Because this phase can efficiently carry electrical currents, researchers link superionic water to the generation of the unusual magnetic fields of ice giants such as Uranus and Neptune. Given the large water inventories inside these planets, superionic water may even be the most widespread form of water in the solar system rather than the familiar liquid or ice.

Previous high pressure experiments had already demonstrated superionic water, but the detailed arrangement of the oxygen atoms remained unclear. Earlier work pointed to either a body centered cubic lattice, with an additional atom in the center of each cube, or a face centered cubic lattice, with atoms on each cube face, as the dominant structures.

The new study reveals a more complex picture in which superionic water adopts a mixed close packed structure that combines face centered cubic regions with hexagonal close packed stacking. In the hexagonal close packed arrangement, layers of closely packed atoms form hexagonal patterns, and their combination with cubic layers produces significant stacking faults throughout the lattice.

Instead of settling into a single regular configuration, the oxygen sublattice forms a hybrid, structurally disordered sequence that can only be resolved using high precision measurements with modern X ray free electron lasers. This misstructured stacking is a key microscopic feature of the phase and shapes how hydrogen ions percolate through the lattice.

To probe this behavior, the team carried out two complementary experiments, one at the Matter in Extreme Conditions instrument at the Linac Coherent Light Source in the United States and another at the HED HIBEF instrument at the European XFEL near Hamburg. At these facilities, powerful laser driven compression waves squeeze water to more than 1.5 million atmospheres while heating it to several thousand degrees Celsius.

The experiments capture the atomic scale structure on timescales of trillionths of a second, fast enough to follow the evolution of the superionic phase under dynamic compression. The resulting diffraction patterns match state of the art computer simulations and confirm that structural diversity in superionic water closely mirrors the variety of crystal forms known for ordinary solid ice.

According to the researchers, the results demonstrate that water, despite its apparent simplicity, continues to reveal unexpected properties when pushed to extreme conditions. The work adds a new structural phase to the phase diagram of water and shows that even within the superionic regime multiple configurations compete, depending sensitively on pressure and temperature.

These findings provide important constraints for refined models of the deep interiors and thermal evolution of ice giants in and beyond the solar system. Better structural information on superionic water will help planetary scientists understand how composition, temperature and phase transitions influence the strength, geometry and temporal variability of magnetic fields in these planets.

The project formed part of a joint initiative between the German Research Foundation DFG and the French research funding agency ANR and brought together more than 60 scientists from institutions across Europe and the United States. Their combined expertise in laser driven compression, X ray diagnostics and theory was crucial to isolate the subtle signatures of the mixed close packed phase in the data.

Research Report:Observation of a mixed close-packed structure in superionic water
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