Structure and properties of two superionic ice phases
Prakapenka, VB, Holtgrewe, N., Lobanov, SS etal. Structure and properties of two superionic ice phases. Nat. Phys. (2021). https://doi.org/10.1038/s41567-021-01351-8
Commentary
Water is predicted to be a superionic ice with highly mobile protons in a stable oxygen sublattice at high pressures, but the existence of superionic ice and the location of the melting line are difficult to determine from both theory and experiment, leading to contradictory results depending on the technique employed and the interpretation of the data.
In this study, we report high-pressure, high-temperature synchrotron X-ray diffraction and optical spectroscopy measurements of water in a laser-heated diamond anvil cell, revealing a first-order phase transition to ice with a body-centered and face-centered cubic oxygen lattice.
Based on clear densities, increased photoconductivity, and greatly reduced fusion enthalpies, we assign these observed structures to the theoretically predicted superionic ice phases. This measurement suggests the presence of face-centered cubic superionic ice on water-rich giant planets such as Neptune and Uranus, and determines the pressure-temperature stability field of the superionic ice phase and the melting line.
The melting line determined here is at a higher temperature than that determined by static compression experiments, but is consistent with theoretical calculations and data from shock wave experiments.
