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, $\left({\mathrm{H}}_2\mathrm{O}\right){\left({\mathrm{H}}_2\right)}_{\mathrm{n}}$, 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 ${\mathrm{C}}_2$-filled ice phase undergoes a transformation to a ${\mathrm{C}}_3$-filled ice phase over a broad pressure range of 47 to 104 GPa at room temperature. The ${\mathrm{C}}_3$ phase contains twice as much molecular ${\mathrm{H}}_2$ as the ${\mathrm{C}}_2$ phase. Heating the ${\mathrm{C}}_2$-filled ice above approximately 1500 K induces the transition to the ${\mathrm{C}}_3$ 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.

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• Crystal structure
• High-pressure studies
• First-principles calculations
• Raman spectroscopy
• X-ray diffraction