Observation of Plastic Ice VII by Quasi Elastic Neutron Scattering

Water stands as the third most abundant molecular system in the universe, and its condensed form, ice, is a key component in the interiors of icy moons, giant planets, and Uranus- and Neptune-like exoplanets [1–3]. Water undergoes complex phase transitions under extreme conditions, leading to the formation of numerous crystalline and amorphous phases [4–6]. Among these, molecular dynam- ics simulations suggest the existence of a plastic phase[7–12] along the melting curve of Ice VII, one of the most relevant high-pressure and high-temperature ice phases[4]. Plastic crystals are hybrid states between liquids and solids, characterized by structural order, typical of crystals, and dynamic molecular orientations, reminiscent of liquids. Here, we present experimental evidence of a plastic phase of Ice VII using quasi-elastic neutron scattering (QENS), conducted at temperatures between 450 and 600 K and pressures up to 6 GPa. Our results reveal the existence of a body centered cubic (bcc) structure, as the one of ice VII, where water molecules exhibit picosecond rotational dynamics, as in liquid water. Comparison with molecular dynamics simulations indicates that plastic Ice VII does not conform to a free rotor phase but rather exhibits rapid orientational jumps, as observed in jump-rotor plastic crystals [13, 14]. The observation of plastic ice VII carries significant implications for the geodynamics of internal hot icy layers in icy planets, and could play a key role in the differen- tiation processes of large icy moons.