Question

Which of the following $fcc$ structure contains cations in alternate tetrahedral voids?

• A. $NaCI$
• B. $ZnS$
• C. $Na_2O$
• D. $CaF_2$

Answer 1

Answer: (B)

$ZnS$

Answer 2

In ZnS, $S^{2-}$ (sulphide ions) are present at fee positions giving four sulphide ions per unit cell. To comply with 1 : 1 stoichiometry, four $Zn^{2+}$ ion must be present in four alternate tetrahedral voids out of eight tetrahedral voids present. In NaCI, $Na^+$ ions are present in octahedral voids
while in $Na_2O , Na^+$ ions are present in all its tetrahedral voids giving the desired 2:1 stoichiometry. In $CaF_2 , Ca^{2+}$ ions occupies fee positions and all the tetrahedral voids are occupied by fluoride ion