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Density functional theory study of phase IV of solid hydrogen

We have studied solid hydrogen up to pressures of 300 GPa and temperatures of 350 K using density functional theory methods and have found "mixed structures" that are more stable than those predicted earlier. Mixed structures consist of alternate layers of strongly bonded molecules and weakly bonded graphene-like sheets. Quasi-harmonic vibrational calculations show that mixed structures are the most stable at room temperature over the pressure range 250-295 GPa. These structures are stabilized with respect to strongly-bonded molecular phases at room temperature by the presence of lower-frequency vibrational modes arising from the graphene-like sheets. Our results for the mixed structures are consistent with the experimental Raman data [M. I. Eremets and I. A. Troyan, Nature Mater. 10 927 (2011) and R. T. Howie et al. Phys. Rev. Lett. 108 125501 (2012)]. We find that mixed phases are reasonable structural models for phase IV of hydrogen.