Physicists have created a previously unknown phase of superionic ice in the laboratory, a breakthrough that could help explain the peculiar magnetic fields of Uranus and Neptune. The new phase, which features a hexagonal close-packed structure, was formed by heating ultrapure water to thousands of kelvin while compressing it to pressures comparable to those deep inside the ice giant planets.
Superionic ice is an exotic state of water that exists only under extreme conditions, with pressures reaching tens or even hundreds of gigapascals and temperatures above 1000 to 2000 kelvin. Under these conditions, hydrogen ions diffuse through a solid lattice of oxygen atoms almost as freely as they would in a liquid, giving the material an ionic conductivity that can exceed 100 Siemens per centimetre — approaching that of metals. The melting temperature of this ice is also extraordinarily high, reaching several thousand kelvin.
The research team, led by physicists Alexis Forestier and Gunnar Weck at the French Alternative Energies and Atomic Energy Commission, used a modified diamond anvil cell to achieve these extreme conditions. They added two cup-shaped discs of boron-doped diamond to the anvil surfaces, which allowed them to heat a 12-micron sample uniformly with lasers while applying pressures between 80 and 229 gigapascals. This setup avoided the large temperature gradients that typically complicate measurements in conventional diamond anvil cells.
Heating the sample from 300 to 2630 kelvin, the researchers obtained high-quality X-ray diffraction spectra using the Extremely Brilliant Source at the European Synchrotron Research Facility in Grenoble. The spectra revealed the new hexagonal close-packed phase emerging alongside a previously observed face-centred cubic phase across a pressure range of 130 to 200 gigapascals and temperatures above 1800 kelvin. The discovery brings the total number of known superionic phases of water to three, following the body-centred cubic phase first observed in 2016.
Beyond the laboratory achievement, the findings may illuminate the interiors of Uranus and Neptune. Unlike Earth’s magnetic field, which is generated by molten iron in the core, the ice giants’ magnetic fields are skewed and off-centre relative to their rotation axes. Scientists have hypothesized that these fields arise from conductive superionic warm dense ice — a mixture of water, ammonia and methane — in the planets’ outer third. The new hexagonal close-packed phase, which becomes more stable at pressures above 150 to 200 gigapascals, could exist deeper within the inner shell of these planets.
«The experimental observation of this new hcp superionic phase now provides a rationale for incorporating its mechanical properties and ionic conductivity into these models of planetary interiors,» Forestier said. «Indeed, this phase can coexist with the fcc superionic phase and become more stable at higher pressures, and so probably exist deeper within the inner shell of these ice giants.»
The researchers plan to map the phase diagram more precisely, defining the regions where pressure and temperature are stable and tracing the transition lines between molecular and superionic phases. They also aim to understand how the new phase behaves as it melts. «We also want to obtain more direct experimental signatures of superionic behaviour, beyond the thermal expansion we observed,» Weck said. «Brillouin spectroscopy measurements, for example, could reveal anomalies in the speed of sound, strengthening the evidence for proton diffusion.»
The results, published in Physical Review Letters, mark a significant step forward in understanding water’s behaviour under extreme conditions and offer a new tool for modelling the interiors of distant worlds.