Order-Disorder in Fe-Si Alloys: Implications for Seismic Anisotropy and Thermal Evolution of Earth's Inner Core

TL;DR

This study uses first-principles modeling to explore Fe-Si alloy phases at core conditions, revealing phase coexistence, anisotropic properties, and implications for Earth's inner core seismic and thermal behavior.

Contribution

It provides the first detailed phase diagram and property analysis of Fe-Si alloys at inner-core pressures and temperatures, highlighting phase coexistence and anisotropy effects.

Findings

Pronounced miscibility gap between hcp and B2 Fe-Si phases.

B2 Fe-Si exhibits strong shear anisotropy (22.9%).

Lower thermal conductivity of B2 Fe-Si compared to pure iron.

Abstract

Understanding the structure and dynamics of Earth's inner core is essential for constraining its composition, thermal evolution, and seismic properties. Silicon is a probable major component of Earth's core. Using first-principles molecular dynamics and thermodynamic modeling, we investigate the structural, elastic, and transport properties of Fe-Si alloys at high pressures and temperatures. By computing the Gibbs free energies of B2, hcp, fcc, and bcc solid solutions, we construct the Fe-Si phase diagram applicable to the Earth's inner core. Our results reveal a pronounced miscibility gap between hcp and B2 Fe-Si, with the two phases coexisting over the compositional range of 6-11 wt% Si at 6000 K. The B2 Fe-Si alloy exhibits strong single-crystal shear anisotropy (22.9% at 6000 K) compared to the nearly isotropic hcp phase (0.6%), and yields a shear wave velocity (3.73 km/s) and Poisson's ratio consistent with seismological observations. Moreover, the computed transport properties reveal substantially lower thermal conductivity of B2 Fe-Si relative to pure iron or hcp Fe-Si under inner-core conditions. These results imply that Earth's inner core likely comprises multiple phases, whose distribution and crystallographic texture critically influence its seismic and thermal properties.