ArticlePublisher preview available

Structure and properties of two superionic ice phases

November 2021
Nature Physics 17(11):1-6

DOI:10.1038/s41567-021-01351-8

Authors:

Nicholas Holtgrewe

University of Chicago

Sergey S Lobanov

GFZ Helmholtz Centre for Geosciences

In the phase diagram of water, superionic ices with highly mobile protons within the stable oxygen sublattice have been predicted at high pressures. However, the existence of superionic ices and the location of the melting line have been challenging to determine from both theory and experiments, yielding contradictory results depending on the employed techniques and the interpretation of the data. Here we report high-pressure and high-temperature synchrotron X-ray diffraction and optical spectroscopy measurements of water in a laser-heated diamond anvil cell and reveal first-order phase transitions to ices with body-centred and face-centred cubic oxygen lattices. Based on the distinct density, increased optical conductivity and the greatly decreased fusion enthalpies, we assign these observed structures to the theoretically predicted superionic ice phases. Our measurements determine the pressure–temperature stability fields of superionic ice phases and the melting line, suggesting the presence of face-centred cubic superionic ice in water-rich giant planets, such as Neptune and Uranus. The melting line determined here is at higher temperatures than previously determined in static compression experiments, but it is in agreement with theoretical calculations and data from shock-wave experiments. Measurements of the phase diagram of water reveal first-order phase transitions to face- and body-centred cubic superionic ice phases. The former is suggested to be present in the interior of ice giant planets.

Phase diagram of water at extreme P–T conditions Solid phases are labelled after ref. ⁶. Fluid phases are labelled following ref. ³⁶ and the results of this work reporting a conducting fluid. All symbols except cyan circles are the results of this work and show P–T conditions of selected XRD measurements and the onset temperatures for optical absorption. The regions labelled as molecular/ionic and conducting fluids differ by the optical properties measured in this work. Cyan circles show Raman data from refs. 5,6 (the error bars are taken from the original data) for the phase line between ice VII and dynamically disordered ice VII′. The one-side pressure error bars and filled symbols (skipped for clarity below 1,600 K, where the error bars are comparable to the symbol size) correspond to our estimation of the thermal pressure (Supplementary Fig. 9). The solid lines (guides to the eye) correspond to the proposed phase lines. The melting line above 60 GPa has a large uncertainty because of deterioration of the temperature control in the regime where water becomes absorptive (Methods). We refer to Supplementary Figs. 1 and 2 for comparisons with other experiments and theoretical calculations and also for the density–temperature phase diagram. The calculated isentropes of Neptune and Uranus are from ref. ⁸. Source data

…

XRD patterns measured on laser heating (LH) a, At 48.5 GPa. b, At 150 GPa. The stated pressures are nominal pressures at 300 K, labelled as RT (room temperature). At 49 GPa, bcc-SI and fcc-SI (the fcc-SI peaks are marked by vertical arrows) phases appear at 1,600 K and 2,000 K, respectively. At 150 GPa, the the partially molten fcc-SI phase was observed at 5,200 K and no bcc-SI is detected. The peaks of the low-temperature phases are visible at high temperatures because of the axial temperature gradients. ‘St’ stands for stishovite phase of SiO2 (which was used as the thermal insulator). The transitions are fully reversible, which is seen based on XRD of the sample quenched to 300 K. The ticks correspond to the Bragg reflections of the refined structures; the lattice parameters in b have been extracted from the patterns in the main panel (see Supplementary Fig. 6 for the lattice parameters and for the patterns where the peaks of the bcc-SI phase can be seen more clearly). The top panels are the XRD images in rectangular coordinates for 49 GPa and in polar coordinates for 150 GPa. The inset b demonstrates diffuse scattering of partially molten water at 5,200 K; it is obtained by subtracting the diffraction pattern of the sample quenched to 300 K (raw data). The X-ray wavelength is 0.3344 Å. Source data

