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

Earth's inner core consists of iron (Fe) alloyed with minor light elements, predominantly silicon (Si), yet the crystal structure and seismic velocities of Fe-Si alloys at inner-core conditions remain poorly constrained. Ab-initio methods struggle with the alloy's vast configurational complexity, limiting reliable property predictions. To overcome this, here we integrate a hybrid Monte Carlo sampling algorithm with a deep-learning interatomic potential to compute the Fe-Si binary phase diagram and sound velocities at inner-core boundary pressures. A complex phase diagram emerges, featuring the re-entrance of a body-centered cubic (bcc) phase stabilized by pronounced short-range ordering of the Si atoms. The bcc phase reproduces key seismic features of the inner core, including low shear-wave velocities and seismic anisotropy, more faithfully than other competing close-packed structures, making it a leading candidate for the inner-core structure. Our results underscore the importance of accurately describing light-element effects on Earth's core properties.