Why does ZnS not crystallize in the NaCl structure ?

Answer

In case ZnS radius ratio is `(r_(Zn^(2+)))/(r_(S^(2))) = 0.402`
That's why `Zn^(2+)` ion is present in tetrahedral void. If `Zn^(2+)` ion is present in octahedral void [For NaCl type structure] then anion-anion contact is there and compound become unstable. That's why. ZnS not crystalize in the NaCl structure.