In everyday life, we typically encounter water in one of three familiar states – solid, liquid or gas. But there are in fact many more phases, some of which – predicted to exist at high temperature and pressure – are so strange they're referred to as exotic. State-of-the-art neutron spectrometers and sample environment infrastructures at the Institut Laue-Langevin (ILL) have enabled the first experimental observation of one of these exotic phases – plastic ice VII.
Plastic ice VII was originally predicted more than 15 years ago by molecular dynamics (MD) simulations as a phase of water that could exist under high temperature and pressure. "Plastic phases are hybrid states that blend properties of both solids and liquids," explains Livia Eleonora Bove, research director at the French National Centre for Scientific Research CNRS, associate professor at La Sapienza University in Rome (Italy) and associated scientist at EPFL – École Polytechnique Fédérale de Lausanne (Switzerland). "In plastic ice, the water molecules form a rigid cubic lattice, as in ice VII, but exhibit picosecond rotational motion reminiscent of liquid water."
For the study of these fast molecular motions, Quasi-Elastic Neutron Scattering (QENS) is a powerful tool. "The ability of QENS to probe both the translational and rotational dynamics is a unique advantage for the exploration of such exotic phase transitions compared to other spectroscopic techniques," explains Maria Rescigno, PhD student at Sapienza University and first author of the published study. QENS enabled the identification of three distinct phases as temperature and pressure were varied: liquid water in which both translational and rotational components are present; solid ice where both translational and rotational dynamics are frozen; and the intermediate plastic ice phase where the molecules, arranged in an ordered crystalline structure, have lost the ability to translate freely but have retained the capacity to rotate.
The experiments revealing plastic ice VII were performed using the time-of-flight spectrometers IN5 and IN6-SHARP at the ILL. Temperatures as high as 450 – 600 K and pressures from 0.1 to 6 GPa (up to about 60 thousand times the normal atmospheric pressure) were required to produce this exotic state of water. The implementation of such demanding thermodynamic conditions in neutron spectroscopy was made possible by recent technological advances achieved in collaboration between Bove, CNRS research director Stefan Klotz, and ILL scientist Michael Marek Koza as part of a long-term project at the ILL. "The success of this study relies on the extensive expertise and unique infrastructure built over the years at the ILL, in particular in terms of complex sample environments and high pressures," underlines Koza, "Additionally, the continuous improvement of ILL's spectrometers – such as those made within the Endurance upgrade programme – has facilitated ever more sophisticated experiments carried out by state-of-the-art instruments,".
A comprehensive analysis of the neutron scattering data also revealed that the molecular dynamics of plastic ice VII could me more intricate than MD simulation had initially predicted. "The QENS measurements suggested a different molecular rotation mechanism for plastic ice VII than the free rotor behaviour initially expected," explains Rescigno. Additional MD simulations, together with Markov chain analysis, provided a more detailed picture of the water molecule dynamics. A 4-fold rotational model, as typically observed in jump-rotor plastic crystals, was identified as the most likely mechanism.
Further investigations – involving neutron and X-ray diffraction measurements, respectively, on the D20 diffractometer at the ILL and at the Institute of Mineralogy, Physics of Materials and Cosmochemistry (IMPMC) – were carried out to explore the nature of the phase transition from ice VII to plastic ice VII. "This transition is predicted to be either first-order or continuous, depending on the simulation method used," explains Bove. "The continuous transition scenario is very intriguing as it hints that the plastic phase could be the precursor of the elusive superionic phase – another hybrid exotic phase of water predicted at even higher temperatures and pressures, where hydrogen can diffuse freely through the oxygen crystalline structure." Both plastic and superionic phases are of high interest in planetary science, with potential implications in our understanding of the internal structure and glacial flow of icy moons like Ganimede and Callisto and icy planets like Uranus and Neptune, where they might dominate.
Neutron scattering hasn't traditionally been a go-to technique in planetary science. Nevertheless, its unique ability to precisely measure the location and dynamics of hydrogen in a material, combined with the recent possibility of conducting experiments at planetary relevant pressures, has enabled neutron scattering to make a substantial impact in this domain. And there may be more exotic phases yet to uncover.
