1204.3304 (Chris J. Pickard et al.)

Density functional theory study of phase IV of solid hydrogen    [PDF]

Chris J. Pickard, Miguel Martinez-Canales, Richard J. Needs

A new structural phase of solid hydrogen has recently been discovered in high-pressure room temperature experiments [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 have studied hydrogen up to pressures of 300 GPa and temperatures of 350 K using density functional theory methods and have found new "mixed structures" which consist of alternate layers of strongly bonded molecules and weakly bonded graphene-like sheets. These structures are stabilized with respect to purely 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, and quasi-harmonic vibrational calculations show mixed structures to be the most stable at room temperature over the pressure range 250-295 GPa. Recent high-pressure low-temperature infra-red (IR) measurements show a drop in the vibron frequency around 300 GPa [C.-S. Zha et al. Phys. Rev. Lett. 108 146402 (2012)] which is consistent with our prediction of a new stable molecular phase under these conditions.

View original: http://arxiv.org/abs/1204.3304