Optical studies of solid hydrogen to 320 GPa and evidence for black hydrogen

Loubeyre, P.; Occelli, F.; LeToullec, R.

doi:10.1038/416613a

Cited by 401 publications

(344 citation statements)

References 23 publications

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“…Indeed, quantum ion motion usually favours phases with lower ion-ion correlations, which would further stabilize the low-temperature liquid phase found here. In addition, our findings are consistent with pressure estimates from experiments where hydrogen is expected to metallize 3, 29 . Finally, we note that the observed increase in MLWF spreads when going from the high-pressure solid to the molecular liquid phase points at the presence of dynamically-induced intermolecular charge transfer (dipole moments); this indicates that the transition from solid to liquid at high pressure is accompanied by an increase of the hydrogen infrared absorption.…”

supporting

confidence: 81%

A quantum fluid of metallic hydrogen suggested by first-principles calculations

Bonev

Schwegler

Ogitsu

et al. 2004

Nature

297

281

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It is generally assumed 1,2,3 that solid hydrogen will transform into a metallic alkali-like crystal at sufficiently high pressure. However, some theoretical models 4,5 have also suggested that compressed hydrogen may form an unusual two-component (protons and electrons) metallic fluid at low temperature, or possibly even a zero-temperature liquid ground state. The existence of these new states of matter is conditional on the presence of a maximum in the melting temperature versus pressure curve (the 'melt line'). Previous measurements 6,7,8 of the hydrogen melt line up to pressures of 44 GPa have led to controversial conclusions regarding the existence of this maximum. Here we report ab initio calculations that establish the melt line up to 200 GPa. We predict that subtle changes in the intermolecular interactions lead to a decline of the melt line above 90 GPa. The implication is that as solid molecular hydrogen is compressed, it transforms into a low-temperature quantum fluid before becoming a monatomic crystal. The emerging low-temperature phase diagram of hydrogen and its isotopes bears analogies with the familiar phases of 3 He and 4 He, the only known zero-temperature liquids, but the long-range Coulombic interactions and the large component mass ratio present in hydrogen would ensure dramatically different properties 9,10 .The possible existence of low-temperature liquid phases of compressed hydrogen has been rationalized with arguments based on the nature of effective pair interactions and of the quantum dynamics at high density, resulting in proton-proton correlations insufficient for the stabilization of a crystalline phase 5 . But so far there has been no conclusive evidence establishing whether hydrogen metallizes at low temperature as a solid (the more widely accepted view to date) or as a liquid. Measurements and theoretical predictions of the near-ground state high-pressure phases 3 of hydrogen have proven to be difficult because of the light atomic mass, significant quantum effects and strong electron-ion interactions. In this regard, the finite temperature liquid-solid phase boundary predicted here is especially valuable for understanding the manner in which hydrogen metallizes.The appearance of a maximum melting temperature in hydrogen is in itself a manifestation of an unusual physical phenomenon. The few systems with a negative melt slope involve either open crystalline structures, such as water and graphite, or in the case of closed packed solids, a promotion of valence electrons to higher orbitals upon compression (6s to 5d in cesium 11 , for example). In these cases, the liquid is denser than the solid when they coexist, possibly because of structural or electronic transitions taking place continuously in the liquid, as a function of pressure, but only at discrete pressure intervals in the solid.In contrast, recent experiments 8 have shown that hydrogen phase I -a solid structure with rotationally-free molecules associated with the sites of a hexagonal closed packed (hcp) lattice -persist...

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supporting

confidence: 81%

A quantum fluid of metallic hydrogen suggested by first-principles calculations

Bonev

Schwegler

Ogitsu

et al. 2004

Nature

297

281

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“…5 that at 400 GPa on our scale, the old MXB86 scale would give a pressure 67 GPa lower, or 333 GPa. Perhaps this explains the conflict between Narayana et al 11 and Loubeyre et al 12 on hydrogen at high pressures.…”

