Question

How can you convert NaCl structure into CsCl structure?

Answer 1

1. To convert the NaCl structure into the CsCl structure, the size of the cations must be increased.
2. In the NaCl structure, which is a face-centred cubic (FCC) lattice, the coordination number is 6 : 6, meaning each Na⁺ ion is surrounded by six Cl⁻ ions and vice versa.
3. This structure is stable when the radius ratio `(r_+/r_-)` is approximately between 0.414 and 0.732.
4. However, the CsCl structure has a body-centred cubic (BCC) arrangement with a coordination number of 8 : 8.
5. For this structure to be stable, the radius ratio must be greater than 0.732. Therefore, by replacing Na⁺ ions with larger Cs⁺ ions, the radius ratio increases sufficiently to allow a transition from the NaCl (FCC) structure to the CsCl (BCC) structure.
6. This substitution changes the ionic arrangement, enabling each ion to be surrounded by eight oppositely charged ions, thus forming the CsCl-type crystal lattice.