Synchrotron infrared measurements of dense hydrogen to 360 GPa

A platform for planar dynamic compression of crystalline hydrogen toward the terapascal regime

We describe a method for laser-driven planar compression of crystalline hydrogen that starts with a sample of solid para-hydrogen (even-valued rotational quantum number j) having an entropy of 0.06 kB/molecule at 10 K and 2 atm, with Boltzmann constant kB. Starting with this low-entropy state, the sample is compressed using a small initial shock (<0.2 GPa), followed by a pressure ramp that approaches isentropic loading as the sample is taken to hundreds of GPa. Planar loading allows for quantitative compression measurements; the objective of our low-entropy compression is to keep the sample cold enough to characterize crystalline hydrogen toward the terapascal range.

Theoretical methods for structural phase transitions in elemental solids at extreme conditions: statics and dynamics

In recent years, theoretical studies have moved from a traditionally supporting role to a more proactive role in the research of phase transitions at high pressures. In many cases, theoretical prediction leads the experimental exploration. This is largely owing to the rapid progress of computer power and theoretical methods, particularly the structure prediction methods tailored for high-pressure applications. This review introduces commonly used structure searching techniques based on static and dynamic approaches, their applicability in studying phase transitions at high pressure, and new developments made toward predicting complex crystalline phases. Successful landmark studies for each method are discussed, with an emphasis on elemental solids and their behaviors under high pressure. The review concludes with a perspective on outstanding challenges and opportunities in the field.

Stress-induced high-Tc superconductivity in solid molecular hydrogen

Solid molecular hydrogen has been predicted to be metallic and high-temperature superconducting at ultrahigh hydrostatic pressures that push current experimental limits. Meanwhile, little is known about the influence of nonhydrostatic conditions on its electronic properties at extreme pressures where anisotropic stresses are inevitably present and may also be intentionally introduced. Here we show by first-principles calculations that solid molecular hydrogen compressed to multimegabar pressures can sustain large anisotropic compressive or shear stresses that, in turn, cause major crystal symmetry reduction and charge redistribution that accelerate bandgap closure and promote superconductivity relative to pure hydrostatic compression. Our findings highlight a hitherto largely unexplored mechanism for creating superconducting dense hydrogen, with implications for exploring similar phenomena in hydrogen-rich compounds and other molecular crystals.

Geometrical picture of the electron-electron correlation at the large-D limit

In electronic structure calculations, the correlation energy is defined as the difference between the mean field and the exact solution of the non relativistic Schrödinger equation. Such an error in the different calculations is not directly observable as there is no simple quantum mechanical operator, apart from correlation functions, that correspond to such quantity. Here, we use the dimensional scaling approach, in which the electrons are localized at the large-dimensional scaled space, to describe a geometric picture of the electronic correlation. Both, the mean field, and the exact solutions at the large-D limit have distinct geometries. Thus, the difference might be used to describe the correlation effect. Moreover, correlations can be also described and quantified by the entanglement between the electrons, which is a strong correlation without a classical analog. Entanglement is directly observable and it is one of the most striking properties of quantum mechanics and bounded by the area law for local gapped Hamiltonians of interacting many-body systems. This study opens up the possibility of presenting a geometrical picture of the electron-electron correlations and might give a bound on the correlation energy. The results at the large-D limit and at D = 3 indicate the feasibility of using the geometrical picture to get a bound on the electron-electron correlations.

High pressure hydrogen and the Potts model on a triangular lattice

We present Monte Carlo studies and analysis of the frustrated antiferromagnetic Potts model of a triangular lattice. This Potts model shows a remarkably rich range of structures, and striking similarities to the high pressure phases of hydrogen which are typified by hexagonal close packed layered structures [1]. There are four known H₂molecular phases, all of which are isostructural to within the resolution of x-ray diffraction. Experimentally, the phase lines have been mapped by spectroscopy, which cannot reveal the structure. Study by density functional theory (DFT) has suggested a large number of candidate structures, based on the hexagonal-close packing of H₂molecules. The Potts model exhibits structures similar to DFT candidate hydrogen phases I, II and III: the range of different Potts model structures suggests that the hydrogen system in the 'phase II' region, may exhibit more than a single phase. It also suggests reorientational excitations which may be detectable in spectroscopy.

