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 phase in which water molecules retain the crystalline structure of ice VII but rotate around their centre of mass.
This challenging study, using a combination of neutron scattering experiments and advanced computational modelling, was performed by an international team from institutions including the Centre for Science at Extreme Conditions (CSEC) at the University of Edinburgh. The investigators observed that at temperatures above approximately 470 K and pressures exceeding 4 GPa, ice VII transitions into this 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.