Formation of Iron-Helium Compounds under High Pressure.

We report the formation of fcc and distorted hcp iron-helium compounds with x in FeHe_{x} 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 FeHe_{x} were still observed by XRD and SIMS measurements. Our first-principles calculations indicate that fcc and hcp FeHe_{x}, 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 ^{3}He.