Hydrogen Clathrate Structures in Uranium Hydrides at High Pressures

Room-temperature superconductivity has always been an area of intensive research. Recent findings of clathrate metal hydrides structures have opened up the doors for achieving room-temperature superconductivity in these materials. Here, we report first-principles calculations for stable H-rich clathrate structures of uranium hydrides at high pressures. The clathrate uranium hydrides contain H cages with stoichiometries of H24, H29, and H32, in which H atoms are bonded covalently to other H atoms, and U atoms occupy the centers of the cages. Especially, a UH10 clathrate structure containing H32 cages is predicted to have an estimated T c higher than 77 K at high pressures.

Probing the Electronic Band Gap of Solid Hydrogen by Inelastic X-Ray Scattering up to 90 GPa

Metallization of hydrogen as a key problem in modern physics is the pressure-induced evolution of the hydrogen electronic band from a wide-gap insulator to a closed gap metal. However, due to its remarkably high energy, the electronic band gap of insulating hydrogen has never before been directly observed under pressure. Using high-brilliance, high-energy synchrotron radiation, we developed an inelastic x-ray probe to yield the hydrogen electronic band information in situ under high pressures in a diamond-anvil cell. The dynamic structure factor of hydrogen was measured over a large energy range of 45 eV. The electronic band gap was found to decrease linearly from 10.9 to 6.57 eV, with an 8.6 times densification (ρ/ρ_{0}∼8.6) from zero pressure up to 90 GPa.

First-principles equation of state of liquid hydrogen and dissociative transition

The properties of dense hot hydrogen, in particular the phase transition between the molecular insulating and atomic conductive states, are important in the fields of astrophysics and high-pressure physics. Previous ab initio calculations suggested the metallization in liquid hydrogen, accompanied by dissociation, is a first-order phase transition and ends at a critical point in temperature range between 1500 and 2000 K and pressure close to 100 GPa. Using density functional theoretical molecular dynamics simulations, we report a first-principles equation of state of hydrogen that covers dissociation transition conditions at densities ranging from 0.20 to 1.00 g/cc and temperatures of 600-9000 K. Our results clearly indicate that a drop in pressure and a sharp structural change still occur as the system transforms from a diatomic to monoatomic phase at temperatures above 2000 K, and support the first-order phase transition in liquid hydrogen would end in the temperature about 4500 K.

New possible candidate structure for phase IV of solid hydrogen

It has been proved in experiments that there are at least five phases of solid hydrogen at high pressure, however, only the structure of phase I has been absolutely determined. We revisited the phase space of solid hydrogen in the pressure range of 200-500 GPa using the particle swarm optimization technique combined with first-principles simulations. A novel orthorhombic structure named Ama2 is proposed as a possible candidate structure for phase IV. The Ama2 structure is a 'mixed structure' with two different types of layers and is distinctly different from the previously reported Pc structure. Enthalpies and Gibbs free energies show that Ama2 and Pc are competitive in the pressure region of phase IV. Nevertheless, the Raman and infrared vibron frequencies of Ama2 calculated by using density functional perturbation theory based on first-principles lattice dynamics show a better agreement with the experimental measurements than those of the Pc structure. And the pressure dependence of these low-frequency Raman vibrons of Ama2 obtained from the first-principles molecular dynamics simulation shows a steeper slope, which resolves the long-standing issue of large discrepancies between the calculated Raman frequencies and the experimental ν ₁ [P. Loubeyre, F. Occelli and P. Dumas, Phys. Rev. B: Condens. Matter Mater. Phys., 2013, 87, 134101 and C. S. Zha, R. E. Cohen, H. K. Mao and R. J. Hemley, Proc. Natl. Acad. Sci. U.S.A., 2014, 111, 4792]. Structural and vibrational analyses show that the hydrogen molecules in the weakly bonded molecular layer of Ama2 form distorted hexagonal patterns, and their vibration can be used to explain the experimental ν ₁ vibron. It is found that the weakly bonded layer is almost the same as the layers in the C2/c structure. This confirms the experimental conclusion [P. Loubeyre, F. Occelli and P. Dumas, Phys. Rev. B: Condens. Matter Mater. Phys., 2013, 87, 134101] that the ordering of hydrogen molecules in the weakly bonded molecular layers of the 'mixed structure' for phase IV is similar to that in the layers of the C2/c structure.

