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

Understanding the behavior of H-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 H 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 H-He mixtures at pressures from 0 GPa to 200 GPa. It is found that there is no sign of miscibility and chemical reactivity in H-He mixtures with any H:He ratio. Moreover, instead of H-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)].