-He Mixture to 75 GPa*

Hydrogen and helium are two most fundamental quantum solids abundant in the Giant planets. The study of dense hydrogen-helium mixture is, therefore, importance to provide insights into the internal structure of the Giant planets as well as the chemistry between these fundamental solids. We have studied hydrogen-helium mixtures to 75 GPa at room temperature as a function of composition using diamond anvil cells and confocal micro-Raman spectroscopy. The results indicate that the binary phase diagram consists of various forms of hydrogen-rich and helium-rich phases, including a metastable H₂-rich solid grown in a He-rich fluid and a structural phase transition of He lattice at ~52 GPa. These results therefore underscore a considerable level of miscibility in dense H₂-He mixtures that can have a significant implication to the current layer structure of the Giant planets.