First experimental evidence of a new form of ice with dynamic behaviour
A crystalline phase of water that forms at pressures above 50000 atmospheres and 300 °C: plastic VII ice, which differs from other forms of ice due to its hybrid nature between a solid and a liquid. The water molecules in this phase are arranged in a dense cubic lattice, but unlike other forms of ice, they are free to rotate around their equilibrium positions in a liquid-like manner. This dynamic behaviour gives the phase a plastic nature, the existence of which has been hypothesised by molecular dynamics simulations but never observed experimentally.
The international research team, led by Livia Eleonora Bove of the Physics Department at Sapienza University of Rome, succeeded in obtaining direct observation of the existence of plastic VII ice. To experimentally demonstrate the exotic behaviour of this water phase, the team of researchers used Quasi-Elastic Neutron Scattering (QENS), a technique that allows direct measurement of rotational properties and diffusive dynamics in molecular systems. The experimental data immediately provided evidence of the existence of the plastic phase. However, further experiments and comparison with molecular dynamics simulations were needed to understand in detail the mechanism by which the molecules rotate. John Russo and Francesco Sciortino of the Physics Department at Sapienza University of Rome contributed to these research activities.
‘By combining experimental data and simulations, we discovered that rotations in plastic ice are not completely free, but rather occur through jumps between preferential positions,’ says Maria Rescigno of Sapienza University of Rome, first author of the paper. ’This behaviour gives plastic VII ice unique properties that distinguish it from other solid phases of water and significantly influence its physical properties.’
The study not only provides new information on the nature of hydrogen bonds under extreme conditions - crucial for better understanding the properties of water and many other chemicals - but also opens up new avenues for understanding the structure of frozen celestial bodies and their evolution.
A particularly interesting case is that of Jupiter's two moons, Ganymede and Callisto, whose internal differentiation remains an open question in planetology. One possible explanation for this phenomenon could depend on the presence of plastic ice in only one of the two moons. This circumstance would have influenced their structural evolution differently.
The research, the result of an international collaboration involving as many as nine institutions, represents an important advance in understanding the complex phase diagram of water under extreme conditions and could open up new research perspectives in the field of planetology.
References:
Rescigno, M., Toffano, A., Ranieri, U. et al. “Observation of Plastic Ice VII by Quasi-Elastic Neutron Scattering”. Nature (2025). https://doi.org/10.1038/
Further Information:
Livia Eleonora Bove
Department of Physics
liviaeleonora.bove@uniroma1.it
Maria Rescigno
Department of Physics