Plastic ice: a new phase of water under extreme conditions

Ice is more than just the frozen stuff in your freezer. Water, thanks to its unique molecular structure, can exist in at least twenty different solid forms, depending on pressure and temperature.

One of the most fascinating of these is ice VII, a high-pressure phase of ice that forms deep inside giant planets and icy moons. In ice VII, water molecules are arranged in a tightly packed, cubic structure.

A mysterious phase finally observed

However, for decades, scientists have theorized that under even higher temperatures and pressures, a new phase of ice VII—plastic ice VII—could emerge. In this state, water molecules remain locked in a solid lattice like in ice VII, but instead of just vibrating around their equilibrium positions, they can rotate freely, similar to molecules in a liquid.

This ‘plastic’ phase of ice could play a key role in shaping the interiors of icy planets like Neptune and Uranus, as well as some of Jupiter’s Galilean moons, influencing their thermal properties and how they transport heat and materials. But plastic ice VII has never been directly observed in experiments—until now.

A team of researchers have used quasi-elastic neutron scattering (QENS) to observe plastic ice VII for the first time. They subjected water to pressures up to 6 GPa (60,000 times Earth’s atmospheric pressure) and temperatures between 450 and 600 K (170–325°C). Their findings confirm that under these extreme conditions, water molecules in ice VII maintain a crystalline lattice while undergoing rapid rotational motion.

The research was led by Livia E. Bove, a scientist from EPFL’s Laboratory of Quantum Magnetism and CNRS, Paris.

Direct experimental evidence of plastic ice VII

To uncover this elusive ice form, the team used the Institut Laue-Langevin’s neutron-scattering instruments, The Quasi-Elastic Neutron Scattering (QENS) technique is particularly suited for detecting hydrogen atom movements, making it ideal for studying water’s behavior at a molecular level.

Neutron scattering allowed the researchers to observe how hydrogen atoms within the ice VII structure moved over time, distinguishing between different types of molecular motion. The technique provided crucial insights into the rotational dynamics of water molecules in the plastic ice phase.

The experiments were conducted using the Paris–Edinburgh press, which is specifically designed to generate extremely high pressures in small samples. By compressing water to pressures of up to 6 GPa while simultaneously heating it, the researchers created the conditions under which plastic ice VII could form. The press ensured that the ice was stable during neutron scattering measurements, allowing for precise observations of its dynamic properties.

By comparing the neutron scattering data to molecular dynamics simulations, the researchers confirmed that the observed phase matched the characteristics of plastic ice VII. The combination of high-pressure generation and advanced neutron analysis provided direct experimental evidence of this long-theorized state of water.

The unique properties of plastic ice VII

In plastic ice VII, water molecules adopt a body-centered cubic structure, like conventional ice VII. But unlike standard ice, where molecules are rigidly locked in place, plastic ice VII allows rapid molecular rotations on a picosecond timescale (one trillionth of a second). Instead of behaving as free rotors, the molecules perform sudden orientational jumps, constrained by weak energy barriers. This dynamic movement sets plastic ice VII apart from other ice phases.

Beyond the novelty of discovering a new ice phase, plastic ice VII has important implications for planetary science. This phase could explain how heat and material transport occur in the interiors of icy planets and moons, affecting their evolution and structure. It may also influence the way large icy bodies differentiate—offering insights into why some moons, like Ganymede, have metallic cores while others, like Callisto, do not.

Understanding plastic ice VII also adds to our knowledge of high-pressure physics and could inform future research on other exotic materials that exist in extreme environments.

List of contributors

• EPFL Laboratory of Quantum Magnetism
• Sapienza Università di Roma
• Institut Laue-Langevin
• CNRS-CEA
• Sorbonne Université
• University of Bristol
• IBM Research Europe
• University of Manchester
• University of Edinburgh