Section: The Calibrationmentioning

confidence: 89%

“…Narayana et al 11 studied hydrogen in a DAC to 342 GPa and observed that the sample remained transparent to this pressure ͑the only measurement made on the sample was this visual one͒. Later, Loubeyre et al 12 studied hydrogen and observed it to become black at a lower pressure, 320 GPa. Two possible solutions to these contradictory results are that Narayana et al did not have a sample in their DAC gasket, but they are confident that they did, 13 and that the pressure scales were incorrect so that the sample of Loubeyre et al was really at a higher pressure than that of Narayana et al Narayana et al used the x-ray-marker method and determined the pressure from that of the gasket adjacent to the sample, making a correction to the pressure in the gasket because it was not embedded in the hydrogen sample.…”

Section: Introductionmentioning

confidence: 99%

The ruby pressure standard to 150GPa

Chijioke

Nellis

Солдатов

et al. 2005

Journal of Applied Physics

249

173

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A determination of the ruby high-pressure scale is presented using all available appropriate measurements including our own. Calibration data extend to 150 GPa. A careful consideration of shock-wave-reduced isotherms is given, including corrections for material strength. The data are fitted to the calibration equation P = ͑A / B͓͒͑ / 0 ͒ B −1͔ ͑GPa͒, with A = 1876± 6.7, B = 10.71± 0.14, and is the peak wavelength of the ruby R1 line.

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“…Our study leads to a radical revision of the DFT phase diagram of hydrogen up to nearly 400 GPa. That the most stable phases remain insulating to very high pressures eliminates a major discrepancy between theory 5 and experiment 6 . One of our new phases is calculated to be stable over a wide range of pressures, and its vibrational properties agree with the available experimental data for phase III.…”

mentioning

confidence: 99%

Structure of phase III of solid hydrogen

Pickard

Needs²

2007

Nature Phys

667

729

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Hydrogen, being the first element in the periodic table, has the simplest electronic structure of any atom, and the hydrogen molecule contains the simplest covalent chemical bond. Nevertheless, the phase diagram of hydrogen is poorly understood. Determining the stable structures of solid hydrogen is a tremendous experimental challenge 1-3 , because hydrogen atoms scatter X-rays only weakly, leading to low-resolution diffraction patterns. Theoretical studies encounter major difficulties owing to the small energy differences between structures and the importance of the zero-point motion of the protons. We have systematically investigated the zerotemperature phase diagram of solid hydrogen using firstprinciples density functional theory (DFT) electronic-structure methods 4 , including the proton zero-point motion at the harmonic level. Our study leads to a radical revision of the DFT phase diagram of hydrogen up to nearly 400 GPa. That the most stable phases remain insulating to very high pressures eliminates a major discrepancy between theory 5 and experiment 6 . One of our new phases is calculated to be stable over a wide range of pressures, and its vibrational properties agree with the available experimental data for phase III.The low-pressure phase I of solid hydrogen, which consists of freely rotating molecules on a hexagonal close-packed lattice 2 , transforms at pressures of about 110 GPa to the broken-symmetry phase II, in which the mean molecular orientations are ordered, and then to phase III at about 150 GPa (ref. 1). However, even the combination of X-ray and neutron scattering data and Raman and infrared vibrational data has not so far yielded the structures of phases II and III of hydrogen.The theoretical prediction of stable crystal structures is very difficult because of the need to search the very large space of possible structures, and the necessity of obtaining accurate energies for each of these structures. First-principles DFT methods have proved an efficient means of calculating quite accurate energies, and they have provided many insights into the properties of materials, including solid hydrogen 5,7 . At present, DFT offers the highest level of theoretical description at which we can carry out searches over many possible candidate structures.Our approach is to relax many random structures to minima in the enthalpy at fixed pressure 8 . This method does not rely on previous theoretical or experimental results, and it allows for the possibility of finding radically new structures. In some cases we used the intuition gained from the random searches to build other candidate structures. We then calculated the enthalpies of the most stable phases at a larger number of pressures, generating the data shown in Fig. 1. We refer to each structure by its short HermannMauguin space-group symbol, giving additional information where an ambiguity might occur.The lowest-enthalpy structures found around 100 GPa were those of space groups Pca2 1 and P2 1 /c, which were considered in previous studies 5,7 , and ...

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Optical studies of solid hydrogen to 320 GPa and evidence for black hydrogen

Cited by 401 publications

References 23 publications

A quantum fluid of metallic hydrogen suggested by first-principles calculations

A quantum fluid of metallic hydrogen suggested by first-principles calculations

The ruby pressure standard to 150GPa

Structure of phase III of solid hydrogen

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