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 namedAma2 is proposed as a possible candidate structure for phase IV. TheAma2 structure is a 'mixed structure' with two different types of layers and is distinctly different from the previously reportedPcstructure. Enthalpies and Gibbs free energies show thatAma2 andPcare competitive in the pressure region of phase IV. Nevertheless, the Raman and infrared vibron frequencies ofAma2 calculated by using density functional perturbation theory based on first-principles lattice dynamics show a better agreement with the experimental measurements than those of thePcstructure. And the pressure dependence of these low-frequency Raman vibrons ofAma2 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 nu(1)[P. Loubeyre, F. Occelli and P. Dumas,Phys. Rev. B: Condens. Matter Mater. Phys., 2013,87, 134101 and C. S. Zha, R. E. Cohen, H. K. Mao and R. J. Hemley,Proc. Natl. Acad. Sci. U.S.A., 2014,111, 4792]. Structural and vibrational analyses show that the hydrogen molecules in the weakly bonded molecular layer ofAma2 form distorted hexagonal patterns, and their vibration can be used to explain the experimental nu(1)vibron. It is found that the weakly bonded layer is almost the same as the layers in theC2/cstructure. This confirms the experimental conclusion [P. Loubeyre, F. Occelli and P. Dumas,Phys. Rev. B: Condens. Matter Mater. Phys., 2013,87, 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 theC2/cstructure.