Examples of ionic structures based on the simple cubic packing of anions are CsCl and CaF2(fluorite). In CsCl all the cubic interstitial sites are occupied by caesium cations(Fig. 1.120b) but in fluorite only half the sites are occupied by the calcium cations. The resulting unit cell is not just one simple cube of flubrine anions, but a larger cube with a cell side double that of the simple cube and containing therefore 2 x 2 x 2 8 cubes, four of which contain calcium cations and four of which are empty.
The distribution of the small calcium cations in the cubic sites is such that they form an fcc array and the fluorite structure can be represented alternatively as an fcc array of calcium cations with all the tetrahedral sites occupied by fluorine anions. It is identical, in terms of the distribution of ionic sites, to the structure of TiH2 or Li2O (Fig. 1.14(b), except that the positions of the cations and anions are reversed; hence Li2O is said to have the antifluorite structure. However, these differences, although in principle quite simple, may not be clear until we have some better method of representing the atom/ion positions in crystals other than the sketches(or clinographic projections) used Figs.1.10-1.15