Hydrogen dynamics under extreme conditions: looking for exotic states in water and hydrates

The main focus of this thesis is the characterization of the self dynamics of simple molecular systems as hydrogen, water, ammonia and methane, and their mixtures, under extreme conditions, employing Quasi-Elastic Neutron Scattering (QENS) as key experimental technique. We investigated the self-dynamics of water confined in AlPO4 zeolites with 1.2 nm uniaxial nanopores as a function of temperature, observing that the molecule re-orientational dynamics is active down to 100 K, i.e. well below the expected glass transition for bulk water, when the translational dynamics freezes at much higher temperature. Thus we observe that in highly confined water the translational and rotational degrees of freedom decouple. Similarly, we obtained the first direct experimental evi- dence of the predicted plastic Ice VII, an exotic phase where molecules are held in the crystalline ice VII cubic structure but rotate around their equilibrium positions on a picosecond timescale. Similar phases are found also in water-ammonia mixtures, in particular we have investigated the plastic phase ammonia mono-hydrate (AMH), namely AMH-VII. We also investigated the hydrogen hydrate system, focusing on the structural and vibrational prop- erties of the high pressure filled ice phases. We characterized the quantum dynamics of hydrogen in the high pressure C2 filled ice structure, and observed how the enhanced Van der Waals interactions between the non-polar hydrogen molecules and the water matrix strongly affect the hydrogen’s quan- tum rotations. At higher pressures we identified a new phase, namely C3, with unprecedentedly high hydrogen to water ratio. Finally, we have focused on water-methane mixtures, in particular their diffusive behaviour at high- pressures and high-temperatures, observing a pressure induced modification of the translational diffu- sion mechanism likely linked to the molecular mixing of methane in water. The molecular systems studied in this thesis are known to be among the main constituents of the inte- riors of ice bodies in our solar systems, as well as exoplanets. Thus, the knowledge of the high-pressure high-temperature behaviours of these systems and their mixtures, not only enlarges our understanding of inter-molecular interactions, but is also fundamental in order to build reliable models for planetary interiors. In particular their self-dynamics, which has been studied experimentally in this thesis, carries significant implications as the molecular scale dynamics strongly influences both the mechanical and thermal properties of these systems, on which planetary interiors modeling strongly relies.