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

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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 (Mₛ) among all 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, e.g., by interstitial impurities, Fe₆ could become the basis material for high Mₛ rare-earth-free permanent 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 explain the enigmatic high Mₛ of Fe₁₆N₂, which is currently attracting an intense research activity.