Motivations

This project aims at investigating the mixtures of water, ammonia and methane ices over a wide range of pressure (1-100 GPa) and temperature (100-4000 K). Such mixtures are ubiquitous in the Universe and are present under extreme conditions of pressure and temperature inside giant icy planets (Neptune, Uranus), their satellites (e.g. Titan, Ganymede) and extra-solar planets. Although they are composed of simple molecules (H2O, NH3 and CH4), their properties remain largely unknown at very high density. Up to now, only the pure ices have been extensively investigated. These studies have highlighted interesting and unexpected properties: three exotic solid phases, called superionic, ionic and symmetric states, have been uncovered in pure water and ammonia ices around 1 Mbar (=100 GPa) (see figure 1). In these states, the chemical bonds, whether covalent or hydrogen (H) bonds, are strongly modified. In particular, the superionic phase is a spectacular state of matter presenting simultaneously a crystalline (the fixed ion lattice) and a liquid (the diffusive ions) behavior. Superionic water and ammonia ices are predicted to be excellent proton conductors, and the existence of superionicity in ice mixtures, if demonstrated, could be a key element to explain the unusual magnetic field of the giant icy planets.

Besides the interests in condensed-matter physics and planetary sciences, this project is expected to have an impact over a wide range of disciplines, including inorganic chemistry, materials science and biology. Indeed, ice mixtures are the most simple and are thus ideal systems to study the four most important hydrogen bonds O-H..O, N-H..N, O-H..N and N-H..O, and the mechanisms of proton transfer along these bonds. This topic has direct implications for our understanding of various phenomena, including the high melting point of water, the shape of proteins and photosynthesis. In this context, high pressure investigations provide a unique tool to study these phenomena at it allows varying the strength of the H bonds through the reduction of the bond length, without the perturbation of changing chemistry. Moreover, understanding the structure-properties relationship and the mechanism of proton delocalization in these simple systems could be useful for the development of superionic compounds as components of solid-state batteries, a topic currently under intensive investigation.