Collective motion in hcp-Fe at Earth's inner core conditions

Area of Science:

• Geophysics
• Materials Science
• Condensed Matter Physics

Background:

• Earth's inner core, primarily solid iron (Fe), exhibits unusual properties like shear softening and a high Poisson's ratio.
• The underlying physical mechanisms for these seismic anomalies in the inner core remain a subject of debate.

Purpose of the Study:

• To investigate the elastic properties of hexagonal close-packed iron (hcp-Fe) under inner core pressure-temperature conditions.
• To elucidate the atomic-level mechanisms responsible for the observed seismic behavior of the inner core.

Main Methods:

• In situ shock experiments were conducted to measure longitudinal and shear wave velocities of hcp-Fe.
• Machine learning molecular dynamics (MLMD) simulations were employed to model atomic behavior at high pressures (230–330 GPa).

Main Results:

• In premelting hcp-Fe (T/T_m > 0.96), shear wave velocity decreased by ~30%, and Poisson's ratio increased to ~0.44.
• MLMD simulations revealed collective atomic motion and fast diffusive migration along specific crystallographic directions in premelting hcp-Fe.
• These atomic dynamics lead to elastic softening and an enhanced Poisson's ratio, consistent with seismic observations.

Conclusions:

• Premelting hcp-Fe with collective diffusive motion behaves as an extremely soft solid, not an ideal solid.
• This premelting behavior provides a physical explanation for the enigmatic seismic and geodynamic characteristics of Earth's inner core.