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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, only one, the simple hexagonal-close-packed (hcp) structure of only spherical disordered H₂, has been previously confirmed in experiments. 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 XRDmore » results found a time-averaged structure model in the space group $$P\bar{6}2c$$ 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. Here, 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.« less

Diamond anvil cell techniques have been improved to allow access to the multimegabar ultrahigh-pressure region for exploring novel phenomena in condensed matter. However, the only way to determine crystal structures of materials above 100 GPa, namely, X-ray diffraction (XRD), especially for low Z materials, remains nontrivial in the ultrahigh-pressure region, even with the availability of brilliant synchrotron X-ray sources. In this work,we perform a systematic study, choosing hydrogen (the lowest X-ray scatterer) as the subject, to understand how to better perform XRD measurements of low Z materials at multimegabar pressures. The techniques that we have developed have been proved tomore » be effective in measuring the crystal structure of solid hydrogen up to 254 GPa at room temperature [C. Ji et al.,Nature 573, 558–562 (2019)]. We present our discoveries and experiences with regard to several aspects of this work, namely, diamond anvil selection, sample configuration for ultrahigh-pressure XRD studies,XRD diagnostics for low Z materials, and related issues in data interpretation and pressure calibration.We believe that these methods can be readily extended to other low Z materials and can pave the way for studying the crystal structure of hydrogen at higher pressures, eventually testing structural models of metallic hydrogen.« less