Solid State - Result Question 21
####21. Which of the following fcc structure contains cations in alternate tetrahedral voids?
(2005, 1M)
(a) $NaCl$
(b) $ZnS$
(c) $Na _2 O$
(d) $CaF _2$
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Answer:
Correct Answer: 21. (b)
Solution:
- In $ZnS, S^{2-}$ (sulphide ions) are present at fcc positions giving four sulphide ions per unit cell. To comply with $1: 1$ stoichiometry, four $Zn^{2+}$ ions must be present in four alternate tetrahedral voids out of eight tetrahedral voids present.
In $NaCl, Na^{+}$ions are present in octahedral voids while in $Na _2 O$, $Na^{+}$ions are present in all its tetrahedral voids giving the desired $2: 1$ stoichiometry. In $CaF _2, Ca^{2+}$ ions occupies fcc positions and all the tetrahedral voids are occupied by fluoride ions.
