Low-frequency vibrational dynamics and structure of hydrogen at megabar pressures

Abstract

Low-frequency rotational and lattice-phonon spectra of solid ${\mathrm{H}}_2$ have been measured to 162 GPa at 77 to 295 K. Rotational bands broaden continuously up to and through the 150-GPa phase transition, with no sign of orientational transitions. The optical phonon, identified at low pressures as the transverse-optical mode of the hexagonal-close-packed structure, persists into the high-pressure phase, which indicates the underlying lattice remains hexagonal to these pressures. The measurements are consistent with the 150-GPa transition being largely electronic in origin and associated with metallization.