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Ultrahigh-pressure crystallographic passage towards metallic hydrogen

Journal Article · · Nature (London)
 [1];  [2];  [3];  [4];  [3];  [5];  [6];  [7];  [6];  [6];  [8];  [2];  [2];  [9];  [3];  [3];  [2];  [2];  [1]
  1. Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing (China); Shanghai Advanced Research in Physical Sciences (SHARPS) (China)
  2. Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing (China)
  3. Jilin Univ., Changchun (China)
  4. Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing (China); Deutsches Elektronen-Synchrotron (DESY), Hamburg (Germany)
  5. Deutsches Elektronen-Synchrotron (DESY), Hamburg (Germany)
  6. Lund Univ. (Sweden)
  7. Lund Univ. (Sweden); Brazilian Center for Research in Energy and Materials (CNPEM), Campinas, SP (Brazil). Brazilian Synchrotron Light Laboratory (LNLS)
  8. Argonne National Laboratory (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
  9. Stanford Univ., CA (United States); SLAC National Accelerator Laboratory (SLAC), Menlo Park, CA (United States). Stanford Institute for Materials and Energy Science (SIMES)
+ Show Author Affiliations

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 XRD 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.

Research Organization:
Argonne National Laboratory (ANL), Argonne, IL (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF)
Grant/Contract Number:
AC02-06CH11357
OSTI ID:
3000427
Journal Information:
Nature (London), Journal Name: Nature (London) Journal Issue: 8064 Vol. 641; ISSN 1476-4687; ISSN 0028-0836
Publisher:
Nature Publishing GroupCopyright Statement
Country of Publication:
United States
Language:
English

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