The CsCl structure contains cations and anions with same valencies similar to those of rock salt-type structure. However, the CsCl crystal structure is essentially different from the NaCl structure. The CsCl structure can be viewed as two interlac­ing, equal-sized simple cubic cation and anion sublattices. It can also be shown as a simple cubic unit cell of cations (Cs+) with an anion (CT) in the body center of the cube and vice versa. The lattice constant (a) remains the same regardless of the way to view the structure (Figure 2.21). Each ion in this structure is surrounded by eight ions of the opposite type (i. e., coordination number 8 for both cations and anions similar to that of BCC structure) as the first nearest neighbors. There is only one ion pair per unit cell of CsCl as per simple cubic structure. Whether a solid of MX type would assume a NaCl-type structure or CsCl-type structure depends largely on the cation/anion radii ratio. For CsCl, the radius ratio is about 0.92, whereas for NaCl it is 0.61. Structures with the radius ratio higher than 0.732 tend to have eight coordination and may assume CsCl-type structure. Other examples of this type of structure are CsI and CsBr. Note that CsI may be found in the spent fuels among other compounds as Cs and I are generated as fission products in the reactor fuels. However, during the reactor operation, it stays in gaseous form (not crystalline)

inside the fuel. In accident situations, CsI can escape the cladding containment and form solid phases (crystalline) if it comes into contact with the cooling water.