The structure description of a certain ceramic compound MaXb, where M is a cation and X an

anion and a and b are integers, is as follows.

- The anions form an FCC structure, and

- the cations occupy all of the tetrahedral interstitial sites.

Determine the ratio a/b, and plot schematically the structure of one unit cell for this compound

(you need to mark your plotting clearly and give reasons whenever needed). What are the

coordination numbers of the cation and anion, respectively?