Stable compounds may have stored helium and xenon during Earth’s formation

Much is still unknown about the conditions present during the Earth’s formation. For instance, previous studies indicate that helium (He) and xenon (Xe) are abundant in the planet’s interior, but the only discovered compounds capable of sequestering these elements exist at extremely high temperatures and pressures not possible during the planet’s early years. Identifying compounds that could have held these noble gases when the Earth formed has proven challenging.

Using computational and theoretical techniques, Zhang et al. followed trends they discovered to find several He/Xe-bearing iron halide compounds stable at lower pressures.

“If the electron-negativity between iron (Fe) and other elements is larger, the reaction pressure is lower,” said author Hanyu Liu. “Furthermore, previous studies did not find any favorable binary He and Xe compounds at moderate pressures. This motivated us to explore more complex compounds, such as ternary He or Xe compounds.”

Using a combination of a crystal structure search approach and first-principles calculations, they found multiple compounds, FeF₂He, FeF₃He, FeF₂Xe, FeF₃Xe, and FeCl₃Xe, that are stable over a wide range of temperatures and at pressures below 60 GPa.

The team hopes their work will shed new light on the conditions present on primordial Earth that caused its composition today.

“The results of this study put forward new stable iron halide compounds, expand the possible forms and ranges of He and Xe to the Earth’s lower mantle, provide a theoretical basis for further study of Earth evolution and the allocation of inert elements, and greatly increase our understanding of the physical and chemical properties of inert elements,” said Liu.

Source: “Sequestration of helium and xenon via iron-halide compounds in early earth,” by Jurong Zhang, Hanyu Liu, Changfeng Chen, and Yanming Ma, Matter and Radiation at Extremes (2024). The article can be accessed at https://doi.org/10.1063/5.0164149 .

This paper is part of the High Pressure Science 2024 Collection, learn more here .