Hydrogen bond symmetrization in high-pressure ice clathrates

TL;DR

This study explores hydrogen bond symmetrization in high-pressure hydrogen hydrate phases using Raman spectroscopy and quantum simulations, revealing a quantum-driven, continuous crossover at lower pressures than in pure ice.

Contribution

It provides the first detailed characterization of hydrogen bond symmetrization in hydrogen hydrates, highlighting the role of quantum fluctuations and host-guest interactions.

Findings

Symmetrization occurs via a continuous crossover at lower pressures.

Quantum fluctuations significantly influence the symmetrization process.

No change in crystal symmetry during hydrogen bond symmetrization.

Abstract

Hydrogen bond symmetrization is a fundamental pressure-induced transformation in which the distinction between donor and acceptor sites vanishes, resulting in a symmetric hydrogen-bond network. While extensively studied in pure ice, most notably during the ice VII to ice X transition, this phenomenon remains less well characterized in hydrogen hydrates. In this work, we investigate hydrogen bond symmetrization in the high-pressure phases of hydrogen hydrate (H2-H2O and H2-D2O) through a combined approach of Raman spectroscopy and first-principles quantum atomistic simulations. We focus on the C2 and C3 filled-ice phases, using both hydrogenated and deuterated water frameworks. Our results reveal that quantum fluctuations and the interaction between the encaged H2 molecules and the host lattice play a crucial role in driving the symmetrization process. Remarkably, we find that in both C2 and C3 phases, hydrogen bond symmetrization occurs via a continuous crossover at significantly lower pressures than in pure ice, without any change in the overall crystal symmetry. These findings provide new insight into the quantum-driven mechanisms of bond symmetrization in complex hydrogen-bonded systems under extreme conditions.