Formation and Stability of Dense Methane-Hydrogen Compounds

Through a series of x-ray diffraction, optical spectroscopy diamond anvil cell experiments, combined with density functional theory calculations, we explore the dense CH4-H-2 system. We find that pressures as low as 4.8 GPa can stabilize CH4(H-2)(2) and (CH4)(2)H-2, with the latter exhibiting extreme hardening of the intramolecular vibrational mode of H-2 units within the structure. On further compression, a unique structural composition, (CH4)(3)(H-2)(25), emerges. This novel structure holds a vast amount of molecular hydrogen and represents the first compound to surpass 50 wt % H-2. These compounds, stabilized by nuclear quantum effects, persist over a broad pressure regime, exceeding 160 GPa.