Hydrogen-rich hydrate at high pressures up to 104 GPa

TL;DR

This study investigates hydrogen-rich hydrate stability at high pressures up to 104 GPa, revealing phase transitions and metastability relevant to planetary interiors and hydrogen storage.

Contribution

It provides new insights into the structural stability and phase transitions of hydrogen hydrates under extreme pressures, combining experimental and theoretical methods.

Findings

C2 to C3 phase transition occurs between 47-104 GPa

C3 phase contains twice as much H2 as C2 phase

Metastability of C3 phase down to 40 GPa after decompression

Abstract

Gas hydrates are considered fundamental building blocks of giant icy planets like Neptune and similar exoplanets. The existence of these materials in the interiors of giant icy planets, which are subject to high pressures and temperatures, depends on their stability relative to their constituent components. In this study, we reexamine the structural stability and hydrogen content of hydrogen hydrates, (H2O)(H2)n, up to 104 GPa, focusing on hydrogen-rich materials. Using synchrotron single-crystal X-ray diffraction, Raman spectroscopy, and first-principles theoretical calculations, we find that the C2-filled ice phase undergoes a transformation to C3-filled ice phase over a broad pressure range of 47 - 104 GPa at room temperature. The C3 phase contains twice as much molecular H2 as the C2 phase. Heating the C2-filled ice above approximately 1500 K induces the transition to the C3 phase at pressures as low as 47 GPa. Upon decompression, this phase remains metastable down to 40 GPa. These findings establish new stability limits for hydrates, with implications for hydrogen storage and the interiors of planetary bodies.