Energy Gap Closure of Crystalline Molecular Hydrogen with Pressure

We study the gap closure with pressure of crystalline molecular hydrogen. The gaps are obtained from grand-canonical quantum Monte Carlo methods properly extended to quantum and thermal crystals, simulated by coupled electron ion Monte Carlo methods. Nuclear zero point effects cause a large reduction in the gap (∼2  eV). Depending on the structure, the fundamental indirect gap closes between 380 and 530 GPa for ideal crystals and 330-380 GPa for quantum crystals. Beyond this pressure the system enters into a bad metal phase where the density of states at the Fermi level increases with pressure up to ∼450-500  GPa when the direct gap closes. Our work partially supports the interpretation of recent experiments in high pressure hydrogen.

Distinguishing the Structure of High-Pressure Hydrogen with Dielectric Constants

Identifying the structures of high-pressure hydrogen has been one of the central goals in high-pressure physics; however, it still presents a fundamental challenge because of the lack of an effective measure for distinguishing the structures. Herein, we address this issue by focusing on the potential candidates of phases II and III of high-pressure hydrogen. We find that the anisotropic dielectric constants of the different hydrogen solids and their responses to pressure behave differently depending on the atomic structures, corresponding to the different polarization responses of the structures to the external electric field. These findings are robust regardless of the quantum and thermal motion of hydrogen solids. Therefore, the anisotropic dielectric property can serve as a potential measure for probing the structures of high-pressure hydrogen as well as other high-pressure materials.

High-pressure integrated synchrotron infrared spectroscopy system at the Shanghai Synchrotron Radiation Facility

We describe a new integrated optical spectroscopy facility for high-pressure research in materials research and mineral science located at the beamline BL01B of the Shanghai Synchrotron Radiation Facility. The system combines infrared synchrotron Fourier-Transform spectroscopy with broadband laser visible/near infrared and conventional laser Raman spectroscopy in one instrument. The system utilizes a custom-built microscope optics designed for a variety of diamond anvil cell experiments, which include low-temperature and ultrahigh pressure studies. We demonstrate the capabilities of the facility for studies of a variety of high-pressure phenomena such as phase and electronic transitions and chemical transformations.

Polyhydride CeH9 with an atomic-like hydrogen clathrate structure

Compression of hydrogen-rich hydrides has been proposed as an alternative way to attain the atomic metallic hydrogen state or high-temperature superconductors. However, it remains a challenge to get access to these states by synthesizing novel polyhydrides with unusually high hydrogen-to-metal ratios. Here we synthesize a series of cerium (Ce) polyhydrides by a direct reaction of Ce and H2 at high pressures. We discover that cerium polyhydride CeH9, formed above 100 GPa, presents a three-dimensional hydrogen network composed of clathrate H29 cages. The electron localization function together with band structure calculations elucidate the weak electron localization between H-H atoms and confirm its metallic character. By means of Ce atom doping, metallic hydrogen structure can be realized via the existence of CeH9. Particularly, Ce atoms play a positive role to stabilize the sublattice of hydrogen cages similar to the recently discovered near-room-temperature lanthanum hydride superconductors.

First-order liquid-liquid phase transition in compressed hydrogen and critical point

One of the fundamental problems relating to the properties of hydrogen is that of insulator-metal transition. Recent theoretical and experimental studies show that the metallization in liquid hydrogen could be a first-order phase transition and involve molecular to atomic transition. However, the location of the critical point is still an unresolved question. Earlier studies reported the critical point at a temperature of 1500-2000 K, but recent experimental observations on diamond-anvil cells show that the discontinuous transition still persists at temperatures well above 2000 K. We have carried out a detailed study on the liquid-liquid phase transition in dense hydrogen by uisng ab initio molecular dynamics simulations and found new evidence for the abrupt metallization between weakly dissociated and strongly dissociated fluid phases at temperatures as high as 3000 and 4000 K. Also, the predicted phase boundary is in excellent agreement with the recent experiments. Our results suggest that this first-order transition in liquid hydrogen likely ends in a critical point around 4000 K, which is significantly higher than the previous theoretical predictions.

Band gap closure, incommensurability and molecular dissociation of dense chlorine

Diatomic elemental solids are highly compressible due to the weak interactions between molecules. However, as the density increases the intra- and intermolecular distances become comparable, leading to a range of phenomena, such as structural transformation, molecular dissociation, amorphization, and metallisation. Here we report, following the crystallization of chlorine at 1.15(30) GPa into an ordered orthorhombic structure (oC8), the existence of a mixed-molecular structure (mC8, 130(10)-241(10) GPa) and the concomitant observation of a continuous band gap closure, indicative of a transformation into a metallic molecular form around 200(10) GPa. The onset of dissociation of chlorine is identified by the observation of the incommensurate structure (i-oF4) above 200(10) GPa, before finally adopting a monatomic form (oI2) above 256(10) GPa.

