Discussion
P, F and I represent primitive, face-centered and body-centered lattices, respectively. The lattice types of NaCl and CsCl, respectively, are
Answer: B
The lattice types are F and P.
NaCl has a face-centred cubic lattice: the Cl- ions form an fcc array, and each lattice point carries a basis of one Na+ and one Cl-, so the lattice type is F.
In CsCl, Cs+ sits at the body centre of a cube of Cl- ions. It looks body-centred, but the body-centre site holds a different ion from the corners, so it is not a lattice point equivalent to the corners. The lattice is simple cubic (P) with a two-ion basis.
Calling CsCl body-centred (I) is the common trap; a true I lattice needs identical atoms at the corners and the centre, as in metallic iron.
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