Experimental evidence for superionic water ice using shock compression


In stark contrast to common ice, Ih, water ice at planetary interior conditions has been predicted to become superionic with fast-diffusing (that is, liquid-like) hydrogen ions moving within a solid lattice of oxygen. Likely to constitute a large fraction of icy giant planets, this extraordinary phase has not been observed in the laboratory. Here, we report laser-driven shock-compression experiments on water ice VII. Using time-resolved optical pyrometry and laser velocimetry measurements as well as supporting density functional theory–molecular dynamics (DFT-MD) simulations, we document the shock equation of state of H2O to unprecedented extreme conditions and unravel thermodynamic signatures showing that ice melts near 5,000 K at 190 GPa. Optical reflectivity and absorption measurements also demonstrate the low electronic conductivity of ice, which, combined with previous measurements of the total electrical conductivity under reverberating shock compression, provides experimental evidence for superionic conduction in water ice at planetary interior conditions, verifying a 30-year-old prediction.

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Fig. 1: Experimental approach and representative ultrafast line-imaging pyrometer (SOP) and interferometric Doppler velocimeter (VISAR) data.
Fig. 2: Shock equation of state and reflectivity along the ice VII Hugoniot.
Fig. 3: Optical absorption of shocked water ice VII.
Fig. 4: Electrical conductivity of shock-compressed water along the liquid Hugoniot (black) and ice VII Hugoniot (blue).
Fig. 5: H2O phase diagram at planetary interior conditions.46


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We gratefully acknowledge S. Uhlich, A. Correa Barrios, C. Davis, J. Emig, E. Folsom, R. Posadas Soriano, T. Uphaus and W. Unites for target preparation, the Omega Laser Facility management, staff and support crew for excellent shot and diagnostic support with special thanks to C. Sorce, A. Sorce and J. Kendrick, discussions with S. Brygoo, R. Chau, Z. Geballe, D. Hicks, P. Loubeyre and P. Sterne, and P. Loubeyre for re-analysing XRD data. Prepared by Lawrence Livermore National Laboratory (LLNL) under contract DE-AC52-07NA27344. Omega shots were allocated by the Laboratory Basic Science program of the Laboratory for Laser Energetics at the University of Rochester, NY. Extensive computational support was provided by the LLNL Computing facility. Partial support was provided by LLNL LDRD program 17-ERD-085, the US Department of Energy through the joint FES/NNSA HEDLP program, the University of California, including UC Berkeley’s Miller Institute for Basic Research in Science, the National Science Foundation (#PHY11-25915) and NASA (#NNH12AU44I).

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M.M. designed the project, prepared the pre-compressed cells, fielded the laser experiments, analysed the data and wrote the manuscript. J.R.R. was the principal investigator of the Omega campaign. S.H. performed DFT–MD simulations. P.M.C., J.H.E., J.R.R., G.W.C. and R.J. developed the laser DAC platform and associated analytical methods. J.R.R., D.E.F., F.C. and D.C.S. contributed to the data analysis. All authors discussed the data and commented on the manuscript.

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Correspondence to Marius Millot.

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Supplementary Information

I. Data analysis methods, II. Optical properties and electrical conductivity, III. Molecular dynamics simulations, IV. Models, simulations and previous experiment, V. Superionic water in planetary interiors and dynamo scaling

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Millot, M., Hamel, S., Rygg, J.R. et al. Experimental evidence for superionic water ice using shock compression. Nature Phys 14, 297–302 (2018). https://doi.org/10.1038/s41567-017-0017-4

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