…

Density versus P for 300 K ices, superionic phases and fluid water The densities of combined ices VII and X at 300 K (crossed-haired blue squares), combined bcc-SI (yellow crossed squares) and fcc-SI (crossed-haired diamonds) in their P–T stability regions (solid lines) and fluid water at the melting line (red dashed lines approximating the data of refs. 19,22) are shown; the details about the parameters of these dependencies presented in the Vinet form are in Supplementary Table 4. The uncertainties of our density measurements are smaller than the symbol size. The one-directional error bars and filled symbols show the uncertainties in thermal pressure measurements of bcc-SI and fcc-SI (Supplementary Fig. 9). Our data are compared with the results of dynamic experiments of refs. 19,33 (the error bars are taken from the original data). A detailed comparison of the data with previous experiments and theoretical calculations is shown in Supplementary Fig. 8. Source data

…

Optical spectroscopy data of SI phases and fluid water a, Optical absorption spectra at various P–T conditions. The solid lines are guides to the eye. b, Optical conductivity determined here using broadband spectroscopy in comparison with the results at 532 nm of ref. ¹⁹ obtained along the Hugoniot. The solid lines are guides to the eye. The sample thickness was determined approximately using finite element calculations (see Methods and Supplementary Figs. 15 and 16 for more detailed information). The error bars for the conductivity values in b reflect this uncertainty. Source data

…

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Articles

https://doi.org/10.1038/s41567-021-01351-8

1Center for Advanced Radiations Sources, University of Chicago, Chicago, IL, USA. 2Earth and Planets Laboratory, Carnegie Institution of Washington,

Washington DC, USA. 3GFZ German Research Center for Geosciences, Telegrafenberg, Potsdam, Germany. ✉e-mail: prakapenka@cars.uchicago.edu;

agoncharov@carnegiescience.edu

Ice at extreme pressure–temperature (P–T) conditions experi-

ences a dramatic modification from a hydrogen-bonded molec-

ular dipole form to non-molecular ‘extended’ structures1–6. On

the breakdown of strong covalent intramolecular bonding and the

formation of ionic solids, for example, symmetric ice X (refs. 1,4,7),

the quantum and thermal proton motions become comparable

in energy. This change in the energy landscape results in stability

of superionic phases3, which are characterized by a large proton

mobility within the solid oxygen sublattice and, thus, ionic con-

ductivity. The theoretically predicted superionic states of H2O are

expected to appear at high pressures and high temperatures and

interface the stability fields of solid ices and fluid water. The exis-

tence of superionic ices in nature has important consequences for

the interior of ice giant planets, where the generation of magnetic

field is thought to be related to the presence of shallow fluid con-

vective layers3,8,9.

Several aspects of the phase diagram of water at high pressure

are immensely controversial: the location of the melting line5,10–18

and the existence, structure, physical nature and location of the

solid phase(s) in equilibrium with the fluid phase. Experimental

and theoretical determinations of the melting line vary by up to

700 K (at approximately 50 GPa) and there are no reported measure-

ments above 90 GPa, except a single point near 5,000 K at 190 GPa

derived from shock-wave experiments in pre-compressed water19

(Supplementary Figs. 1 and 2). The experiments agree that there

is a sudden increase in the slope of the melting line at 20-47 GPa

(refs. 5,10,12,13,17,18). However, the origin of this anomaly and its loca-

tion remain controversial. It has been assigned to a triple point

between the fluid, ice VII and ice X (refs. 10,12) (or dynamically

disordered ice VII′ (refs. 4,6,20)), while other works suggest that it

is related to a triple point between the fluid, ice VII and superionic

ice5,21–23. Moreover, there are reports about the existence of another

triple point near 20 GPa and 800 K and an additional solid phase

with unknown properties12. Rigid water models and ab initio calcu-

lations predict the existence of plastic ice phases with body-centred

and face-centred cubic (bcc and fcc, respectively) oxygen lattices

and freely rotating molecules at pressures above 2 GPa and 300 K

(refs. 24–26). Hereafter, ‘bcc’ and ‘fcc’ refer to both plastic and supe-

rionic phases. On the other hand, above 20 GPa and 1,000 K, other

ab initio simulations suggest that ice VII and the fluid are interfaced

in the phase diagram by the superionic phase(s) characterized by a

large proton diffusivity3,8,21,25–30 (Supplementary Fig. 2). The theoret-

ically predicted superionic phases are also expected to show poly-

morphism above 100 GPa (refs. 30–32). Recent dynamic compression

X-ray diffraction (XRD) experiments between 160 and 420 GPa

report a transformation from a bcc ice X to an fcc superionic ice33.

