Theoretical Studies On Exotic Structures And Properties Of Solid Hydrogen At Ultra-high Pressures

Hydrogen, the first element in the periodic table, is the most abundant material in theinteriors of stars and giant planets. High-pressure studies of hydrogen are offundamental interest and are relevant to our understanding of the physics and chemistryof planetary interiors. At ambient condition, molecular hydrogen is the most stable state.And molecular hydrogen is expected to dissociate into atomic phase and exhibitsmetallic properties under megabar pressures. This metal is predicted to besuperconducting with a very high critical temperature, and it may acquire a newquantum state as a metallic or superconducting superfluid. It may potentially berecovered metastable at ambient pressures. However, it is unknown that how molecularhydrogen dissociated into atomic phase. Therefore, the studies of solid hydrogen atultra-high pressures are of fundament interest for physics, chemistry and astrophysicsetc. Here, by using particle swarm optimized, metadynamics, two-phase coexistencemethods we explore the structures, metallization, melting curve and exotic chemicalbondings of solid hydrogen, and details as follow:1. Insulating molecular phases of solid hydrogen have been identified in staticdiamond anvil cell experiments up to pressures of over300GPa. Previous theoreticalstudies suggest solid hydrgeon would undergo a phase transition from molecular phaseinto atomic phase. Here we uncovered a novel crystal structure of solid hydrogen at470GPa by using particle swarm optimized methodology. This structure is aquasi-molecular phase, consisting of two different bond-lengths. The changing ofbond-lengths for two different bonds shows a new dissociation strategy of solid hydrogen: the bond-length of short molecules remains invariant with increasing pressure,but the bond-length of long molecules increases with increasing pressure. Moreover, theintermolecular distance becomes shorter with increasing pressure. When the distancesof inter-molecules and intra-molecules are almost equal, the molecular phase dissociatesinto atomic phase. The finding of this structure provides a deeper understanding ofmolecular dissociation in solid hydrogen, which has been a mystery for decades. Athigher pressure (~1500GPa), we predicted a new low energy structure consisting ofplanar H3clusters. Further calculations on electron-localized function reveal thathydrogen atoms in H3are bonding and interactions between H3clusters are very weak.These finding advanced this field and improving our understanding on high-pressurechemical bondings of solid hydrogen.2. Recently, it was claimed that compression at room temperature offers an easierpath to reach metal hydrogen: Conductive dense hydrogen was reported above220GPaat room temperature, evolving to a strong metallic character above260-270GPa. Asubsequent study could not confirm the metallic character but instead demonstrated asemiconducting nature of phase-IV with a1.8eV direct electronic gap at315GPa, quitesimilar to what was reported at low temperature. The metallization of solid hydrogen atroom temperature is thus of debate sources. Here we performed the metadynamicssimulations at150-300GPa and room temperature. At200GPa, a new partial orderedhcp (po-hcp or phase-IV) structure is uncovered and this structure contains alternatedisorders short molecular layers and graphene-like3-molecules layers. The furtherelectronic calculations show insulating nature of this phase. At275GPa, themetadynamics simulations show phase-IV transformed into a metallic structure. Thefinding of the two structures could help to improve our understanding of experimentalobservations on new insulating phase and metallization of solid hydrogen at highpressures. The molecular dynamics simulations without considering quantum nucleareffects surprisingly found the atoms in graphene-like layers of phase-IV can diffusefrom one site to other sites. Further molecular dynamics simulations with consideringquantum nuclear effects confirmed this proton transfer in this new phase. Moreover, the diffusive constant increased with increasing temperature and pressure. These findingscould well explain the experimental observation of increased half maximum width ofRaman with increasing pressure. We demonstrated that temperature effects are criticallyimportant to an understanding of the high-pressure behavior of solid hydrogen. Oursimulations provide theoretical evidence on the feasible metallization of solid hydrogenat300K below300GPa, in agreement with recent300K experiment, but havingdiscrepancy with other experiments. Further experimental and theoretical studies onsolid hydrogen at300K are greatly stimulated to understand this discrepancy.3. At ultra-high pressures solid hydrogen has been predicted to transform into aquantum fluid, because of its high zero-point motion. Here we report first-principlestwo-phase coexistence and Z-method determinations of the melting line of solidhydrogen in a pressure range spanning from30to600GPa. Our results suggest that themelting line of solid hydrogen, as derived from classical molecular dynamicssimulations, reaches a minimum of367K at~430GPa; at higher pressures the meltingline of the atomic Cs-IV phase regains a positive slope. In view of the possibleimportance of quantum effects in hydrogen at such low temperatures, we alsodetermined the melting temperature of the atomic Cs-IV phase at pressures of400,500and600GPa, employing Feynman path integral simulations. These result in adownward shift of the classical melting line by~100K, and hint at a possible secondarymaximum in the melting line in the region between500and600GPa, testifying to theimportance of quantum effects in this system. Combined, our results imply that thestability field of the zero-temperature quantum liquid phase, if it exists at all, wouldonly occur at higher pressures than previously thought.