The first experimental observation of an exotic water phase: VII plastic ice

In everyday life, we generally find water in one of the three family states: solid, liquid or gas. But, in fact, there are many more phases, some of which predicted to exist at high temperature and pressure, are so strange that they are known as exotic.

The avant-garde neutrons spectrometers and sample environment infrastructures at the Institut Laue-Langevin (Ill) have allowed the first experimental observation of one of these exotic phases: VII plastic ice. The work has been published in Nature.

VII plastic ice was originally predicted more than 15 years for simulations of molecular dynamics (MD) as a water phase that could exist at high temperature and pressure.

“The plastic phases are hybrid states that combine solid and liquid properties,” explains Livia Eleonora Bove, research director of the National Center for Scientific Research of French CNRS, associate professor at the University of La Sapienza in Rome (Italy) and associated scientist In EPFL, École Polytechnique Fédérala de Lausanne (Switzerland).

“On plastic ice, water molecules form a rigid cubic network, as in the Ice VII, but exhibit the pepper rotation movement that remind of liquid water.”

For the study of these rapid molecular movements, the quasiastic neutron dispersion (qens) is a powerful tool.

“The ability of those to probe both translational and rotational dynamics is a unique advantage for the exploration of such exotic phase transitions compared to other spectroscopic techniques,” explains Maria Rescigno, Ph.D. Student of the University of Sapienza and first author of the published study.

Those who allowed the identification of three different phases as the temperature and pressure were varied: liquid water in which 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 orderly crystalline structure, have lost the ability to translate freely but have preserved the ability to rotate.

The experiments that reveal the VII plastic ice were performed using flight time spectrometers in 5 and in6-Sharp in the IFL. Temperatures as high as 450-600 k and pressures from 0.1 to 6 GPA (up to approximately 60 thousand times normal atmospheric pressure) were required to produce this exotic state of water.

The implementation of such demanding thermodynamic conditions in neutron spectroscopy was possible thanks to the recent technological advances achieved in the collaboration between Bove, CNRs Research Director Stefan Klotz, and scientist Ill Michael Marek Kozo as part of a project A long term in the Ill.

“The success of this study is based on extensive experience and unique infrastructure built over the years in the Ill, particularly in terms of complex sample environments and high pressures,” says Koza.

“In addition, the continuous improvement of Ill spectrometers, such as those carried out in the resistance update program, has facilitated the increasingly sophisticated experiments carried out by the last generation instruments.”

An thorough analysis of neutron dispersion data also revealed that the molecular dynamics of plastic ice VII could be more intricate than the MD simulation had initially predicted.

“Qens measurements suggested a different molecular rotation mechanism for plastic ice VII that the behavior of the initially expected free rotor,” explains Resign.

The additional MD simulations, together with the analysis of the Markov chain, provided a more detailed image of the dynamics of the water molecule. A rotation model was identified four times, as typically observed in the plastic crystals of the jump rotor, as the most likely mechanism.

ADDITIONAL INVESTIGATIONS: The Difraction and X -ray diffraction measurements, respectively, in the D20 diffractometer in the Ill and in the Institute of Mineralogy, Materials and Cosmochemistry Physics (IMPMC)) were carried out to explore the nature of the transition of the transition of ICE VII to plastic ice VII.

“It is predicted that this transition will be first order or continuous, depending on the simulation method used,” explains Bove.

“The continuous transition scenario is very intriguing, since it suggests that the plastic phase could be the precursor of the elusive superionic phase, another hybrid exotic phase of the water predicted at even higher temperatures and pressures, where hydrogen can be released freely through of the crystalline structure of oxygen.

The plastic and superionic phases are of great interest in planetary science, with possible implications in our understanding of the internal structure and the glacial flow of frozen moons such as Ganimede and Callisto and icy planets such as Uranus and Neptune, where they could dominate.

Neutron dispersion has not traditionally been a reference technique in planetary science. However, its unique ability to accurately measure the location and dynamics of hydrogen in a material, combined with the recent possibility of carrying out experiments in relevant planetary pressures, has allowed neutron dispersion to have a substantial impact on this domain. And there may be more exotic phases to discover.