# Question: a prove that the ideal ca ratio for the hexagonal...

###### Question details

a) Prove that the ideal c/a ratio for the hexagonal close-packed (hcp) structure is given by c/a≈ 1.633.

b) Sodium transforms from bcc to hcp at about 23 K (the “martensitic” transformation). Assuming that the density remains fixed through this transition, find the lattice constant a of the hexagonal phase, given that a = 4.23 Å in the cubic phase and that the c/a ratio is indistinguishable from its ideal value.