Sodium iodide crystal, illustration. Sodium iodide (NaI) adopts the rock salt crystal lattice, Here, several cubic unit cells have been cut from a larger crystal. Each cell contains four units of NaI. The sodium ions (Na+, grey) form a face centred cube, using the eight corners and the six faces. Iodide ions (I? purple) form a simple cube using the twelve edge positions and the full site at the centre of the crystal.