High-pressure Raman study of solid hydrogen up to 300 GPa

The high-pressure behavior of solid hydrogen has been investigated by in situ Raman spectroscopy upon compression to 300 GPa at ambient temperature. The hydrogen vibron frequency begins to decrease after it initially increases with pressure up to 38 GPa. This softening behavior suggests the weakening of the intramolecular bond and the increased intermolecular interactions. Above 237 GPa, the vibron frequency softens very rapidly with pressure at a much higher rate than that of phase HI, corresponding to transformation from phase III into phase IV. The phase transition sequence has been confirmed from phase I to phase III and then to phase IV at 208 and 237 GPa, respectively. Previous theoretical calculations lead to the proposal of an energetically favorable monoclinic C2/c structure for phase HI and orthorhombic Pbcn structure for phase IV. Up to 304 GPa, solid hydrogen is not yet an alkali metal since the sample is still transparent.