Electronic structure, entropy and the high-pressure stability of bcc iron

Abstract

The high‐pressure electronic structures BCC, FCC, HCP and BCT iron are investigated using the full‐potential linearized augmented plane‐wave (FLAPW) method solved in the local density approximation. The static total energies suggest that, upon compression, BCC iron is dynamically unstable with respect to FCC iron. No BCT (or FCT) structure is found to be stable under compression. From the density of states, however, BCC Fe is found to have a larger electronic entropy than HCP Fe. Moreover, both FCC and HCP iron are non‐magnetic (low‐spin) while BCC iron is paramagnetic at pressures up to 400 GPa. The magnetic entropy of Fe in the BCC structure is large enough to plausibly stabilize BCC iron near 200 GPa and 4000 K and account for the observed solid‐solid phase transition in the shock‐wave Hugoniot of iron. This excess entropy will disappear, however, as the Fe spin moment in BCC iron decreases to zero at pressures between 300 and 400 GPa.