Dense Solid Hydrogen Research Articles

Visual experimental study of slush hydrogen production by freezing-melting method

Slush hydrogen is a mixture of liquid hydrogen and solid hydrogen particles, and considered as an attractive propulsion fuel. The preparation process of slush hydrogen is complex. This paper describes a slush hydrogen preparation device which enables hydrogen liquefaction, liquid hydrogen storage, liquid hydrogen transfer, pumping decompression of liquid hydrogen, slush hydrogen observation, and hydrogen emission. In an open area, many tests using liquid hydrogen are conducted. The results show that 1) The scheme of hydrogen liquefaction using two-stage Gifford- Mcmahon (G-M) refrigerator is feasibility, and the liquefying rate of hydrogen is 0.45 L/h, which meets the consumption of slush hydrogen preparation. 2) In the triple-phase point state, average pressure and temperature of liquid hydrogen are measured as 7.1 kPa and 14.03 K respectively. Compared with NIST physical property data, the relative errors of pressure and temperature are 3.4% and 0.5%, respectively. 3) The formation process of solid hydrogen by the pumping decompression is observed. It is found that the solid hydrogen first nucleates on the Dewar wall. With the continuous run of the vacuum pump, the solid hydrogen gradually grows until dense solid hydrogen is formed, resulting that the light could not pass through it. It indicates that liquid hydrogen in the area near the gas–liquid interface could be completely frozen by the pumping decompression method. 4) The formation of slush hydrogen by a freezing-melting method is observed. It is found that the solid hydrogen forms porous structures due to the boiling fluctuation of liquid hydrogen during the freezing process, and the porous solid hydrogen in the liquid hydrogen forms a fragment or sheet shape again under the boiling action of liquid hydrogen during the melting process. When the vacuum pump is opened again, the violent boiling of the liquid hydrogen causes the fragmented solid hydrogen in the liquid hydrogen to be broken into irregular solid hydrogen particles again. Under the action of Gibbs free energy, irregular solid hydrogen particles form small spherical solid hydrogen particles suspended in liquid hydrogen. 5) Three states of liquid hydrogen, solid hydrogen in liquid hydrogen and slush hydrogen are visually compared. It is found that the observed slush hydrogen is a turbid and viscous state, which is a solid–liquid mixture of tiny solid hydrogen particles suspended in liquid hydrogen. At the same time, the weak motion behavior of viscous fluid is also observed. 6）The leakage heat of the slush hydrogen Dewar is calculated as 4.13 W by the liquid level drop rate, which is 1.66 times of the theoretical calculation, and it could provide a technical reference for the design of liquid hydrogen Dewar.

Synchrotron infrared spectroscopic evidence of the probable transition to metal hydrogen.

Hydrogen has been an essential element in the development of atomic, molecular and condensed matter physics1. It is predicted that hydrogen should have a metal state2; however, understanding the properties of dense hydrogen has been more complex than originally thought, because under extreme conditions the electrons and protons are strongly coupled to each other and ultimately must both be treated as quantum particles3,4. Therefore, how and when molecular solid hydrogen may transform into a metal is an open question. Although the quest for metal hydrogen has pushed major developments in modern experimental high-pressure physics, the various claims of its observation remain unconfirmed5-7. Here a discontinuous change of the direct bandgap of hydrogen, from 0.6electronvolts to below 0.1electronvolts, is observed near 425gigapascals. This result is most probably associated with the formation of the metallic state because the nucleus zero-point energyis larger than this lowest bandgap value. Pressures above 400gigapascals are achieved with the recently developed toroidal diamond anvil cell8, and the structural changes and electronic properties of dense solid hydrogen at 80kelvin are probed using synchrotron infrared absorption spectroscopy. The continuous downward shifts of the vibron wavenumber and the direct bandgap with increased pressure point to the stability of phase-III hydrogen up to 425gigapascals. The present data suggest that metallization of hydrogen proceeds within the molecular solid, in good agreement with previous calculations that capture many-body electronic correlations9.

Topological Surface States in Dense Solid Hydrogen.

