Effect of non-uniform stresses on bcc Fe and Fe-Ni alloys at Earth's core conditions

• Elucidation of Earth's core composition and structure is needed for the understanding of various phenomena, from geodynamics to geomagnetism. Geochemical data suggest that solid Fe alloyed with Ni is Earth's core main constituent, however, the exact content and crystal structure remain unclear. We report the results of ab initio molecular dynamics study of the mechanical stability of the body-centered cubic (bcc) phase of Fe and Fe0.89Ni0.ll. A striking saw-toothed effect in the dependence of stresses on finite distortions is revealed. The calculated region of mechanical stability of the bcc iron under non- hydrostatic stress at Earth's core conditions is narrow, and about 5 GPa stresses should destabilize it. While it is unlikely that stresses in Earth's core are sufficient to mechanically destabilize bcc iron, experimental controversies and puzzling stabilization of diverse phases may be well explained by the observed saw-toothed stress effect. It might be common for different materials close to dynamical instability and has crucial importance for determination of equilibrium phase boundaries.

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