image: The phase boundary is marked by the thick black line and different structures within each phase are determined based on the molecular dynamics results, where triangles, stars, and circles represent, respectively, the solid, superionic (with diffusive H atoms), and liquid structures. Also presented are the phase coexistence probabilities, as indicated by the colored scale bar shown in the inset, in the context of the Boltzmann distribution for the FeO2H2He structures relative to the dehydrogenation products FeO2H, H2 and He at selected points along the geotherm in the pressure range of 100-140 GPa. The yellow region presents the geotherm of the Earth's core. view more
Credit: ©Science China Press
This study is led by Prof. Hanyu Liu, Prof. Yanming Ma (College of Physics, Jilin University) and Prof. Changfeng, Chen (Department of Physics and Astronomy, University of Nevada). Their simulations present evidence of direct and prevalent chemical association of hydrogen and helium facilitated by their reaction with recently discovered iron peroxide FeO2 in forming rare quaternary compound FeO2H2He, which has been predicted to be viable in a large region of the pressure-temperature phase diagram. Most interestingly, in a wide swath of the phase space corresponding to Earth's lowest-mantle regions, this quaternary compound stays in a superionic state hosting liquid-like hydrogen inside the FeO2He sublattice that remains intact in crystalline form. This exotic solid-liquid mixture state of matter makes an unusually conducive environment promoting close coalescence of hydrogen and helium. “These results highlight a compelling case of hydrogen-helium chemical association, which may be harbored in deep-Earth regions, providing crucial guidance for exploring the novel solid and superionic phases of FeO2H2He in laboratory experiments and also for modeling interiors of giant solar and extrasolar planets”, Liu says.
Hydrogen and helium are the most abundant elements in the universe and play crucial roles in geological and astrophysical environments, but they are known to be inert toward each other across wide pressure-temperature and concentration ranges and remain largely immiscible up to multi-megabar pressures and 3,000-4,000 K temperatures. Given their prominent presence and influence on the formation and evolution of celestial bodies, it is of great interest and significance to explore and decipher the nature of interactions between hydrogen and helium, especially possible chemical association that would have considerable impacts in many scientific fields, from chemistry, physics, geoscience to astrophysics.
By employing an advanced crystal structure search method as called CALYPSO, they identified a quaternary compound FeO2H2He, which could be stabilized in a wide range of pressure and temperature conditions. Further molecular dynamics simulations indicate a novel superionic state of FeO2H2H, which hosted liquid-like diffusive hydrogen in the FeO2He sublattice. Meanwhile, a conducive environment for hydrogen-helium chemical association was identified, where the pressure and temperature conditions correspond to the Earth's lowest mantle regions. “These results suggested the surprising chemically facilitated coalescence of otherwise immiscible molecular species highlights a promising avenue for exploring this long-sought but hitherto unattainable state of matter”, Liu says, “this finding raises strong prospects for exotic H-He mixtures inside Earth, as well as providing implications for other astronomical bodies.
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See the article:
Direct H-He Chemical Association in Superionic FeO2H2He at Deep-Earth Conditions
https://doi.org/10.1093/nsr/nwab168
Journal
National Science Review