Metallization of dense hydrogen and associated possible high-temperature superconductivity represents one of the key problems of physics. Recent theoretical studies indicate that before becoming a good metal, compressed solid hydrogen passes through a semimetallic stage. We show that such semimetallic phases predicted to be the most stable at multimegabar (∼300 GPa) pressures are not conventional semimetals: they exhibit topological metallic surface states inside the bulk "direct" gap in the two-dimensional surface Brillouin zone; that is, metallic surfaces may appear even when the bulk of the material remains insulating. Examples include hydrogen in the Cmca-12 and Cmca-4 structures; Pbcn hydrogen also has metallic surface states but they are of a nontopological nature. The results provide predictions for future measurements, including probes of possible surface superconductivity in dense hydrogen.

Quasiparticle energies and excitonic effects in dense solid hydrogen near metallization

We investigate the crucial metallization pressure of the $Cmca\text{\ensuremath{-}}12$ phase of solid hydrogen (H) using many-body perturbation theory within the $GW$ approximation. We consider the effects of self-consistency, plasmon-pole models, and the vertex correction on the quasiparticle band gap $({E}_{\mathrm{g}})$. Our calculations show that self-consistency leads to an increase in ${E}_{\mathrm{g}}$ by 0.33 eV over the one-shot ${G}_{0}{W}_{0}$ approach. Because of error cancellation between the effects of self-consistency and the vertex correction, the simplest ${G}_{0}{W}_{0}$ method underestimates ${E}_{\mathrm{g}}$ by only 0.16 eV compared with the prediction of the more accurate $GW\ensuremath{\Gamma}$ approach. Employing the plasmon-pole models underestimates ${E}_{\mathrm{g}}$ by 0.1--0.2 eV compared to the full-frequency numerical integration results. We thus predict a metallization pressure around 280 GPa, instead of 260 GPa predicted previously. Furthermore, we compute the optical absorption including the electron-hole interaction by solving the Bethe-Salpeter equation (BSE). The resulting absorption spectra demonstrate substantial redshifts and enhancement of absorption peaks compared to the calculated spectra neglecting excitonic effects. We find that the exciton binding energy decreases with increasing pressure from 66 meV at 100 GPa to 12 meV at 200 GPa due to the enhanced electronic screening as solid H approaches metallization. Because optical measurements are so important in identifying the structure of solid H, our BSE results should improve agreement between theory and experiment.

Raman measurements of phase transitions in dense solid hydrogen and deuterium to 325 GPa

Raman spectroscopy of dense hydrogen and deuterium performed to 325 GPa at 300 K reveals previously unidentified transitions. Detailed analysis of the spectra from multiple experimental runs, together with comparison with previous infrared and Raman measurements, provides information on structural modifications of hydrogen as a function of density through the I-III-IV transition sequence, beginning near 200 GPa at 300 K. The data suggest that the transition sequence at these temperatures proceeds by formation of disordered stacking of molecular and distorted layers. Weaker spectral changes are observed at 250, 285, and 300 GPa, that are characterized by discontinuities in pressure shifts of Raman frequencies, and changes in intensities and linewidths. The results indicate changes in structure and bonding, molecular orientational order, and electronic structure of dense hydrogen at these conditions. The data suggest the existence of new phases, either variations of phase IV, or altogether new structures.

Electronic excitations and metallization of dense solid hydrogen

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.

High-Pressure Measurements of Hydrogen Phase IV Using Synchrotron Infrared Spectroscopy

Phase IV of dense solid hydrogen has been identified by its infrared spectrum using high-pressure synchrotron radiation techniques. The spectrum exhibits a sharp vibron band at higher frequency and lower intensity than that for phase III, indicating the stability of molecular H(2) with decreased intermolecular interactions and charge transfer between molecules. A low-frequency vibron having a strong negative pressure shift indicative of strongly interacting molecules is also observed. The character of the spectrum is consistent with an anisotropic, mixed layer structure related to those recently predicted theoretically. Phase IV was found to be stable from 220 GPa (300 K) to at least 340 GPa (near 200 K), with the I-III-IV triple point located. Infrared transmission observed to the lowest photon energies measured places constraints on the electronic properties of the phase.

Strong Isotope Effect in Phase II of Dense Solid Hydrogen and Deuterium

Quantum nuclear zero-point motions in solid H(2) and D(2) under pressure are investigated at 80 K up to 160 GPa by first-principles path-integral molecular dynamics calculations. Molecular orientations are well defined in phase II of D(2), while solid H(2) exhibits large and very asymmetric angular quantum fluctuations in this phase, with possible rotation in the (bc) plane, making it difficult to associate a well-identified single classical structure. The mechanism for the transition to phase III is also described. Existing structural data support this microscopic interpretation.

