In a cubic ZnS lattice if the radii of Zn^2+ and *^2- ions are 0.83 A and 1.74 A, what is the edge length of the unit cell?

Question

In a cubic ZnS lattice if the radii of Zn2+ and S2- ions are 0.83 A and 1.74 A, what is the edge length of the unit cell?

Answer 1

S2- are in CCP i.e. at the corners and Zn2+ are in alternate tetrahedral voids, i.e. at the centre of half the body diagonal. So S2- and Zn2+ touch each other at the midpoint of half the body diagonal.