Searching for high magnetization density in bulk Fe: The new metastable Fe₆ phase

We report the discovery of a new allotrope of iron by first principles calculations. This phase has Pmn2₁ symmetry, a six-atom unit cell (hence the name Fe₆), and the highest magnetization density (M_s) among all the known crystalline phases of iron. Obtained from the structural optimizations of the Fe₃C-cementite crystal upon carbon removal, Pmn2₁ Fe₆ is shown to result from the stabilization of a ferromagnetic FCC phase, further strained along the Bain path. Although metastable from 0 to 50 GPa, the new phase is more stable at low pressures than the other well-known HCP and FCC allotropes and smoothly transforms into the FCC phase under compression. If stabilized to room temperature, for example, by interstitial impurities, Fe₆ could become the basis material for high M_s rare-earth-free permament magnets and high-impact applications such as light-weight electric engine rotors or high-density recording media. The new phase could also be key to explaining the enigmatic high M_s of Fe₁₆N₂, which is currently attracting intense research activity.