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.

Identification of high-pressure phases III and IV in hydrogen: Simulating Raman spectra using molecular dynamics

“…The transition to Cmca-4 was also found by Liu et al 34 in their metadynamics calculations. We conclude that the already-achievable experimental conditions 17 are already close to the transition to a Cmca-4 "phase V" state. Phase IVa, stacked BGBG, 220 GPa, 220 K, P c. Bottom: Phase IVb, stacked BG BG , 270 GPa, 220 K, P c. The time averaging was chosen large enough to capture the rotation of the B-layer molecules and G -layer motifs, making these smaller.…”

“…6). The simulation which 17 Note that the sensitivity of the detector is reduced at high frequency, 21 so the peak amplitudes are not directly comparable.…”

“…Experimental work also finds two vibron modes, lending support to a two-layer model. 17 The true nature of phase IV and its actual Raman spectrum can only reliably be calculated by including temperature effects, which is the subject of this paper.…”

Graphene physics and insulator-metal transition in compressed hydrogen

“…A new phase (phase IV) has been discovered above 220 GPa in the higher temperature regime 4,5,8 . Though several different structures have been proposed for phase IV from first-principles calculations, 11,12,15 there is agreement that this phase can be viewed as a mixed-layer structure, where layers of weakly interacting H 2 molecules are sandwiched between graphene-like layers.…”

“…Recently, solid hydrogen has been intensively investigated in a new pressure-temperature domain (200-360 GPa, and just above 300 K), both experimentally [4][5][6][7][8][9] and theoretically [10][11][12][13][14][15] . A new phase (phase IV) has been discovered above 220 GPa in the higher temperature regime 4,5,8 .…”