Formation of Iron-Helium Compounds under High Pressure

TL;DR

This study combines experiments and first-principles calculations to demonstrate the formation and stability of iron-helium compounds under high pressure, suggesting Earth's core may contain primordial helium.

Contribution

It provides new experimental and theoretical evidence for stable iron-helium compounds at high pressures, revealing helium's potential role in Earth's core composition.

Findings

FeHex compounds form at 5-54 GPa and ~1000-2820 K.

FeHex remains stable after pressure release at room temperature.

Helium occupies interstitial sites in FeHex, stable up to 50 GPa.

Abstract

We report the formations of fcc and distorted hcp iron-helium compounds with x in FeHex up to 0.13 and 0.48, respectively, based on experiments at 5-54 GPa and ~1000-2820 K. Upon releasing pressure under room temperature, these fcc and distorted hcp FeHex were still observed by XRD and SIMS measurements. Our first-principles calculations indicate that fcc and hcp FeHex, with helium atoms occupying the tetrahedral and trigonal-planar interstitial sites (instead of the octahedral sites), are dynamically stable throughout 0-50 GPa. These results support that the Earth's core can be a large reservoir of primordial 3He.