Symmetry Breaking in Dense Solid Hydrogen: Mechanisms for the Transitions to Phase II and Phase III

Spectroscopic data for the high-pressure phases of hydrogen together with the topology of the phase diagram provide new insight on the behavior of the material at megabar pressure. Structural mechanisms are proposed for the transitions to phases II and III. These mechanisms include a partially ordered structure (possibly incommensurate) and an ordered isotranslational structure for the two phases, respectively. The analysis supports the existence of an additional phase, isostructural to phase III, with boundary that meets to form a second triple point with phases I and III.

Structure and spectroscopic properties of dense solid hydrogen at 160 GPa

The structure and spectroscopic properties of dense solid hydrogen structures are re-examined. The objective was to critically examine several candidate structures including the structure with space group C 2 / c recently proposed by Pickard and Needs. All of these have a unique similar structural feature and their spectroscopic properties such as high infrared intensity agree with experimental findings near 160 GPa. The Raman and infrared spectra as well as the detailed phonon stability of the C 2 / c structure are examined. The pressure dependence of the infrared absorption intensity is shown to be in excellent agreement with experiment.

Enhanced Friedel structure and proton pairing in dense solid hydrogen.

The mechanism of proton pairing in dense solid hydrogen, and its progression with density, are both studied using effective potentials between protons which include electronic response up to quadratic terms. For high pressures nonlinear effects originating with different pairs are crucial in establishing the net attraction within a given pair, and in this picture Friedel oscillations are considerably enhanced by quadratic response, subsequently playing a very important role in the overall pairing. Calculated vibron frequencies also show substantial agreement with experiment, reflecting at the same time significant changes in the physical character of the pairing itself.

Highly compressed molecular hydrogen near the melting line

We studied the radial and angular molecular distribution functions of dense solid hydrogen within non-empirical atom-atom potential (AAP) approximation. Lines of translational and orientational melting were located in the Monte Carlo computer simulation and compared with experiment. Significance of the non-central short-range part of intermolecular repulsion and of molecular non-rigidity in the description of translational and orientational phase transitions in dense condensed hydrogen was demonstrated.

Summary

Summary

Charge transfer and electron-vibron coupling in dense solid hydrogen

Abstract We examine the role of charge transfer (CT) interactions in dense molecular hydrogen in relation to recently observed spectroscopic properties at megabar pressures. Specifically, we consider virtual H 2 + H 2 − states in which an electron is transferred to a neighbor. The Mulliken CT integral t admixes H 2 + H 2 − fluctuations of overlapping molecules. The amplitude of the charge fluctuations is the ionicity γ, which is found in H 2 dimers and lattices with nonoverlapping valence and conduction bands for t small compared to the CT excitation energy ω CT . Vibronic coupling within a dimer is found using a Herzberg-Teller expansion and gives explicit expressions for vibron shifts in Raman and IR spectra and for IR oscillator strength due to electron-vibron coupling. We estimate linear electron-vibron coupling constants and other parameters required to interpret the vibrational data at and above the 150-GPa transition of dense hydrogen within a CT model. We discuss important structural implications of equal IR and Raman shifts at the transition and relate anisotropic CT processes to the orientational state of the dense molecular solid.

REFLECTANCE EFFECTS CAUSED BY REFRACTIVE INDEX GRADIENTS IN DIAMOND CELL SAMPLES OF H2 AND Al2O3

At high pressures, optical spectroscopic techniques are the principal means for probing dense solid hydrogen, including both its vibrational and electronic excitations. Ruoff et al. [Phys. Rev.B47, 11039 (1993)] proposed that refractive index gradients of H 2 samples loaded with 5% Al 2 O 3 powder in diamond anvil cells could cause a 4% rise in infrared reflectivity which interfered with the measurements on hydrogen reported by Mao et al. [Phys. Rev. Lett.65, 484 (1990)]. We have analyzed this alternative proposal, including computer simulations of the appropriate sample configuration. Our calculations show that effects of a graded index on the rise of reflectivity from 1.0 to 0.5 eV are less than 0.1% and are therefore insignificant.