Melting curve and isostructural transition in superionic ice, J.-A. Queyroux (a,b), J.-A. Hernandez (c), G. Weck (b), S. Ninet (a), T. Plisson (b), S. Klotz (a), G. Garbarino (d), N. Guignot (e), M. Mezouar (d),
M. Hanfland (d), J.-P. Itié (e) and F. Datchi (a), Phys. Rev. Lett. 125, 195501 (2020); https://doi.org/10.1103/ PhysRevLett.125.195501. (a) Sorbonne Université, IMPMC, Paris
(France) (b) CEA, Arpajon (France) (c) Oslo University, Oslo (Norway) (d) ESRF (e) Soleil Synchrotron, Saint-Aubin (France)
 C. Cavazzoni et al., Science 283, 44 (1999).  J.-A. Hernandez & R. Caracas, Phys. Rev. Lett. 117, 135503 (2016); J. Chem. Phys. 148, 214501 (2018).
PRINCIPAL PUBLICATION AND AUTHORS
(bcc) phases. The large entropy increase at the transition implied a larger number of possible proton positions in the high-T phase, and thus a higher proton disorder compatible with superionic conduction.
The experimental data were compared to a recent theoretical study , which predicted a first-order transition between two high-pressure ice phases noted ice VII and ice VII . The latter shares the same bcc oxygen sublattice as ice VII (the stable phase of ice above 2.2 GPa at room temperature), and thus only differs in the dynamics and average position of the H atoms (Figure 20). In ice VII , which is stable in the range 600-800 K along the melting line, protons are dynamically disordered but remain localised along the diagonals of the bcc oxygen sublattice; in ice VII , stable above 1000 K, the protons may delocalise out of the diagonals into the interstitial space. This delocalisation induces a sudden shift in volume, responsible for the first- order character of the VII -VII transition. Ice VII is always superionic, with the highest electrical conductivity, whereas ice VII is only superionic close to the transition pressure. The P-T position and volume jump of the VII -VII transition, calculated here at 900 K, matched those found in the experiment very well, strongly suggesting that the observed high-T phase is the superionic ice VII . The triple point between the liquid, ice VII and ice VII was located between 800 and 900 K, again in very good agreement with experiments.
The present study thus provides a clear experimental detection of the long-sought bcc superionic ice and shows that the P-T conditions at which it becomes stable are lower than initially expected. The new melting data should be helpful to refine models of giant icy planets such as Neptune and Uranus. Measuring the melting line and characterising the structure of ice at more extreme conditions remains a challenging project that should be within reach of new synchrotron sources such as the ESRF- EBS.
Fig. 19: Phase diagram of ice. Red squares and blue circles are the new experimental data for melting and bcc-bcc transition, respectively. Black lines are predicted boundaries from .
Fig. 20: X-ray diffraction patterns of ice at 33 GPa. The two sets of bcc peaks in the black pattern correspond to ice VII and ice VII . The insets give representations of these ices from computer simulations, showing the trajectories of H atoms (in grey) .