Physicists discover hexagonal superionic ice at extreme pressures

Physicists have experimentally demonstrated a new form of superionic ice featuring a hexagonal close-packed (hcp) structure. The discovery provides a critical missing piece in the understanding of how water behaves under the crushing pressures found in the deep interiors of planets.

A research team led by Alexis Forestier of the French Alternative Energies and Atomic Energy Commission (CEA) achieved the result using laser-heated diamond anvil cells and synchrotron x-ray diffraction. By subjecting water samples to pressures exceeding 200 gigapascals—roughly 2 million atmospheres—and temperatures above 1,800 kelvins, the team observed a transition from a face-centered cubic (fcc) structure to the hcp arrangement. This shift, described as a martensitic transition, confirms that hcp ice becomes the dominant and more stable phase under these extreme conditions.

The nature of superionic ice

Superionic ice is an exotic state of matter that defies the traditional boundaries between solids and liquids. In this phase, the oxygen atoms are locked into a rigid crystal lattice, while the hydrogen nuclei, or protons, move freely through that lattice like a liquid. This dual nature allows the material to maintain a structural shape while simultaneously conducting ions.

Until now, superionic ice had been primarily observed in a face-centered cubic arrangement. The confirmation of the hcp structure adds a new layer of complexity to the phase diagram of water, showing that the material can reorganize its fundamental geometry when pushed to the limits of pressure and heat.

Implications for ice giants

This discovery has direct consequences for planetary science, specifically regarding the "ice giants" Uranus and Neptune. These planets are believed to harbor vast amounts of superionic ice in their interiors. Because different crystal structures conduct electricity in different ways, the prevalence of hcp ice over fcc ice at extreme pressures could offer a new explanation for the unusual, lopsided magnetic fields observed on these worlds.

Refining the internal models of these planets depends on knowing exactly which phase of ice exists at specific depths. The transition to a hexagonal structure suggests that the internal conductivity of these planets may be more varied than previously modeled, potentially explaining why their magnetic poles are so far offset from their centers.

Future research

While the existence of hcp superionic ice is now experimentally verified, its full physical profile remains a subject for study. Researchers are now calling for further theoretical work to determine the material's specific properties, particularly its electrical conductivity and mechanical plasticity. Understanding these traits will be essential for creating accurate simulations of planetary cores.