First principles molecular dynamics study of filled ice hydrogen hydrate

TL;DR

This study uses first principles molecular dynamics to explore the structural stability, phase transitions, and vibrational properties of hydrogen hydrate filled-ice phase C2, revealing temperature and pressure effects on its symmetry and hydrogen bonding.

Contribution

It provides new insights into the stability and phase behavior of filled-ice hydrates under various conditions using first principles simulations.

Findings

Cubic structure is unstable at low temperature and high pressure.

Thermal effects stabilize cubic symmetry at room temperature below 30 GPa.

Hydrogen bonds become symmetrized above 60 GPa, eliminating order-disorder transitions.

Abstract

We investigated structural changes, phase diagram, and vibrational properties of hydrogen hydrate in filled-ice phase C2 by using first principles molecular dynamics simulation. It was found that the experimentally reported 'cubic' structure is unstable at low temperature and/or high pressure. The 'cubic' structure reflects the symmetry at high (room) temperature where the hydrogen bond network is disordered and the hydrogen molecules are orientationally disordered due to thermal rotation. In this sense, the 'cubic' symmetry would definitely be lowered at low temperature where the hydrogen bond network and the hydrogen molecules are expected to be ordered. At room temperature and below 30 GPa, it is the thermal effects that play an essential role in stabilizing the structure in 'cubic' symmetry. Above 60 GPa, the hydrogen bonds in the framework would be symmetrized and the hydrogen bond order-disorder transition would disappear. These results also suggest the phase behavior of other filled-ice hydrates. In the case of rare gas hydrate, there would be no guest molecues rotation-nonrotation transition since the guest molecules keep their spherical symmetry at any temperature. On the contrary methane hydrate MH-III would show complex transitions due to the lower symmetry of the guest molecule. These results would encourage further experimental studies, especially NMR spectroscopy and neutron scattering, on the phases of filled-ice hydrates at high pressures and/or low temperatures.