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) Na2O

(d) CaF2

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Answer:

Correct Answer: 21. (b)

Solution:

  1. In ZnS,S2 (sulphide ions) are present at fcc positions giving four sulphide ions per unit cell. To comply with 1:1 stoichiometry, four Zn2+ 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 Na2O, Na+ions are present in all its tetrahedral voids giving the desired 2:1 stoichiometry. In CaF2,Ca2+ ions occupies fcc positions and all the tetrahedral voids are occupied by fluoride ions.



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