title: Mixed molecular and atomic phase of dense hydrogen.
abstract: We used Raman and visible transmission spectroscopy to investigate dense hydrogen (deuterium) up to 315 (275) GPa at 300 K. At around 200 GPa, we observe the phase transformation, which we attribute to phase III, previously observed only at low temperatures. This is succeeded at 220 GPa by a reversible transformation to a new phase, IV, characterized by the simultaneous appearance of the second vibrational fundamental and new low-frequency phonon excitations and a dramatic softening and broadening of the first vibrational fundamental mode. The optical transmission spectra of phase IV show an overall increase of absorption and a closing band gap which reaches 1.8 eV at 315 GPa. Analysis of the Raman spectra suggests that phase IV is a mixture of graphenelike layers, consisting of elongated H2 dimers experiencing large pairing fluctuations, and unbound H2 molecules.
authors.affiliation: Centre for Science at Extreme Conditions and School of Physic and Astronomy, University of Edinburgh, Edinburgh, EH9 3JZ, United Kingdom.
authors.forename: Ross T
authors.initials: RT
authors.lastname: Howie
authors.forename: Christophe L
authors.initials: CL
authors.lastname: Guillaume
authors.forename: Thomas
authors.initials: T
authors.lastname: Scheler
authors.forename: Alexander F
authors.initials: AF
authors.lastname: Goncharov
authors.forename: Eugene
authors.initials: E
authors.lastname: Gregoryanz