Ab initio study of the miscibility for solid hydrogen-helium mixtures at high pressure

Area of Science:

• Condensed matter physics
• Materials science
• Computational chemistry

Background:

• High-pressure behavior of hydrogen-helium (H2-He) mixtures is crucial for planetary science.
• Conflicting experimental results exist regarding H2-He miscibility in solids at high pressures.
• Previous studies lack definitive explanations for observed experimental phenomena.

Purpose of the Study:

• To investigate the miscibility and reactivity of solid H2-He mixtures across a wide pressure range (0-200 GPa).
• To resolve discrepancies between experimental findings on H2-He mixtures.
• To identify the source of characteristic vibrational peaks observed in high-pressure experiments.

Main Methods:

• First-principles calculations were employed to model H2-He mixtures.
• Structure prediction methods were combined with theoretical calculations.
• Calculated Raman modes were compared with experimental data.

Main Results:

• No evidence of miscibility or chemical reactivity was found in H2-He mixtures for any H:He ratio.
• Calculated Raman modes for nitrogen-hydride (N-H) mixtures better matched experimental observations than H2-He modes.
• The findings support the experimental results reported by Turnbull et al.

Conclusions:

• Solid H2-He mixtures are immiscible and unreactive under high pressure.
• Observed experimental vibrational modes are likely attributable to N-H interactions, not H-He.
• Theoretical calculations provide a framework for reinterpreting experimental data in high-pressure hydrogen-based systems.