Hanyu Liu, Li Zhu, Wenwen Cui, Yanming Ma
Metallic hydrogen has long been thought to be a potential room-temperature (300 K) superconductor; however, its experimental realization has eluded researchers for decades. Metallization of solid hydrogen was recently claimed in a 300 K experiment, albeit at a surprisingly low pressure of 260-270 GPa that contradicted earlier expectations that metallization should occur above 400 GPa. Here, we employed first-principles metadynamics simulations to explore the 300 K structures of solid hydrogen over the pressure range 150-300 GPa. At 200 GPa, we find the ambient-pressure disordered hexagonal close-packed (hcp) phase transited into an insulating partially disordered hcp phase (pd-hcp), a mixture of ordered graphene-like H2 layers and the other layers of weakly coupled, disordered H2 molecules. Within this phase, hydrogen remains in paired states with creation of shorter intra-molecular bonds, which are responsible for the very high experimental Raman peak above 4000 cm-1. At 275 GPa, our simulations established a transformation from pd-hcp into the ordered molecular metallic Cmca phase (4 molecules/cell) that was previously proposed to be stable only above 400 GPa. Gibbs free energy calculations at 300 K confirmed the energetic stabilities of the pd-hcp and metallic Cmca phases over all known structures at 220-242 GPa and >242 GPa, respectively. Our simulations highlighted the major role played by temperature in tuning the phase stabilities and provided the direct theoretical evidence on the metallization of solid hydrogen below 300 GPa at 300 K.
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http://arxiv.org/abs/1205.6092
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