Mixed Molecular and Atomic Phase of Dense Hydrogen

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 ${\mathrm{H}}_2$ dimers experiencing large pairing fluctuations, and unbound ${\mathrm{H}}_2$ molecules.

Figure 1

Proposed phase diagram of ${\mathrm{H}}_2$. The melting curve (thick solid line) is a Ketchin fit from Ref. 12, and the symbols along it are from Refs. 12, 27. The solid lines separating the phases I, II, and III are from Ref. 28. The thin solid lines show the $P\mathrm{\text{−}}T$ paths taken in this study, and the stars along them show phase I to III and III to IV transformations. The dashed lines are proposed boundaries between I–III, III–IV, and I–IV phases. The calculated transition lines between molecular and atomic liquid are shown with dashed lines and crosses [24] and triangles [25]. The microphotographs show the appearance of deuterium in phase I and hydrogen in phase IV.

Figure 2

Representative Raman spectra of ${\mathrm{H}}_2$ (a) and ${\mathrm{D}}_2$ (b) at different pressures (shifted vertically for clarity). The weak broad peaks at $\sim 2400{\mathrm{cm}}^{-1}$ on spectra at 120 GPa for ${\mathrm{H}}_2$ and 90 GPa for ${\mathrm{D}}_2$ are due to the 2nd-order Raman from diamond. The inset in (a) shows the stressed diamond peak at 315 GPa.

Figure 3

${\mathrm{H}}_2$ and ${\mathrm{D}}_2$ phonons frequency shifts as a function of pressure. Open squares are the ${\nu }_1$, ${\nu }_2$, and low energy modes of hydrogen; open circles are ${\nu }_1$ and ${\nu }_2$ modes of deuterium. The dashed lines are infrared and Raman measurements at low temperatures from Refs. 6, 8, 20. The solid line is the ${\nu }_1$ mode of deuterium at 300 K from Ref. 11. Inset: Hydrogen ${\nu }_1$ (solid circles) and ${\nu }_2$ (open squares) modes FWHM as a function of pressure.

Figure 4

Optical absorption spectra of ${\mathrm{H}}_2$ in a visible spectral range at 300 K and different pressures. The reference transmission spectrum was measured at 75 GPa. Inset: Estimated band gap as a function of pressure at 300 K (solid squares) and at 77 K from Ref. 8 (open triangles).