Competing Phases of Iron at Earth's Core Conditions From Deep-Learning- Aided ab-initio Simulations

Z Li and S Scandolo, GEOPHYSICAL RESEARCH LETTERS, 51, e2024GL110357 (2024).

DOI: 10.1029/2024GL110357

The properties and relative stability of different structures of iron at the extreme conditions of pressure and temperature of relevance for the Earth's core were determined with ab-initio atomistic simulations aided by a machine-learning force-field. We find that the body-centered cubic (bcc) structure is mechanically stable at core temperatures, but its free energy is marginally higher than those of the hexagonal close- packed and face-centered cubic structures. The bcc structure is the only structure whose shear sound velocity matches seismic data. The small free-energy difference between competing structures suggests that the role of impurities could be crucial in stabilizing the bcc structure in the inner core. Plain Language Summary Determining the crystal structure and the elastic properties of the compound that forms the Earth's solid inner core is crucial to interpret seismic data. We know that the inner core is predominantly composed of iron, but laboratory-based experiments and theoretical modeling haven't yet been able to constrain the crystal structure and the properties of pure Fe at the conditions of pressure and temperature found in the inner core. We have recently developed a deep-learning-aided atomistic simulation method that is able to determine Gibbs free energies of solids with quantum-chemical accuracy (a few meV/atom). We find that although body-centered cubic Fe is energetically slightly less favored than the hexagonal close-packed form, the shear wave velocity of bcc Fe matches seismic data much better than all other crystal structures, suggesting that bcc is a strong candidate for the crystal structure of Fe in the Earth's inner core and could be stabilized by the presence of light elements in the core.

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