Metallic hydrogen

Hydrogen is the simplest and most abundant element in the Universe. There are two pathways for creating metallic hydrogen under high pressures. Over 80 years ago Wigner and Huntington predicted that if solid molecular hydrogen was sufficiently compressed in the T  =  0 K limit, molecules would dissociate to form atomic metallic hydrogen (MH). We have observed this transition at a pressure of 4.95 megabars. MH in this form has probably never existed on Earth or in the Universe; it may be a room temperature superconductor and is predicted to be metastable. If metastable it will have an important technological impact. Liquid metallic hydrogen can also be produced at intermediate pressures and high temperatures and is believed to make up ~90% of the planet Jupiter. We have observed this liquid-liquid transition, also known as the plasma phase transition, at pressures of ~1-2 megabar and temperatures ~1000-2000 K. However, in this paper we shall focus on the Wigner-Huntington transition. We shall discuss the methods used to observe metallic hydrogen at extreme conditions of static pressure in the laboratory, extending our understanding of the phase diagram of the simplest atom in the periodic table.

Strong Electron-Phonon and Band Structure Effects in the Optical Properties of High Pressure Metallic Hydrogen

The recent claim of having produced metallic hydrogen in the laboratory relies on measurements of optical spectra. Here, we present first-principles calculations of the reflectivity of hydrogen between 400 and 600 GPa in the I4_{1}/amd crystal structure, the one predicted at these pressures, based on both time-dependent density functional and Eliashberg theories, thus, covering the optical properties from the infrared to the ultraviolet regimes. Our results show that atomic hydrogen displays an interband plasmon at around 6 eV that abruptly suppresses the reflectivity, while the large superconducting gap energy yields a sharp decrease of the reflectivity in the infrared region approximately at 120 meV. The experimentally estimated electronic scattering rates in the 0.7-3 eV range are in agreement with our theoretical estimations, which show that the huge electron-phonon interaction of the system dominates the electronic scattering in this energy range. The remarkable features in the optical spectra predicted here encourage extending the optical measurements to the infrared and ultraviolet regions as our results suggest optical measurements can potentially identify high-pressure phases of hydrogen.

Strain-induced modulations of electronic structure and electron–phonon coupling in dense H3S

External stress is an effective tool to modulate the Fermi surface topology, logarithmic average frequency, and electron–phonon coupling parameter of dense H₃S and thus has a sensitive and considerable effect to the superconducting critical temperature.

A topological study of chemical bonds under pressure: solid hydrogen as a model case

It is now well recognized that a fundamental understanding of the rules that govern chemistry under pressure is still lacking. Hydrogen being the "simplest" element as well as a central core to high pressure physics, we undertake a general study of the changes in the chemical bonding under pressure. We start from a simple trimer unit that has been found in high pressure phases, whose behavior has been found to reveal the basics of hydrogen polymerization under pressure. Making use of bond analysis tools, mainly the NCI (noncovalent interactions) index, we show that polymerization takes place in three steps: dipolar attraction, repulsion and bond formation. The use of a 1D Wigner-Seitz radius allowed us to extend the conclusions to 3D networks and to analyze their degree of polymerization. On the one hand, this approach provides new insight into the polymerization of hydrogen. On the other hand, it shows that complicated molecular solids can be understood from cluster models, where correlated methods can be applied, main differences in solid state arising at the transition points, where breaking/forming of bonds happens at once instead of continuously like in the cluster model.

High-Pressure Behavior of Hydrogen and Deuterium at Low Temperatures

In situ high-pressure low-temperature high-quality Raman data for hydrogen and deuterium demonstrate the presence of a novel phase, phase II^{'}, unique to deuterium and distinct from the known phase II. Phase II^{'} of D_{2} is not observed in hydrogen, making it the only phase that does not exist in both isotopes and occupies a significant part of P-T space from ∼25 to 110 GPa and below 125 K. For H_{2}, the data show that below 30 K the transition to phase II happens at as low as 73 GPa. The transformation from phase II to III commences at around ∼155  GPa and is completed by 170 GPa with the average pressure of ∼160  GPa being slightly higher than previously thought. The updated phase diagrams of H_{2} and D_{2} demonstrate the difference between the isotopes at low temperatures and moderate pressures, providing new information on the phase diagrams of both elements.

