Density functional theory study of phase IV of solid hydrogen

Abstract

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. Quasiharmonic 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, Nat. 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.