Ultrahigh Pressure Crystallographic Passage Toward Metallic Hydrogen

Crystallographic evolution of hydrogen under multi-megabar compression is a key problem in condensed-matter physics and unsolved challenge for experimentalists and theoreticians. Although dozens of crystal structures have been proposed by theory, only one, the simple hexagonal-close-packed (hcp) structure, has been previously confirmed in experiments. Utilizing new-generation synchrotron nano-focused X-ray probes developed for micron-sized hydrogen crystals at pressures above 212 GPa, we are able to observe the transition from hcp H2 to a larger hexagonal supercell with lattice parameters expanding to √3✕a and 2✕c of hcp, in which the characteristic hcp peaks (1 0 0), (0 0 2), and (1 0 1) become (1 1 0), (0 0 4), and (1 1 2), respectively, and three new peaks, (1 0 1), (1 0 3), and (2 0 1), of the supercell appear. Theoretical calculations based on our XRD results found a time-averaged structure model in space group P6 ̅2c (190) with alternating layers of spherically disordered H2 molecules and ordered molecules toward a graphene-like atomic H network. Such a hexagonal supercell has not been reported by any previous theoretical study for post-hcp phase, but is close to a number of theoretical models with mixed-layer structures. The clear evidence of a structural transition beyond hcp marks the polymerization nature of molecular-to-atomic hydrogen transition.