High-pressure transitions are thought to modify hydrogen molecules to a molecular metallic solid and finally to an atomic metal1, which is predicted to have exotic physical properties and the topology of a two-component (electron and proton) superconducting superfluid condensate2,3. Therefore, understanding such transitions remains an important objective in condensed matter physics4,5. However, measurements of the crystal structure of solid hydrogen, which provides crucial information about the metallization of hydrogen under compression, are lacking for most high-pressure phases, owing to the considerable technical challenges involved in X-ray and neutron diffraction measurements under extreme conditions. Here we present a single-crystal X-ray diffraction study of solid hydrogen at pressures of up to 254 gigapascals that reveals the crystallographic nature of the transitions from phase I to phases III and IV. Under compression, hydrogen molecules remain in the hexagonal close-packed (hcp) crystal lattice structure, accompanied by a monotonic increase in anisotropy. In addition, the pressure-dependent decrease of the unit cell volume exhibits a slope change when entering phase IV, suggesting a second-order isostructural phase transition. Our results indicate that the precursor to the exotic two-component atomic hydrogen may consist of electronic transitions caused by a highly distorted hcp Brillouin zone and molecular-symmetry breaking.

Ji, C., Li, B., Liu, W., Smith, J., Majumdar, A., Luo, W., Ahuja, R., Shu, J., Wang, J., Sinogeikin, S., Menmg, Y., Prakapenka, V.B., Greenberg E., Xu, R., Huang, X., Yang, W., Shen, G., Mao, W.L., Mao, H -K.,  Ultrahigh-pressure isostructural electronic transitions in hydrogen, Nature 573, 558-562 (2019) DOI : https://doi.org/10.1038/s41586-019-1565-9

a) Microscope image of the hydrogen sample at 212 GPa and room temperature (phase III conditions), illuminated by both transmitted and reflected light. The inset shows the measured Raman vibron.
b) Merged raw XRD images showing the XRD spots. The colour code defined in the key—with red, green and blue representing the (100), (002) and (101) reflections, respectively—is used in b–d.
c) Montage of the 26 XRD spots, showing the quality of the data.
d) Quality of indexing, showing the d spacing of the (100), (002) and (101) reflections measured at different Ω angles. Dashed lines show the d values calculated using fitted unit cell parameters.