The rock salt or sodium chloride (NaCl) lattice structure consists of two inter­mingling cation (Na+) and anion (Cl-) FCC sublattices, as illustrated in Figure 2.20. Alternatively, the rock salt structure can also be described as an FCC anion lattice in which all its octahedral interstitial sites are filled up by the cations. Here nearest neighbors to each ion are the six ions of the opposite charge. Coordination numbers of cations and anions are six each in this struc­ture similar to that for simple cubic structure. The number of octahedral sites in an FCC unit cell is the same as the number of atoms (in this case, anions). Therefore, the stoichiometry of this crystal type is MX (M = metal, X = non­metal). This means that valencies of both cations and anions in this crystal type need to be the same. Some examples of the rock salt structure are KCl, LiF, and KBr (monovalent ions); MgO, CoO, and MnO (divalent ions); and UC (tetravalent ions). Note that UC is a potential nuclear fuel. The Struckturbericht notation of a rock salt structure is B1. There are four ion pairs per unit cell similar to an FCC structure. The lattice constant of a NaCl-type crystal struc­ture is given by 2(R+ + R-), where R+ and R — are the cation and anion radii,

respectively. In rock salt structures, the cation to anion radius ratio can vary between 0.414 and 0.732.