Nature | Letter

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Evidence for a new phase of dense hydrogen above 325 gigapascals

Journal name:
Nature
Volume:
529,
Pages:
63–67
Date published:
DOI:
doi:10.1038/nature16164
Received
Accepted
Published online

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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  1. Representative Raman spectra of three hydrogen isotopes.
    Figure 1: Representative Raman spectra of three hydrogen isotopes.

    a–c, Spectra of hydrogen are shown in black (a), hydrogen deuteride in green (b) and deuterium in blue (c). The spectra at different pressures (as labelled) are plotted as a waterfall; the offset is for clarity. The low-frequency modes L1–4 and the vibrational modes ν1,2 are labelled. The second-order diamond band spanning 2,300–2,600 cm−1 is visible on some spectra; its central position is marked by the dashed vertical line in a. The diamond edge for each isotope, which was used to determine the pressure, is labelled on the highest-pressure spectra (1,917 cm−1 for H2, 1,921 cm−1 for HD and 1,912 cm−1 for D2). HD was formed from a mixture of H2 (75%) and D2 (25%); see Methods. The spectra were collected using a 647.1-nm excitation wavelength. The arrows in a and c represent the splitting of the L3 mode and the onset of IV′ (for clarity see Extended Data Fig. 4).

  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 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.

    a, Relative intensities of the vibrational (ν1) and four low-frequency modes (L1–4) of hydrogen represented as a percentage of the total Raman activity of the sample; error bars reflect the accuracy of the measurement (see Methods and Extended Data Fig. 7). The low-frequency modes L2 and L3 disappear at around 325 GPa. b, The full-width at half-maximum (FWHM) of the low-frequency mode L1 of H2, D2 and HD as function of pressure; the dashed curve is a guide to the eye. The dashed vertical lines in a and b indicate the transformations to phases IV′ and V.

  3. Frequencies of the vibrational and low-energy modes of the isotopes as functions of pressure.
    Figure 3: Frequencies of the vibrational and low-energy modes of the isotopes as functions of pressure.

    a–c, The data for the different isotopes are shown as open stars (D2; a), triangles with centre dots (HD; b) and open circles (H2; c), with different colours representing different experimental runs. Data from previous studies16, 18 are shown as grey symbols. The vertical dashed lines denote the phase transitions in the corresponding isotope. d, The theoretically calculated frequencies of hydrogen for the metallic and non-molecular (atomic) structures of I41/amd (red) and (yellow) from ref. 22. The insets are photos of the HD sample at 50 GPa and at 388 GPa, as labelled, taken in transmitted and reflected light.

  4. Proposed phase diagram of hydrogen up to 400 GPa.
    Figure 4: Proposed phase diagram of hydrogen up to 400 GPa.

    The coloured, filled symbols and solid phase lines below 300 GPa in the main figure are from ref. 14, and show phases I(I′), III and IV(IV′). The solid black diamonds (phase V) are from this study, the vertical, grey dashed line indicates the transition from phase IV′ to phase V and the dashed-dotted arrow is the proposed continuation of the melting curve. The inset shows a sketch of the phase diagram of D2. The coloured, filled symbols and solid lines were obtained by us in another, unreported study. The dashed lines are the proposed melting curves of deuterium, which have not been measured experimentally, but are assumed to follow the same trend as those of hydrogen. The red open triangles are from ref. 26 and separate the metallic and semi-conducting liquids.

  5. Calculating pressure.
    Extended Data Fig. 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.

  6. Comparison of pressure calibrations.
    Extended Data Fig. 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.

  7. HD compressed to 218 GPa.
    Extended Data Fig. 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.

  8. Low-energy-mode splitting from phase IV to phase IV′.
    Extended Data Fig. 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.

  9. Comparison with previous data.
    Extended Data Fig. 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.

  10. Heating at about 360 GPa.
    Extended Data Fig. 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).

  11. Calculating relative integrated intensities.
    Extended Data Fig. 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).

References

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Author information

  1. Present address: Center for High Pressure Science & Technology Advanced Research, Shanghai 201203, China.

    • Ross T. Howie

Affiliations

  1. School of Physics and Centre for Science at Extreme Conditions, University of Edinburgh, Edinburgh EH9 3JZ, UK

    • Philip Dalladay-Simpson,
    • Ross T. Howie &
    • Eugene Gregoryanz

  2. Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China

    • Eugene Gregoryanz

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.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

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Author details

Extended data figures and tables

Extended Data Figures

  1. Extended Data Figure 1: Calculating pressure. (131 KB)

    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.

  2. Extended Data Figure 2: Comparison of pressure calibrations. (144 KB)

    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.

  3. Extended Data Figure 3: HD compressed to 218 GPa. (328 KB)

    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.

  4. Extended Data Figure 4: Low-energy-mode splitting from phase IV to phase IV′. (243 KB)

    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.

  5. Extended Data Figure 5: Comparison with previous data. (302 KB)

    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.

  6. Extended Data Figure 6: Heating at about 360 GPa. (323 KB)

    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).

  7. Extended Data Figure 7: Calculating relative integrated intensities. (363 KB)

    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).

Additional data