Bonding, structures, and band gap closure of hydrogen at high pressures

Goncharov, Alexander F.; Tse, John S.; Wang, Hui; Yang, Jianjun; Struzhkin, Viktor V.; Howie, Ross T.; Gregoryanz, Eugene

doi:10.1103/physrevb.87.024101

Cited by 64 publications

(99 citation statements)

References 34 publications

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“…Our 768-and 288-atom results are significantly different from previous MD work 34,35 which has been equivocal about the structure of phase IV, due to reorienting of molecules, transformation of one layer type to another, and "mixed" phases of apparently random B-and G-layer stacking. In our own calculations with 24 or 48 molecules per unit cell we find the same behavior as described in previous work.…”

Section: Discussioncontrasting

confidence: 57%

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Identification of high-pressure phases III and IV in hydrogen: Simulating Raman spectra using molecular dynamics

2013

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We present a technique for extracting Raman intensities from ab initio molecular dynamics (MD) simulations at high temperature. The method is applied to the highly anharmonic case of dense hydrogen up to 500 K for pressures ranging from 180 to 300 GPa. On heating or pressurizing we find first-order phase transitions under the experimental conditions of the phase III-IV boundary. At even higher pressures, close to 350 GPa, we find a second phase transformation to the previously proposed Cmca-4. Our method enables, for the first time, a direct comparison of Raman vibrons between theory and experiment at finite temperature. This turns out to provide excellent discrimination between subtly different structures found in MD. We find candidate structures whose Raman spectra are in good agreement with experiment. The new phase obtained in hightemperature simulations adopts a dynamic, simple hexagonal structure with three layer types: freely rotating hydrogen molecules, static hexagonal trimers, and rotating hexagonal trimers. We show that previously calculated structures for phase IV are inconsistent with experiment, and their appearance in simulation is due to finite-size effects.

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Section: Discussioncontrasting

confidence: 57%

“…In our own calculations with 24 or 48 molecules per unit cell we find the same behavior as described in previous work. 34,35 These finitesize effects could be responsible for apparent rapid diffusion.…”

Section: Discussionmentioning

confidence: 99%

Identification of high-pressure phases III and IV in hydrogen: Simulating Raman spectra using molecular dynamics

2013

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“…Thus, the distorted hexagons are clearly seen in the layered C2/c and Cmca-4 structures. The former is the most plausible candidate for the low temperature phase III and the latter is a predicted at higher pressures and low temperatures 7,11,12 . The intermolecular distance in graphene-like layeres of high-pressure structures like C2/c,Cc, Pc and Pbcn is on the order of 1.0-1.1Å [11][12][13][14] .…”

Section: Analysis Of Candidate Structuresmentioning

confidence: 99%

“…Recently, solid hydrogen has been intensively investigated in a new pressure-temperature domain (200-360 GPa, and just above 300 K), both experimentally [4][5][6][7][8][9] and theoretically [10][11][12][13][14][15] . A new phase (phase IV) has been discovered above 220 GPa in the higher temperature regime 4,5,8 .…”

Section: Introductionmentioning

confidence: 99%

Graphene physics and insulator-metal transition in compressed hydrogen

2013

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Compressed hydrogen passes through a series of layered structures in which the layers can be viewed as distorted graphene sheets. The electronic structures of these layered structures can be understood by studying simple model systems-an ideal single hydrogen graphene sheet and threedimensional model lattices consisting of such sheets. The energetically stable structures result from structural distortions of model graphene-based systems due to electronic instabilities towards Peierls or other distortions associated with the opening of a band gap. Two factors play crucial roles in the metallization of compressed hydrogen: (i) crossing of conduction and valence bands in hexagonal or graphene-like layers due to topology and (ii) formation of bonding states with 2pz π character.

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“…Recent theoretical work has predicted new bounds on stability and has predicted new structures and their stabilities at megabar pressures (15)(16)(17)(18)(19)(20)(21)(22). First-principles calculations predict stability of a structure with space group symmetry C2/c in the pressuretemperature (P-T) region found experimentally for phase III (15).…”

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confidence: 99%

Electronic excitations and metallization of dense solid hydrogen

Cohen

Naumov

Hemley

2013

Proc. Natl. Acad. Sci. U.S.A.

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Theoretical calculations and an assessment of recent experimental results for dense solid hydrogen lead to a unique scenario for the metallization of hydrogen under pressure. The existence of layered structures based on graphene sheets gives rise to an electronic structure related to unique features found in graphene that are well studied in the carbon phase. The honeycombed layered structure for hydrogen at high density, first predicted in molecular calculations, produces a complex optical response. The metallization of hydrogen is very different from that originally proposed via a phase transition to a close-packed monoatomic structure, and different from simple metallization recently used to interpret recent experimental data. These different mechanisms for metallization have very different experimental signatures. We show that the shift of the main visible absorption edge does not constrain the point of band gap closure, in contrast with recent claims. This conclusion is confirmed by measured optical spectra, including spectra obtained to low photon energies in the infrared region for phases III and IV of hydrogen.density functional theory | diamond anvil cells | optical spectroscopy | semimetal absorption N ot only is hydrogen the simplest of all of the elements, it also reflects much of the range of electronic structures observed in materials, ranging from a Mott-Hubbard insulator for an expanded atomic lattice, to a van der Waals bonded condensed phase in the low-pressure molecular crystal, to an atomic metal at low temperatures and extreme compression. At high temperatures hydrogen forms the plasma that is the primary visible component of the universe. Because hydrogen is the most abundant element in the universe, forming most physical bodies, and is subjected to the entire range of pressures in the universe where atomic matter is stable, and because of its fundamental role in the history and structure of quantum mechanics, hydrogen at high pressure has been of intensive theoretical and experimental interest for the last century (1, 2). It has become evident from first-principles quantum-mechanical simulations that hydrogen passes through a set of layered structures as it is compressed. Important new advances in theory and experiment clarify the underlying framework of dense hydrogen physics and chemistry. We show here, from electronic structure calculations and analysis of experimental data, that metallization occurs through an evolution from a molecular solid to an extended crystalline solid in which distorted graphene-like layers provide the primary structural motif.At low temperatures hydrogen forms a simple hexagonal closepacked structure with freely rotating molecules, called phase I (1). At low temperature, the material transforms to the quantum broken symmetry phase (phase II) at pressures of 10 (50) GPa for H 2 (D 2 ) and phase III at 150 (165) GPa (1). Vibrational spectroscopy and X-ray diffraction of the lower pressure phases help constrain the structures of the higher pressure phases. The tra...

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Bonding, structures, and band gap closure of hydrogen at high pressures

Cited by 64 publications

References 34 publications

Identification of high-pressure phases III and IV in hydrogen: Simulating Raman spectra using molecular dynamics

Identification of high-pressure phases III and IV in hydrogen: Simulating Raman spectra using molecular dynamics

Graphene physics and insulator-metal transition in compressed hydrogen

Electronic excitations and metallization of dense solid hydrogen

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