Ab initio study of the miscibility for solid hydrogen–helium mixtures at high pressure

Understanding the behavior of H₂–He binary mixtures at high pressure is of great importance. Two more recent experiments [J. Lim and C. S. Yoo, Phys. Rev. Lett. 120, 165301 (2018) and R. Turnbull et al., ibid. 121, 195702 (2018)] are in conflict, regarding the miscibility between H₂ and He in solids at high pressure. On the basis of first-principles calculations combined with the structure prediction method, we investigate the miscibility for solid H₂–He mixtures at pressures from 0 GPa to 200 GPa. It is found that there is no sign of miscibility and chemical reactivity in H₂–He mixtures with any H:He ratio. Moreover, instead of H₂–He mixtures, the calculated Raman modes of the N–H mixtures can better explain the characteristic peaks observed experimentally, which were claimed to be the H–He vibrational modes. These calculation results are more in line with the experimental findings by Turnbull et al. [Phys. Rev. Lett. 121, 195702 (2018)].