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Evidence for a new phase of dense hydrogen above 325 gigapascals

Abstract

Almost 80 years ago it was predicted that, under sufficient compression, the H–H bond in molecular hydrogen (H2) would break, forming a new, atomic, metallic, solid state of hydrogen1. Reaching this predicted state experimentally has been one of the principal goals in high-pressure research for the past 30 years. Here, using in situ high-pressure Raman spectroscopy, we present evidence that at pressures greater than 325 gigapascals at 300 kelvin, H2 and hydrogen deuteride (HD) transform to a new phase—phase V. This new phase of hydrogen is characterized by substantial weakening of the vibrational Raman activity, a change in pressure dependence of the fundamental vibrational frequency and partial loss of the low-frequency excitations. We map out the domain in pressure–temperature space of the suggested phase V in H2 and HD up to 388 gigapascals at 300 kelvin, and up to 465 kelvin at 350 gigapascals; we do not observe phase V in deuterium (D2). However, we show that the transformation to phase IV′ in D2 occurs above 310 gigapascals and 300 kelvin. These values represent the largest known isotropic shift in pressure, and hence the largest possible pressure difference between the H2 and D2 phases, which implies that the appearance of phase V of D2 must occur at a pressure of above 380 gigapascals. These experimental data provide a glimpse of the physical properties of dense hydrogen above 325 gigapascals and constrain the pressure and temperature conditions at which the new phase exists. We speculate that phase V may be the precursor to the non-molecular (atomic and metallic) state of hydrogen that was predicted 80 years ago.

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Figure 1: Representative Raman spectra of three hydrogen isotopes.
Figure 2: Relative intensities of the vibrational, low-frequency modes and the full-width at half-maximum of the L1 mode as a function of pressure.
Figure 3: Frequencies of the vibrational and low-energy modes of the isotopes as functions of pressure.
Figure 4: Proposed phase diagram of hydrogen up to 400 GPa.

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Acknowledgements

We are grateful to M. Frost for assistance during experiments. This work is supported by a Leadership Fellowship from the UK Engineering and Physical Sciences Research Council (EPSRC), reference number EP/J003999/1. P.D-S. acknowledges studentship funding from EPSRC grant number EP/G03673X/1.

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Authors

Contributions

R.T.H. and P.D-S. carried out the experiments, analysed the data and wrote the paper. E.G. conceived and designed the project, carried out the experiments, analysed the data and wrote the paper.

Corresponding author

Correspondence to Eugene Gregoryanz.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Calculating pressure.

a, A typical example of a spectrum from the first-order Raman band of diamond when probing the sample (orange). The frequency edge is given by the vertical dashed line at 1,796 rel. cm−1, which corresponds to a pressure of 275 GPa (ref. 27). This stressed edge is defined as the frequency that minimizes (purple). b, H2 vibrational-mode (vibron) frequency (ν1) plotted as a function of the stressed-diamond-edge frequency.

Extended Data Figure 2 Comparison of pressure calibrations.

a, Vibration-mode (vibron) frequency plotted using the three pressure gauges of the stressed-diamond frequency proposed by Akahama et al.: blue squares27, green circles28 and red triangles11. b, The three pressure gauges plotted as a function of pressure, coloured as in a; the dashed line marks the highest frequency of the stressed diamond recorded on a HD sample, 1,936 rel. cm−1.

Extended Data Figure 3 HD compressed to 218 GPa.

Representative Raman spectra from HD, a mixture of hydrogen (75%) and deuterium (25%), as a function of pressure at 300 K. The spectra show the evolution of the ν1 vibrational modes of HD (labelled ‘HD-ν1’) and H2 (labelled ‘H2-ν1’) from loading at 0.5–218 GPa, as labelled. Above 47 GPa, there is an observed transfer of integrated intensity from the ν1 band of H2 to the ν1 band of HD, with the latter vibrational mode becoming stronger than the former at 150 GPa, and the only resolvable ν1 band above 218 GPa. The spectra were collected using a 514-nm excitation wavelength. The spectra from this run above 218 GPa are shown in Fig. 1b.

Extended Data Figure 4 Low-energy-mode splitting from phase IV to phase IV′.

Representative Raman spectra of the low-frequency excitations of the three isotopes (left, H2; centre, D2; right, HD) as functions of pressure during the transition from phase IV to phase IV′. The low-frequency mode L3 splits to produce mode L4.

Extended Data Figure 5 Comparison with previous data.

a, Frequencies of the vibrational modes versus pressure from ref. 21 (black circles) and the current study and our previous study16 (violet squares). The open symbols represent data for H2; the symbols enclosing pluses represent data for D2. b, Representative Raman spectra of hydrogen from ref. 21 (black) and ref. 16 (violet). The dashed vertical line indicates the lowest vibrational-mode frequency (and therefore the highest pressure) observed in ref. 21.

Extended Data Figure 6 Heating at about 360 GPa.

a, Raman spectra for a pure hydrogen sample, taken using a probe laser with a wavelength of 647 nm, as function of temperature at pressures between 367 GPa and 350 GPa (black). The Raman spectrum collected 2 μm away on the rhenium gasket is shown in red. The vertical dashed lines indicate the frequency space occupied by the second-order diamond band. DE, diamond edge. b, Example spectrum of the sample (black) and the gasket (2 μm away, red), collected at 361 GPa, and the difference between them (blue).

Extended Data Figure 7 Calculating relative integrated intensities.

Representative Raman spectrum demonstrating how intensities were calculated for a given spectrum. The best fit (black curve) to the experimental data (green points) is shown (measured by the left axis) along with the integrated intensities for each excitation (indicated by the height of the blue bars, measured by the right axis). The inset shows the raw value of the integrated intensities of each excitation in the main figure as a percentage of the total Raman activity. 2nd Dmd., second-order diamond band (unlabelled in the main figure).

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Dalladay-Simpson, P., Howie, R. & Gregoryanz, E. Evidence for a new phase of dense hydrogen above 325 gigapascals. Nature 529, 63–67 (2016). https://doi.org/10.1038/nature16164

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