Short-range order stabilizes a cubic Fe alloy in Earth's inner core

Zhi Li, Sandro Scandolo
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Abstract

The phase diagram and sound velocities of the Fe-Si binary alloy, crucial for understanding the Earth's core, are determined at inner core boundary pressure with \textit{ab-initio} accuracy through deep-learning-aided hybrid Monte Carlo simulations. A complex phase diagram emerges close to the melting temperature, where a re-entrance of the body-centered cubic (bcc) phase is observed. The bcc structure is stabilized by a pronounced short-range ordering of the Si atoms. The miscibility gap between the short-range ordered bcc structure and the long-range ordered cubic B2 structure shrinks with increasing temperature and the transition becomes continuous above 6000 K. We find that a bcc Fe-Si solid solution reproduces crucial geophysical data such as the low shear sound velocity and the seismic anisotropy of the inner core much better than other structures.
短程秩序稳定了地球内核中的立方铁合金
通过深度学习辅助的混合蒙特卡洛模拟,确定了内核边界压力下Fe-Si二元合金的相图和声速,这对理解地核至关重要。在接近熔化温度时,出现了复杂的相图,并观察到体心立方(bcc)相的重新进入。短程有序的 bcc 结构与长程有序的立方 B2 结构之间的混溶隙随着温度的升高而缩小,在 6000 K 以上转变变得连续。我们发现,bcc Fe-Si 固溶体比其他结构更好地再现了关键的地球物理数据,例如内核的低剪切声速和地震各向异性。
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