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

Abstract

Earth’s inner core is predominantly composed of solid iron (Fe) and displays intriguing properties such as strong shear softening and an ultrahigh Poisson’s ratio. Insofar, physical mechanisms to explain these features coherently remain highly debated. Here, we have studied longitudinal and shear wave velocities of hcp-Fe (hexagonal close-packed iron) at relevant pressure–temperature conditions of the inner core using in situ shock experiments and machine learning molecular dynamics (MLMD) simulations. Our results demonstrate that the shear wave velocity of hcp-Fe along the Hugoniot in the premelting condition, defined as T/T m ( T m : melting temperature of iron) above 0.96, is significantly reduced by ~30%, while Poisson’s ratio jumps to approximately 0.44. MLMD simulations at 230 to 330 GPa indicate that collective motion with fast diffusive atomic migration occurs in premelting hcp-Fe primarily along [100] or [010] crystallographic direction, contributing to its elastic softening and enhanced Poisson’s ratio. Our study reveals that hcp-Fe atoms can diffusively migrate to neighboring positions, forming open-loop and close-loop clusters in the inner core conditions. Hcp-Fe with collective motion at the inner core conditions is thus not an ideal solid previously believed. The premelting hcp-Fe with collective motion behaves like an extremely soft solid with an ultralow shear modulus and an ultrahigh Poisson’s ratio that are consistent with seismic observations of the region. Our findings indicate that premelting hcp-Fe with fast diffusive motion represents the underlying physical mechanism to help explain the unique seismic and geodynamic features of the inner core.

Thermoelastic Properties of B2‐Type FeSi Under Deep Earth Conditions: Implications for the Compositions of the Ultralow‐Velocity Zones and the Inner Core

“…Additionally, V ϕ of H‐bearing B2‐Fe displays no premelting phenomena at the temperature of ICB up to 6,500 K, aligning with the observations of V ϕ for hcp Fe under high pressure and temperature conditions, as reported by Zhang et al. (2023). Consequently, given the high V ϕ value of ∼11.8 km/s for B2‐FeSi at ICB conditions, we conclude that neither H‐free nor H‐bearing B2‐FeSi is a major component in the inner core.…”

Reassessment of Birch's Law on hcp‐Fe From Ultrasonic Sound Velocity Measurement and Implications on the Velocity Profiles of Earth's Inner Core

“…Initially, fitting all data sets together (Case 1, Figure S5 in Supporting Information S1) yields ( ∂ 2 K S / ∂P∂T ) P = 1.16(49) × 10 −4 K −1 and ( ∂ 2 G / ∂P∂T ) P = 3.81(47) × 10 −4 K −1 . The results are plotted in Figure 8 against density together with some reported results on V P and V S at high temperatures (Antonangeli et al., 2012; Brown & Mcqueen, 1986; Huang et al., 2022; Lin et al., 2005; Z. Mao et al., 2012; Ohtani et al., 2013; Zhang et al., 2023). This fitting provides a good estimate of V P for over the density range while the deviation in V S is slightly larger, likely due to the inherent challenges in accurately measuring shear wave velocities, leading to increased uncertainties in the corresponding data.…”

“…(2022) and Zhang et al. (2023) have shown minimal effects on V P but significant shear softening due to melting or pre‐melting effects as conditions approach those of the core. With the current data, which provides precise anchor points and constraints on K S 0 , G 0 , and their first pressure/temperature derivatives, we performed the joint fitting with various data sets into Equations 1–11.…”

“…Instead of using the empirical relation (e.g., Birch's law), we take the finite strain approach to extrapolate our results of both V P and V S to the core conditions. To constrain the cross derivatives of pressure and temperature on bulk and shear moduli [( ∂ 2 K S / ∂P∂T ) P and ( ∂ 2 G / ∂P∂T ) P ], we included three recent studies (Huang et al., 2022; Sakamaki et al., 2016; Zhang et al., 2023) which reached over 3000 K for the sound velocity measurements at high P‐T conditions for a joint analysis. Huang et al.…”

“…(2022) and Zhang et al. (2023) used shock compression to obtain density, V P and bulk sound velocity V B directly, and then derived the shear wave velocity through V S = [3( V P 2 − V B 2 )/4] 0.5 . Sakamaki et al.…”