indicated by their chemical formulae. Such solids are called stoichiometric solids. Defects caused in such solids without changing its stoichiometry are called stoichiometric defects.
-There are two types of stoichiometric defects. One is schottky defect and the other is frenkel. Schottky defect occurs when equal numbers of cations and anions are missing from the lattice.
-Frenkel defect arises when an ion is missing from its actual lattice site and it occupies any interstitial site. The whole electrical neutrality of the solid is maintained because the number of cation and number of anions remains the same. This defect mainly occurs when there is a large difference in the size of cation and anion. This defect mainly occurs in compounds having low coordination numbers and the anion and cation has different sizes.
-In case of zinc sulphide, there is a huge difference in the size of zinc cation and size of sulphur anion. So, it shows a frenkel defect. Size of \[Z{{n}^{2+}}\]is much smaller than the size of \[{{S}^{2-}}\]. Here ZnS has a lower coordination number and due to this defect, there is not significant change in the density of the solid. Electric neutrality is also maintained and no changes in its properties.

Thus, ZnS shows a frenkel defect and the correct option is (b).

Note: Important thing to note is that in the frenkel defect, there is no change in stoichiometry of the solid. There is only displacement of cations from its lattice site.