Elastic Properties of <i>hcp</i> ‐Structured Fe‐C and Fe‐C‐H Alloys at High Pressures: Implications on Earth's Core

Abstract Carbon and hydrogen are both potential light elements in the Earth's core. However, the stability and elastic properties of Fe‐C‐H ternary alloy under high pressure have rarely been explored. In this study, we synthesized a new hcp ‐structured Fe‐C‐H alloy (FeC 0.065 H 1.05 ) at high pressure using the laser‐heated diamond anvil cell. Hydrogen incorporation significantly expands the volume and decreases the bulk modulus of hcp Fe and Fe‐C alloy (FeC 0.065 ). It also elevates the c / a ratio, potentially explaining the anisotropy of the Earth's inner core. Furthermore, the incorporation of carbon and hydrogen considerably decreases the density of hcp Fe. 0.3–0.7 wt.% H and 0.2–0.5 wt.% C in Fe alloys could account for the observed density deficit of the inner core with 7,000–5,000 K inner core boundary temperature. A tradeoff relationship was established that the effect of 4 wt.% carbon is equivalent to 1 wt.% hydrogen on the density of inner core.