Helium and methane are the important constituents of ice giants in the solar system. As such, the behavior of methane-helium mixtures at high pressure offers insights into the structure and composition of interiors of ice giants. Here, we report on the room-temperature high-pressure phase diagram of methane-helium mixtures with helium concentrations from 10% to 98% examined by Raman spectroscopy and visual observation. Several two-phase equilibria and a liquid-liquid-solid three-phase equilibrium are observed upon loading and unloading pressure. Furthermore, based on the Raman vibrational frequency and corresponding full width at half maximum (FWHM) of C-H stretching vibration mode $v_1$, we estimated helium concentrations of methane-rich fluid $\left({\mathrm{F}}_1\right)$ and helium-rich fluid $\left({\mathrm{F}}_2\right)$ in different methane-helium mixtures. In addition, Raman measurements for solid phase $\left({\mathrm{S}}_1\right)$ across all mixtures suggest helium can hardly dissolve in ${\mathrm{S}}_1$ at high pressure.

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• High-pressure studies
• Phase diagrams
• Phase transitions
• Liquids
• Molecular solids
• Raman spectroscopy