Plastic Ice VII: A New Exotic Phase of Water Ice

Water continues to reveal unexpected behaviours under extreme conditions. In a recent study published in Nature, first experimental evidence of plastic ice VII was reported — a long-predicted orientationally disordered phase in which water molecules retain the crystalline structure of ice VII but rotate around their centre of mass.

In this extremely challenging study, using a combination of high-pressure neutron scattering experiments and advanced computational modelling, the investigators observed that at temperatures above approximately 470 K and pressures exceeding 4 GPa, ice VII transitions into its plastic state. Unlike traditional ice, where molecular motion is frozen, plastic ice VII exhibits rapid molecular reorientations akin to those in a liquid. The combined experimental data and simulations revealed that the water molecules in plastic ice VII do not rotate freely; instead, they jump randomly between a few favoured orientations.

This discovery has significant implications for condensed matter physics as well as planetary science. Notably, plastic ice VII may exist in the deep interiors of icy moons and exoplanets.

Phase diagram of water covering the temperature-pressure ranges of interest. The region of stability of plastic ice VII as inferred from the current study is highlighted in green.