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 CH_{4}-H_{2} system. We find that pressures as low as 4.8 GPa can stabilize CH_{4}(H_{2}){2} and (CH{4}){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, (CH_{4}){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.