New possible candidate structure for phase IV of solid hydrogen.

It has been proved in experiments that there are at least five phases of solid hydrogen at high pressure, however, only the structure of phase I has been absolutely determined. We revisited the phase space of solid hydrogen in the pressure range of 200-500 GPa using the particle swarm optimization technique combined with first-principles simulations. A novel orthorhombic structure named 2 is proposed as a possible candidate structure for phase IV. The 2 structure is a 'mixed structure' with two different types of layers and is distinctly different from the previously reported structure. Enthalpies and Gibbs free energies show that 2 and are competitive in the pressure region of phase IV. Nevertheless, the Raman and infrared vibron frequencies of 2 calculated by using density functional perturbation theory based on first-principles lattice dynamics show a better agreement with the experimental measurements than those of the structure. And the pressure dependence of these low-frequency Raman vibrons of 2 obtained from the first-principles molecular dynamics simulation shows a steeper slope, which resolves the long-standing issue of large discrepancies between the calculated Raman frequencies and the experimental [P. Loubeyre, F. Occelli and P. Dumas, , 2013, , 134101 and C. S. Zha, R. E. Cohen, H. K. Mao and R. J. Hemley, , 2014, , 4792]. Structural and vibrational analyses show that the hydrogen molecules in the weakly bonded molecular layer of 2 form distorted hexagonal patterns, and their vibration can be used to explain the experimental vibron. It is found that the weakly bonded layer is almost the same as the layers in the 2/ structure. This confirms the experimental conclusion [P. Loubeyre, F. Occelli and P. Dumas, , 2013, , 134101] that the ordering of hydrogen molecules in the weakly bonded molecular layers of the 'mixed structure' for phase IV is similar to that in the layers of the 2/ structure.