Ultrahigh-pressure isostructural electronic transitions in hydrogen

High-pressure transitions are thought to modify hydrogen molecules to a molecular metallic solid and finally to an atomic metal 1 , which is predicted to have exotic physical properties and the topology of a two-component (electron and proton) superconducting superfluid condensate 2 , 3 . Therefore, understanding such transitions remains an important objective in condensed matter physics 4 , 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.