Is there an expansion or contraction as iron transform from HCP to BCC? The atomic radius of iron is — Solid State Chemistry Question

💡 Solution & Explanation

HCP volume per atom = $\frac{4\sqrt{2} r_{HCP}^3}{6} \times \text{(atoms per unit cell)} = 4\sqrt{2} (125)^3 \approx 11.04 \times 10^6 \text{ pm}^3$. BCC volume per atom = $\frac{32 r_{BCC}^3}{3\sqrt{3}} = \frac{32 (50\sqrt{2})^3}{3\sqrt{3}} \approx 2.17 \times 10^6 \text{ pm}^3$. The volume per atom decreases, indicating a contraction. Therefore, correct answer is B.