Finally, recent static experiments reported an isostructural transi-

tion of ice VII at high temperatures to a bcc structure with larger

volume and entropy, suggesting that it is superionic34. Overall, exist-

ing experimental data and theoretical calculations show an extreme

diversity concerning proton dynamics and conductivity and poly-

morphism of water and ices (Supplementary Figs. 1 and 2) and thus

call for further experimental investigations.

In this Article, we report the results of combined synchrotron

XRD and optical spectroscopy studies in the laser-heated diamond

anvil cell (DAC) up to 150 GPa and 6,500 K. The measurements

probe in situ structural and electronic properties of H2O ices and

fluid at these conditions, shedding light on the phase diagram and

the transport properties of water at extremes. Our experiments

reveal and map out the stability fields of two solid phases at elevated

temperatures above 20 GPa, which are distinct in density from the

familiar ices and the fluid. We assign these phases to the theoreti-

cally predicted superionic ices based on their excessive entropy and

the P–T conditions of stability. The superionic nature of these phases

is supported by our optical spectroscopy measurements, revealing

that these phases are moderately absorptive. The same experiments

Structure and properties of two superionic

ice phases

Vitali B. Prakapenka 1 ✉ , Nicholas Holtgrewe1,2, Sergey S. Lobanov 2,3 and

Alexander F. Goncharov 2 ✉

In the phase diagram of water, superionic ices with highly mobile protons within the stable oxygen sublattice have been pre-

dicted at high pressures. However, the existence of superionic ices and the location of the melting line have been challenging

to determine from both theory and experiments, yielding contradictory results depending on the employed techniques and

the interpretation of the data. Here we report high-pressure and high-temperature synchrotron X-ray diffraction and optical

spectroscopy measurements of water in a laser-heated diamond anvil cell and reveal first-order phase transitions to ices with

body-centred and face-centred cubic oxygen lattices. Based on the distinct density, increased optical conductivity and the

greatly decreased fusion enthalpies, we assign these observed structures to the theoretically predicted superionic ice phases.

Our measurements determine the pressure–temperature stability fields of superionic ice phases and the melting line, suggest-

ing the presence of face-centred cubic superionic ice in water-rich giant planets, such as Neptune and Uranus. The melting line

determined here is at higher temperatures than previously determined in static compression experiments, but it is in agree-

ment with theoretical calculations and data from shock-wave experiments.

NATURE PHYSICS | VOL 17 | NOVEMBER 2021 | 1233–1238 | www.nature.com/naturephysics 1233

Content courtesy of Springer Nature, terms of use apply. Rights reserved

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Raman and optical spectroscopy are versatile tools for nondestructive characterization of a wide range of properties of novel materials and minerals in situ at extreme and ambient conditions. These techniques are genuinely complementary to X-ray tools (diffraction and spectroscopy) in the probe energy, momentum transfer, and time scale, making concomitant X-ray and optical probes available for advanced sample analysis. We have built a state-of-the-art, user-friendly integrated Raman and optical spectroscopy system at Sector 13 (GeoSoilEnviroCARS, University of Chicago, IL) of the Advanced Photon Source (APS), Argonne National Laboratory (ANL), where optical probes are available now in combination with high resolution in-situ synchrotron X-ray diffraction and spectroscopy tools (XRD, IXS, XES, NFS, and others) for extensive sample investigation. The integrated optical system enables a variety of techniques including multi-colored (five laser lines: 266, 473, 532, 660, and 946 nm) confocal Raman, fluorescence, and optical spectroscopy from ultraviolet (UV) to near infrared (IR) spectral ranges (266–1600 nm), and Coherent Anti-Stokes Raman spectroscopy (CARS) in combination with near IR double sided laser heating.