Three ionic crystal forms, illustration. Three of the most common ionic crystal forms are shown here as space filling models. In sodium chloride (NaCl, left), the Na+ (red) and Cl?(mint green) ions form interlocking face-centred cubic structures. Calcium fluoride (CaF2, centre) in the fluorite lattice, has Ca(2+) cations (blue) in a face-centre cubic structure, with fluoride ions (F? yellow-green) in the eight tetrahedral voids. The fluorite lattice is used by Zirconium dioxide (ZrO2), lithium dioxide (Li2O), sodium sulphide (Na2S), strontium fluoride (SrF2) and others. In the caesium chloride lattice (right), chloride anions (Cl? pink) form a primitive cube with a caesium cation (Cs+, purple) in the centre.