Ultrahigh-pressure crystallographic passage towards metallic hydrogen

The structural evolution of molecular hydrogen H₂ under multi-megabar compression and its relation to atomic metallic hydrogen is a key unsolved problem in condensed-matter physics. Although dozens of crystal structures have been proposed by theory^1-4, only one, the simple hexagonal-close-packed (hcp) structure of only spherical disordered H₂, has been previously confirmed in experiments⁵. Through advancing nano-focused synchrotron X-ray probes, here we report the observation of the transition from hcp H₂ to a post-hcp structure with a six-fold larger supercell at pressures above 212 GPa, indicating the change of spherical H₂ to various ordered configurations. Theoretical calculations based on our XRD results found a time-averaged structure model in the space group $$ with alternating layers of spherically disordered H₂ and new graphene-like layers consisting of H₂ trimers (H₆) formed by the association of three H₂ molecules. This supercell has not been reported by any previous theoretical study for the post-hcp phase, but is close to a number of theoretical models with mixed-layer structures. The evidence of a structural transition beyond hcp establishes the trend of H₂ molecular association towards polymerization at extreme pressures, giving clues about the nature of the molecular-to-atomic transition of metallic hydrogen. Considering the spectroscopic behaviours that show strong vibrational and bending peaks of H₂ up to 400 GPa, it would be prudent to speculate the continuation of hydrogen molecular polymerization up to its metallization.