The 4th International Conference on Matter and Radiation at Extremes (ICMRE 2019)

440 / 2019-03-06 09:15:26

Synchrotron X-ray diffraction of solid hydrogen at ultrahigh pressures

Developments of advanced synchrotron X-ray diffraction techniques for studying crystal structures of solid hydrogen at multi-megabar pressures/ Direct measurements of crystal structures of novel hydrogen high pressure phases/Electronic topological transit

Plenary track > High Pressure Physics and Materials Science

Achieving pressure-induced metallization of hydrogen is the holy-grail of high pressure physics and attracted extensive interests over the past 80 years. Understanding the route of hydrogen metallization remains an important challenge, because even the very basic crystallographic information of most of the high pressure phases of hydrogen is still unclear, due to the daunting experimental difficulties for direct crystal structure measurements of hydrogen. By developing ultra-high pressure sample preparation and diagnostic techniques tailored for measuring X-ray diffraction (XRD) of hydrogen under multi-megabar pressures, we successfully measured synchrotron single-crystal XRD of hydrogen up to 254 GPa, and revealed the crystallographic nature of transitions from phases I to III and IV. By doubling the previous pressure range of SXRD of H2 at room-temperature, we observed that hydrogen molecules remain in the hexagonal close-packed (hcp) lattice accompanied by a monotonic increase in anisotropy. However, a kink of both c/a ratio and the compressibility was apparently observed in phase IV, suggesting a second-order iso-structural phase transition, in sharp contrast to the previously predicted first order phase transition. We performed theoretical calculations and discovered the appearance of Dirac cone in the electronic band structure, correlating the iso-structural phase transition with an electronic topological transition (ETT). Our study suggests that massive distortion of the hcp Brillouin zone may lead to a series of ETT phases prior to the hydrogen band closure.