Structures of hydrogen at megabar pressures

Nagao, Keiko; Nagara, Hitose; Matsubara, Satοshi

doi:10.1103/physrevb.56.2295

Cited by 41 publications

(26 citation statements)

References 29 publications

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“…Their work suggested that after molecular dissociation, hydrogen should adopt a structure similar to Phase IV of cesium, a bodycentered tetragonal structure with space group I4 1 /amd and a c/a ratio greater than unity, consistent with earlier predictions (Nagao et al, 1997;Pickard and Needs, 2007). This structure was found to be stable at least up to ∼1 TPa, including a harmonic estimation of ZPE.…”

Section: Atomic Crystal Phasessupporting

confidence: 70%

“…Anisotropic structures were also considered by Barbee III et al, 1989 andCohen, 1991, the latter study drawing a structural analogy with the 9R ground-state structure of ambientpressure lithium (Overhauser, 1984). Other studies, however, predicted isotropic structures (Ceperley and Alder, 1987a;Nagao et al, 1997;Natoli et al, 1993;.…”

Section: Atomic Crystal Phasesmentioning

confidence: 99%

“…Bonds are drawn between nearest neighbors. simple harmonic approximation can fail, using a family of face-centered tetragonal structures (similar, in fact, to those considered again in later studies (Nagao et al, 1997)). Only by including anharmonicity, for example, were some structures found to be energetically stabilized, in this case the isotropic fcc structure.…”

Section: Atomic Crystal Phasesmentioning

confidence: 99%

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The properties of hydrogen and helium under extreme conditions

McMahon¹,

Morales²,

Pierleoni³

et al. 2012

Rev. Mod. Phys.

479

415

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Hydrogen and helium are the most abundant elements in the universe and, in principle, are the simplest elements. Nonetheless, they display remarkable properties under pressure that have fascinated theoreticians and experimentalists for over a century. Recent advances in computational methods have made it possible to elucidate many of these properties. We review some of the computational methods that have been applied to dense hydrogen and helium in recent years, mainly those that perform a simulation directly from the physical picture of electrons and ions; primarily, those based on density functional theory and quantum Monte Carlo methods. We then discuss the predictions from such methods as applied to the phase diagram of hydrogen, including the solid and fluid phases, with particular focus on the crystal structures, the liquidliquid transition and comparison of the results with experimental shock-wave data. We then discuss predictions of ordered quantum states, including a possible low-temperature fluid and high-temperature superconductivity in the atomic state. We also briefly discuss pure helium, and then focus on hydrogen-helium mixtures, with particular focus on properties of relevance to planetary science.

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Section: Atomic Crystal Phasessupporting

confidence: 70%

Section: Atomic Crystal Phasesmentioning

confidence: 99%

Section: Atomic Crystal Phasesmentioning

confidence: 99%

See 1 more Smart Citation

The properties of hydrogen and helium under extreme conditions

McMahon¹,

Morales²,

Pierleoni³

et al. 2012

Rev. Mod. Phys.

479

415

View full text Add to dashboard Cite

show abstract

“…The results indicate the stability of the ordered molecular phase III to the highest pressure reached and provide constraints on the insulator-to-metal transition pressure. R ecent theoretical predictions for the transformation pressure of solid hydrogen to its metallic state are still uncertain and range between 260 and 410 GPa (1)(2)(3). Metallization by band overlap is predicted to occur before breakdown to a monoatomic solid.…”

mentioning

confidence: 99%

Spectroscopic studies of the vibrational and electronic properties of solid hydrogen to 285 GPa

Goncharov

Gregoryanz

Hemley

et al. 2001

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

104

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We report Raman scattering and visible to near-infrared absorption spectra of solid hydrogen under static pressure up to 285 GPa between 20 and 140 K. We obtain pressure dependences of vibron and phonon modes consistent with results previously determined to lower pressures. The results indicate the stability of the ordered molecular phase III to the highest pressure reached and provide constraints on the insulator-to-metal transition pressure. R ecent theoretical predictions for the transformation pressure of solid hydrogen to its metallic state are still uncertain and range between 260 and 410 GPa (1-3). Metallization by band overlap is predicted to occur before breakdown to a monoatomic solid. The ability of this theory to calculate the band gap at relevant densities is substantially impaired by the uncertainty of the structure of the high-pressure molecular modification of solid hydrogen-phase III (e.g., ref. 4). The nature of this phase and its related infrared activity have been the topic of numerous theoretical and experimental studies (e.g., ref. 5), but definitive knowledge of its crystal and electronic structure is not yet in hand.Diamond-anvil cells have been used successfully to reach static pressures on the order of 360 GPa for compressed metals (6-8). However, numerous attempts to compress solid hydrogen to transform it to conducting states (9-15) have not been successful in reaching the critical pressure range, while at the same time characterizing the sample and pressure definitively. The claim of compressing solid hydrogen to 342 GPa (15), for instance, showed no evidence for the presence of hydrogen in the sample chamber, indicating that instead the ''soft'' hydrogen was most likely lost by developing small leaks in diamonds and gasket or by reaction with the gasket material (see ref. 4). In addition, the large stress-induced increase in optical absorption and fluorescence in diamond anvils (6, 16, 17) poses a major obstacle in optical measurements of hydrogen samples and pressure calibration by ruby fluorescence. Control measurements for optical absorption experiments are essential. For example, comparison of the absorption of the transparent ruby grains adjacent to hydrogen can be used to ascertain that the pressure-induced changes in absorption occurs is in hydrogen but not in the diamond windows (9). At pressures beyond 180 GPa, the ruby fluorescence becomes extremely weak and can be overwhelmed by diamond fluorescence, thus presenting another serious problem for pressure calibration. As a result, the pressure for our previously reported onset of absorption in hydrogen could be determined as being above 200 GPa, but the upper limit could not be established. Indirect methods of pressure calibration, such as x-ray diffraction measurements on the gasket (15), do not indicate the pressure of the sample, and pressure calibration based on the pressure shift of diamond Raman band (18) may vary by as much as a factor of 3, depending on the local nonhydrostatic stress condition on the pressure-bearin...

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“…Various candidate structures for the high-pressure phases (called "phase II" or "BSP" below ഠ150 GPa and "phase III" or "HA" above ഠ150 GPa) have been proposed based on static [7][8][9][10][11][12][13][14][15], and dynamic [16][17][18] density-functional calculations and on QMC [6] investigations. Most of these structures have hexagonal and orthorhombic unit cells with up to four molecules.…”

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confidence: 99%

Metallization of Molecular Hydrogen: Predictions from Exact-Exchange Calculations

2000

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We study metallization of molecular hydrogen under pressure using exact-exchange (EXX) Kohn-Sham density-functional theory in order to avoid well-known underestimates of band gaps associated with standard local-density or generalized-gradient approximations. Compared with the standard methods, the EXX approach leads to considerably (1-2 eV) higher gaps and significant changes in the relative energies of different structures. Metallization is predicted to occur at a density of greater, approximately greater than 0.6 mol/cm(3) (corresponding to a pressure of greater, approximately greater than 400 GPa), consistent with all previous measurements.

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Structures of hydrogen at megabar pressures

Cited by 41 publications

References 29 publications

The properties of hydrogen and helium under extreme conditions

The properties of hydrogen and helium under extreme conditions

Spectroscopic studies of the vibrational and electronic properties of solid hydrogen to 285 GPa

Metallization of Molecular Hydrogen: Predictions from Exact-Exchange Calculations

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