Bravais phases of Fe under pressure from first principles

Abstract

The structure and stability of four Bravais phases of Fe under pressure aredetermined by a procedure which first looks for minima of the internal energyE at constantvolume V and then tests the states at the minima for stability (or instability) by showing that the Gibbs free energyG at constantpressure p is a minimum (or not a minimum) with respect to all possible strains. The phasesconsidered here are either body-centered tetragonal (bct), which includes body-centeredcubic (bcc) and face-centered cubic (fcc), or rhombohedral (rh). The results includeshowing that bcc Fe becomes unstable at 1500 kbar, that fcc Fe is stable atp = 0,that a phase transition from bcc to fcc is thermodynamically favored at 290 kbar, that a bct phaseat c/a = 0.89 is unstable up to 2700 kbar and that a rh phase with angleα = 60.5° is stableat p = 0 with E slightly higher than that for fcc Fe.