Ceramics and glasses
Which statement about the CsCl structure is correct?
- ACs⁺ and Cl⁻ each form a simple cubic lattice, interpenetrating each other, and the coordination number is 8
- BIt is a kind of body-centered cubic (BCC) lattice with the same kind of ion at the corners and the body center, giving 2 ions of the same kind per unit cell
- CIt is the same structure as rock salt, with Cs⁺ and Cl⁻ alternating on an FCC lattice, and each ion has a coordination number of 6
Show answer and explanation
Answer: A. Cs⁺ and Cl⁻ each form a simple cubic lattice, interpenetrating each other, and the coordination number is 8
The CsCl structure has Cl⁻ at the cube corners and Cs⁺ at the body center. Because the radius ratio (about 0.94) exceeds 0.732, each Cs⁺ is surrounded by 8 Cl⁻ and the coordination number is 8. It looks like BCC, but the corner and body-center ions are of different kinds and cannot be brought into coincidence by a translation, so the Bravais lattice is simple cubic. In other words, it is two interpenetrating simple cubic lattices, and calling it BCC is a common mistake. Two kinds of ions alternating on an FCC lattice with a coordination number of 6 describes the NaCl (rock salt) structure.