Abstract
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 theory1,2,3,4, 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 \(P\bar{6}2c\) 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. 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.
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Data availability
The data that support the findings of this study are available from the corresponding author upon request. Raw SCXRD data are available at Zenodo (https://zenodo.org/uploads/14032134)53. Source data are provided with this paper.
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Acknowledgements
We thank H. Shu for assistance with gas loading systems. We acknowledge financial supports from the National Science Foundation of China (NSFC) 12074014, the National Key Research and Development Program of China 2023YFA1608902 and 2022YFA1402301, the NSFC U2230401, 52288102, 52090024 and T2495233, the Shanghai Science and Technology Committee, China (no. 22JC1410300), the Shanghai Key Laboratory of Material Frontiers Research in Extreme Environments, China (no. 22dz2260800) and the Fundamental Research Funds for the Central Universities. This is also a contribution to the project of Theory of Hydrocarbon Enrichment under Multi-Spheric Interactions of the Earth (THEMSIE04010102). Portions of this work were performed at P02.2, Petra III, Deutsches Elektronen Synchrotron (DESY) in Germany, NanoMAX, MAX IV in Sweden, 34-ID-E, Advanced Photon Source (APS), Argonne National Laboratory (ANL) in the United States, as well as BL15U1, Shanghai Synchrotron Radiation Facility (SSRF) in China. We acknowledge the MAX IV Laboratory for time on Beamline NanoMAX under proposals 20190637, 20220481 and 20230465. Research conducted at MAX IV, a Swedish national user facility, is supported by the Swedish Research Council under contract no. 2018-07152, the Swedish Governmental Agency for Innovation Systems, under contract no. 2018-04969 and Formas under contract no. 2019-02496. This research used resources of the Advanced Photon Source, a US Department of Energy, Office of Science User Facility, operated by Argonne National Laboratory under contract no. DE-AC02-06CH11357.
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H.-K.M. conceived and supervised the project; C.J., B.L., Y.Z., K.G., A.B., L.A.B.M. and Y.G. performed synchrotron SCXRD measurements; K.G., A.B., M.K., S.K. and W.L. developed and set up the synchrotron nano-probe beamlines; C.J. and J.W. improved the sample preparation techniques for nano-probe SCXRD measurements with C.J. preparing ultrahigh-pressure hydrogen samples; C.J., H.-K.M. and Y.D. performed data analysis; J.L., Y.L., H.L. and Y.M. performed theoretical calculations. H.-K.M., C.J. and W.L.M. wrote the paper in consultation with K.G., L.A.B.M., J.L., H.L. and W.Y.
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Extended data figures and tables
Extended Data Fig. 1 Post-hcp data points in phase diagram.
(a) Phase diagram of hydrogen. Lines represent phase boundary, stars represent pressure points at which SCXRD data indicate a post-hcp structure. All data were collected at room temperature. Similarity in colours of phases IV, IV’, and V represents their similarity in crystal structure as indicated by spectroscopic data. The drawing of this phase diagram takes references of previous studies11,18,20. (b) Enlarged view of the phase diagram included in the red dash box in (a).
Extended Data Fig. 2 Qualities of rotation center alignment with and without Au position marker.
(a) Micro-image of H2 sealed in MgO-epoxy insert gasket without Au position marker. (b) Rotation center alignment by scanning MgO (2 0 0) peak at three different angles. Position with no MgO peak is where H2 is located. Dash lines mark the center positions of H2 at different angles. (c) and (d) show micro-images of H2 sealed in MgO-epoxy insert gasket with Au position marker with transmitted light and reflected light illumination, respectively. A piece of 2 to 3 μm Au was loaded nearby H2 sample. (e) Rotation center alignment by scanning Au (1 1 1) Bragg peak at three different angles. Dash lines mark the center positions of Au at different angles. With Au position marker, the precision of rotation center alignment is substantially improved.
Extended Data Fig. 3 Rocking curves of Bragg peaks measured in ten crystal grains.
(a) and (b) belong to the data measured at 221 GPa and 245 GPa (corresponding to Fig. 1g and f, respectively). (c) to (j) belong to grains #8, #1, #2, #3, #4, #6, #7, and #9 measured at 233 GPa, 212 GPa, 212 GPa, 213 GPa, 218 GPa, 223 GPa, 232 GPa, and 243 GPa, respectively. Such sharp rocking curves are typical for hydrogen at two megabar pressures measured using nano-probe. Miller indexes are in the \(\sqrt{3}\) × a 2 × c supercell. In each crystal grain, intensity is normalized against the strongest Bragg peak, of which the value is set to 100.
Extended Data Fig. 4 Raw XRD images of the other seven crystal grains measured at 212 GPa (a), 212 GPa (b), 213 GPa (c), 218 GPa (d), 223 GPa (e), 232 GPa (f), 243 GPa (g), and 233 GPa (h), respectively.
