Ultrahigh-pressure crystallographic passage towards metallic hydrogen

Area of Science:

• Condensed-matter physics
• Materials science under extreme conditions

Background:

• The structural evolution of molecular hydrogen (H2) under extreme pressures is a fundamental unsolved problem.
• Previous experiments confirmed only the simple hexagonal-close-packed (hcp) structure of disordered spherical H2.
• Understanding this transition is crucial for comprehending the path to atomic metallic hydrogen.

Purpose of the Study:

• To investigate the structural changes of molecular hydrogen (H2) at multi-megabar pressures.
• To identify new crystal structures beyond the previously known hcp phase.
• To provide insights into the molecular-to-atomic transition of hydrogen.

Main Methods:

• Utilized advanced nano-focused synchrotron X-ray probes for structural analysis.
• Experimentally observed structural transitions at pressures above 212 GPa.
• Performed theoretical calculations and structural modeling based on experimental X-ray diffraction (XRD) data.

Main Results:

• Observed a transition from the hcp H2 structure to a novel post-hcp structure with a larger supercell.
• The post-hcp structure features alternating layers of disordered H2 and graphene-like H6 (H2 trimers).
• This newly identified supercell structure was not predicted by prior theoretical studies.

Conclusions:

• The experimental evidence demonstrates a structural transition beyond hcp, indicating molecular association and polymerization of H2 at extreme pressures.
• The findings suggest a continuation of hydrogen molecular polymerization up to its metallization phase.
• This study provides critical clues regarding the nature of the molecular-to-atomic transition in hydrogen.