Atomic diffusion in solid molecular hydrogen.

Area of Science:

• Condensed matter physics
• Materials science under extreme conditions
• Computational quantum chemistry

Background:

• The C2c and Cmca-12 phases of hydrogen are theoretically predicted to be stable above 200 GPa.
• Previous studies focused on the stability of these phases at 0 K, neglecting temperature effects.
• Systematic investigations into the impact of temperature on hydrogen phase properties at high pressures were lacking.

Purpose of the Study:

• To investigate the influence of temperature on the C2c and Cmca-12 phases of hydrogen at extreme pressures.
• To explore the mechanism of diffusion in these phases and its implications.
• To compute the melting curve of hydrogen at high pressures.

Main Methods:

• Ab initio molecular dynamics simulations were employed.
• Simulations covered pressures ranging from 210 to 350 GPa.
• Analysis focused on the behavior of C2c and Cmca-12 hydrogen phases with increasing temperature.

Main Results:

• Diffusion was observed to occur in the Cmca-12 phase upon increasing temperature, while it remained absent in the C2c phase.
• The mechanism of diffusion was explored.
• The melting curve of hydrogen at extreme pressures was computed.

Conclusions:

• The observed diffusion in the Cmca-12 phase at high temperatures could explain recent experimental findings of conductive hydrogen around 260 GPa.
• Diffusion may also account for discrepancies in Raman spectra observed in recent experiments.
• Temperature plays a crucial role in the properties of high-pressure hydrogen phases.