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Observing hot dense superionic water


Under high pressure and high temperature, solid water takes a form called superionic ice, in which hydrogen atoms migrate through the oxygen crystal lattice as in a fluid. X-ray diffraction measurements on beamline ID27 reveal that this superionic ice could occupy a large part of the interior of the planets Uranus and Neptune.

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There is currently great interest in studying the phases and the properties of warm dense ice, present in the planetary interiors of Uranus, Neptune and other giant icy exoplanets being discovered. Under the temperature and pressure conditions of these planetary interiors, part of the ice layer is predicted to be in a superionic (SI) state, where a rigid oxygen lattice coexists with mobile hydrogen atoms, which could influence the magnetic field of Neptune-like planets.

Over the last two decades, in the high-pressure physics community, much work has aimed at finding evidence for the existence of SI ice, its lattice structure and the stability of that phase under different pressures and temperatures. It is known that above pressures of 3 GPa, ice structures are based on body-centered cubic (bcc) oxygen sublattices, the first one being ice VII. Above 60 GPa, ice becomes an ionic solid, called ice X, in which the hydrogen atoms are located halfway between the nearest neighbouring oxygen atoms. Upon heating, the hydrogen bond weakens and ice is expected to transform into the superionic (SI) phase, where the hydrogen atoms become mobile. Various structures of superionic ice have been predicted from ab-initio simulations; however, the properties of these structures and their stability at different pressures and temperatures are still debated.

Recent experimental work using either dynamic [1,2] or static [3,4] compression of water ice more firmly established the existence of superionic ice and demonstrated a transition from an insulating bcc structure to bcc-SI, on one hand, and from the bcc-SI to a face-centered cubic (fcc) superionic phase called ice XVIII, on the other hand. However, the dynamic [2] and static [4] experiments strongly disagreed on the stability of the SI phases. Further measurements were thus essential to resolve these issues.

In this work, X-ray diffraction was combined with the laser-heated diamond anvil cell at beamline ID27 to investigate the structural changes in water ice under a range of pressure and temperature conditions.


Fig. 1: XRD patterns collected at 57 GPa under CO2 laser heating, showing the presence of fcc-SI diffraction peaks in the spectra collected at 1560 K. The double peak structure of bcc (110) at 1320 K and 1420 K is indicative of the presence of the bcc-SI phase at these temperatures. The inset shows a sketch of a cross-sectional view of the H2O sample cavity under direct CO2 laser heating, with the hot ice volume in red.

First, water was heated in the diamond anvil cell by a CO2 laser directly absorbed by the H2O sample. X-ray diffraction patterns were collected up to pressures of around 60 GPa, with the resulting structural data demonstrating the presence of fcc superionic ice, as shown in Figure 1. Higher pressures required indirect heating of the H2O sample using an ytterbium laser, as well as boron-doped diamond disks in the anvil cell, which absorb the radiation of the high-power laser but are transparent to the X-ray beam. This method allowed X-ray diffraction data to be collected for pressures of up to 160 GPa and temperatures of up to 2500 K.



Fig. 2: Phase diagram of dense H2O drawn from present work (blue/red circle and diamond symbols) and shock wave measurements (blue/red triangle). The various coloured domains correspond to the stability fields of: in grey, the molecular fluid; in blue, the bcc ices (ice VII, VII', X and bcc-SI); in yellow, the fcc-SI. The white zones represent the uncertainties on the transition lines. The dot lines correspond to recent large-scale first-principle calculations using a machine-learning potential trained on DFT-PBE simulations.

The resulting data provide clear structural observations of superionic ice at a wide range of temperatures and pressures, allowing a full phase diagram to be reported, as shown in Figure 2. It proposes that dense H2O is composed of insulating bcc ice (VII/VII’/X) at low temperatures, bcc-SI ice from 14 GPa up to at least 120 GPa for temperatures in the range of 850-2000 K, and of fcc-SI ice above ∼50 GPa and ∼1400 K.

Although the findings clearly differ from the phase diagram published in [4], possibly due to difficulties in measuring the sample temperature, for example, they agree very well with previous dynamic compression experiments and theoretical calculations based on advanced simulations methods. This work, therefore, narrows the range of possible predictions for expected phase transitions, advancing our understanding of the phases of hot dense water.


Principal publication and authors
Evidence and stability field of fcc-superionic water ice using static compression, G. Weck (a,b), J.-A. Queyroux (a), S. Ninet (c), F. Datchi (c), M. Mezouar (d), P. Loubeyre (a,b), Phys. Rev. Lett 128, 165701 (2022);
(a) CEA, DAM, DIF, F-91297 Arpajon (France)
(b) Univ. Paris Saclay, Laboratoire Matière Conditions Extrêmes, CEA, Bruyeres Le Chatel (France)
(c) Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Sorbonne Université, CNRS UMR 7590, IRD UMR 206, MNHN, 4 place Jussieu, Paris (France)
(d) ESRF

[1] M. Millot et al., Nat. Phys. 14, 297 (2018).
[2] M. Millot et al., Nature 569, 251 (2019).
 [3] J.-A. Queyroux et al., Phys. Rev. Lett. 125, 195501 (2020).
[4] V.B. Prakapenka et al., Nat. Phys. 17, 1233 (2021).


About the beamline: ID27

The recently upgraded beamline ID27 addresses the most exciting and challenging questions related to science at very high pressures and temperatures, such as the conditions deep inside planets, the quest for room-temperature superconductivity, or synthesising new super-hard materials. The beamline accommodates complex sample environments, such as the double-sided laser-heating system, the Paris-Edinburgh press, the nano-stage and the high-pressure helium cryostat. With a much higher flux and smaller beam than its predecessor (300x300 mm2), as well as better detectors, it enables a new class of ultra-high-pressure experiments (P> 4 Matm), time-resolved experiments with millisecond resolution, 2D micro-fluorescence mapping and  in-situ X-ray imaging.


Top image: Full disk view of the planet Neptune, as taken from the Voyager 2 narrow angle camera. Image credit: NASA/JPL