Earth’s inner core composition inferred from the equations of state of Fe-Si-C-H alloys

The Earth’s inner core is primarily composed of iron alloyed with lighter elements, yet their precise concentrations remain uncertain. Understanding the effect of light elements on core properties is crucial for constraining its composition. We conducted synchrotron X-ray diffraction experiments on Fe-Si-C-H alloys compressed up to 128 gigapascal (GPa) to derive their equations of state and extrapolate the density and bulk sound velocity to inner core conditions. Our results, substantiated by metal-silicate partitioning and phase relations constraints, indicate that a hexagonal close-packed iron alloy containing ~3–4 weight % silicon, with up to 0.5 weight % carbon, and 0.4 weight % hydrogen, aligns best with seismic observations . These findings suggest silicon is an essential component of the inner core, while carbon and hydrogen can substitute for each other within limited amounts. This refined compositional model advances understanding of the Earth’s inner core and provides geophysical interpretations of its properties.