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Structure and properties of two superionic ice phases

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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.
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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 (PT) conditions experi-
ences a dramatic modification from a hydrogen-bonded molec-
ular dipole form to non-molecular ‘extended’ structures16. 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 line5,1018
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
ice5,2123. 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. 2426). 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 diffusivity3,8,21,2530 (Supplementary Fig. 2). The theoret-
ically predicted superionic phases are also expected to show poly-
morphism above 100 GPa (refs. 3032). 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 PT 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
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... Furthermore, the transition pathways amongst the high-pressure ice phases remain largely elusive. Understanding how such transitions occur and in particular the magnitude of the kinetic barrier provides additional in-sights on the structural relationships between the ice phases, and is also crucial for designing and interpreting high-pressure experiments [3,4,8,15,16]. ...
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Most experimentally known high-pressure ice phases have a body-centred cubic (bcc) oxygen lattice. Our atomistic simulations show that, amongst these bcc ice phases, ices VII, VII' and X are the same thermodynamic phase under different conditions, whereas superionic ice VII'' has a first-order phase boundary with ice VII'. Moreover, at about 300 GPa, ice X transforms into the Pbcm phase with a sharp structural change but no apparent activation barrier, whilst at higher pressures the barrier gradually increases. Our study thus clarifies the phase behaviour of the high-pressure insulating ices and reveals peculiar solid-solid transition mechanisms not known in other systems.
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