Those XRD images were also merged from selected step scan images. Small boxes in XRD images mark the peak positions, with magnified views of the peaks in the red rectangles. Numbers are Miller indices in the \(\sqrt{3}\) × a 2 × c hexagonal supercell. Red indices mark peaks uniquely belonging to the \(\sqrt{3}\) × a 2 × c hexagonal supercell, while white indices mark peaks which are also allowed by the hcp symmetry.
Extended Data Fig. 5 Atomic trajectories and free energy of \({\boldsymbol{P}}\bar{{\bf{6}}}{\bf{2}}{\boldsymbol{c}}\).
(a) Atomic trajectory of H from MD simulations with a total time of 2 ps. (b) Atomic trajectory of H from PIMD simulations with a total time of 2 ps. Red and purple dots represent atomic trajectory in G1 and G2 layers, respectively. Blue dots represent atomic trajectory in B layers. (c) Free energies of the P\(\bar{6}2\)c, Cmca-4 and I41/amd structures relative to C2/c−24 at 250 GPa using the phonon DOS derived from MD simulations. The shaded region indicates the error range through comparison with results obtained from static phonon calculations.
Extended Data Fig. 6 Simulated XRD pattern based on the \({\boldsymbol{P}}\bar{{\bf{6}}}{\bf{2}}{\boldsymbol{c}}\) model calculated at 250 GPa.
Red and blue bars represent calculated intensities. Red bars mark peaks uniquely belonging to the \(P\bar{6}2c\) model. Blue bars mark peaks also allowed by the parent hcp unit cell. Spheres mark at what pressures the corresponding Bragg peaks were observed. Spheres are only to demonstrate the statistics of observed peaks, and do not reflect the true d-spacing of those peaks at their observed pressures.
Extended Data Fig. 7 Comparison of Bragg peaks of different structural models in a d-spacing range of 1.15 Å to 2.5 Å to that of \({\boldsymbol{P}}\bar{{\bf{6}}}{\bf{2}}{\boldsymbol{c}}\).
Red bars represent intensity of Bragg peaks. Blue dash lines mark unique supercell peaks, red dash lines mark peaks which are also allowed by hcp-like unit cell. Intensities of Bragg peaks were calculated by using software PCW. Peak intensity generated by PCW of one peak was divided by the multiplicity of that peak. Unit cell parameters of all models were derived based on hcp-like unit cell parameters measured at 245 GPa (a0 = 1.6969 Å and c0 = 2.6124 Å), assuming those lattices being perfect supercell of the hcp lattice (in reality, most theoretical models are distorted from the perfect supercell geometry). Specifically, the geometry relationships are listed as the following: Pca21 (phase II), \(a=\sqrt{3}\cdot \)a0 b = a0 c = c0 α = 90.0° β = 90.0° γ = 90.0°; P63/m (phase II), \(a=2\cdot \)a0 b = 2∙a0 c = c0 α = 90.0° β = 90.0° γ = 120.0°; P6122 (phase III), \(a=\sqrt{3}\)∙a0 \(b=\sqrt{3}\)∙a0 c = 3∙c0 α = 90.0° β = 90.0° γ = 120.0°; B2/n (phase III), \(a=\sqrt{3}\)∙a0 b = 2∙c0 c = 3∙a0 α = 90.0° β = 90.0° γ = 90.0°; Ibam (phase V), \(a=\sqrt{3}\)∙a0 b = a0 c = 2∙c0 α = 90.0° β = 90.0° γ = 90.0°; Pc (phase IV), \(a=\sqrt{3}\)∙a0 b = 3∙a0 c = 2∙c0 α = 90.0° β = 90.0° γ = 90.0°; Cc (phase IV), \(a=2\cdot \sqrt{3}\)∙a0 \(b=2\cdot \sqrt{3}\cdot \)a0 c = 2∙c0 α = 90.0° β = 90.0° γ = 120.0°; Pca21 (phase V) \(a=\sqrt{3}\cdot \)a0 b = 3∙a0 c = 2∙c0 α = 90.0° β = 90.0° γ = 90.0°; P62/c \(a=\sqrt{3}\cdot \)a0 \(b=\sqrt{3}\)∙a0 c = 2∙c0 α = 90.0° β = 90.0° γ = 120.0°; Pna21 (phase V), a = \(2\)∙c0 b = 3∙a0 \(c=\sqrt{3}\)∙a0 α = 90.0° β = 90.0° γ = 90.0°. Atomic coordinates of models from literatures1,26,27,44 were used to calculate XRD patterns. P62/c (phase V) in (d) has very weak (1 0 3) peak, which cannot be displayed in the plotted figure. B2/n is another setting of the so-called C2/c structure with c axis as the unique axis. The calculated intensity of the reflection corresponding to the (1 1 0) of P\(\bar{6}2c\), based on B2/n, is 0.
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Ji, C., Li, B., Luo, J. et al. Ultrahigh-pressure crystallographic passage towards metallic hydrogen. Nature 641, 904–909 (2025). https://doi.org/10.1038/s41586-025-08936-w
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DOI: https://doi.org/10.1038/s41586-025-08936-w
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