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Raman spectroscopy of hot hydrogen above 200 GPa

Abstract

It has been theorized that at high pressure the increased energy of the zero-point oscillations in hydrogen would destabilize the lattice and form a ground fluid state at 0 K (ref. 1). Theory has also suggested that this fluid state, representing a new state of matter, might have unusual properties governed by quantum effects, such as superfluidity or superconductivity2,3. Here, by combining Raman spectroscopy and in situ high-temperature, high-pressure techniques, we demonstrate that above 200 GPa a new phase transition occurs as temperature is increased, for example 480 K at 255 GPa. If the transformation is interpreted as melting, it would be the lowest melting temperature of any material at these high pressures. We also find a new triple point between phases I and IV and the new phase, and demonstrate that hydrogen retains its molecular character around this point. These data may require a significant revision of the phase diagram of hydrogen above 200 GPa.

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Figure 1: Representative Raman spectra, position and FWHM of the vibrational band in hydrogen.
Figure 2: Representative Raman spectra of hydrogen on heating at different pressures.
Figure 3: Proposed phase diagram of hydrogen up to 325 GPa.

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Acknowledgements

The authors are grateful to A. Hermann and G. Ackland for discussions and to C. Guillaume and M. Frost for help with the experiments. This work is supported by a research grant from the UK Engineering and Physical Sciences Research Council.

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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.

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Correspondence to Eugene Gregoryanz.

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The authors declare no competing financial interests.

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Howie, R., Dalladay-Simpson, P. & Gregoryanz, E. Raman spectroscopy of hot hydrogen above 200 GPa. Nature Mater 14, 495–499 (2015). https://doi.org/10.1038/nmat4213

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