Emergence of superconductivity in doped H2O ice at high pressure

We investigate the possibility of achieving high-temperature superconductivity in hydrides under pressure by inducing metallization of otherwise insulating phases through doping, a path previously used to render standard semiconductors superconducting at ambient pressure. Following this idea, we study H2O, one of the most abundant and well-studied substances, we identify nitrogen as the most likely and promising substitution/dopant. We show that for realistic levels of doping of a few percent, the phase X of ice becomes superconducting with a critical temperature of about 60 K at 150 GPa. In view of the vast number of hydrides that are strongly covalent bonded, but that remain insulating up to rather large pressures, our results open a series of new possibilities in the quest for novel high-temperature superconductors.

Infrared Peak Splitting from Phonon Localization in Solid Hydrogen

We show that the isotope effect leads to a completely different spectroscopic signal in hydrogen-deuterium mixtures, compared to pure elements that have the same crystal structure. This is particularly true for molecular vibrations, which are the main source of information about the structure of high-pressure hydrogen. Mass disorder breaks translational symmetry, meaning that vibrations are localized almost to single molecules, and are not zone-center phonons. In mixtures, each observable infrared (IR) peak corresponds to a collection of many such molecular vibrations, which have a distribution of frequencies depending on local environment. Furthermore discrete groups of environments cause the peaks to split. We illustrate this issue by considering the IR spectrum of the high-pressure phase III structure of hydrogen, recently interpreted as showing novel phases in isotopic mixtures. We calculate the IR spectrum of hydrogen-deuterium mixtures in the C2/c and Cmca-12 structures, showing that isotopic disorder gives rise to mode localization of the high-frequency vibrons. The local coordination of the molecules leads to discrete IR peaks. The spread of frequencies is strongly enhanced with pressure, such that more peaks become resolvable at higher pressures, in agreement with the recent measurements.

The role of van der Waals and exchange interactions in high-pressure solid hydrogen

Our study of the van der Waals interactions in solid molecular hydrogen structures indicates two candidates for phase III.

Structural, vibrational, and electronic properties of BaReH9 under pressure

We present a study of the high-pressure behavior of BaReH₉, a novel hydrogen-rich compound, using optical, Raman, and infrared spectroscopy as well as synchrotron x-ray diffraction. The x-ray diffraction measurements demonstrate that BaReH₉ retains its hexagonal structure on room temperature compression up to 40 GPa. Optical absorption shows the absence of a gap closure to 80 GPa. Raman and IR spectra reveal the pressure evolution of a newly observed phonon peak, and large peak broadening with increasing pressure. These data constrain the disorder present in the material following the P-T paths explored.

Anharmonic enhancement of superconductivity in metallic molecular Cmca - 4 hydrogen at high pressure: a first-principles study

First-principles calculations based on density-functional theory including anharmonicity within the variational stochastic self-consistent harmonic approximation are applied to understand how the quantum character of the proton affects the candidate metallic molecular Cmca  -  4 structure of hydrogen in the 400-450 GPa pressure range, where metallization of hydrogen is expected to occur. Anharmonic effects, which become crucial due to the zero-point motion, have a large impact on the hydrogen molecules by increasing the intramolecular distance by approximately a 6%. This induces two new electron pockets at the Fermi surface opening new scattering channels for the electron-phonon interaction. Consequently, the electron-phonon coupling constant and the superconducting critical temperature are approximately doubled by anharmonicity and Cmca  -  4 hydrogen becomes a superconductor above 200 K in all the studied pressure range. Contrary to many superconducting hydrides, where anharmoncity tends to lower the superconducting critical temperature, our results show that it can enhance superconductivity in molecular hydrogen.

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.

Pressure-induced exotic states in rare earth hexaborides

Finding the exotic phenomena in strongly correlated electron systems (SCESs) and understanding the corresponding microphysics have long been the research frontiers of condensed matter physics. The remarkable examples for the intriguing phenomena discovered in past years include unconventional superconductivity, heavy Fermion behaviors, giant magneto-resistance and so on. A fascinating type of rare earth hexaboride RB6 (R  =  Sm, Yb, Eu and Ce) belongs to a strongly correlated electron system (SCES), but shows unusual ambient-pressure and high-pressure behaviors beyond the phenomena mentioned above. Particularly, the recent discovery of the coexistence of an unusual metallic surface state and an insulating bulk state in SmB6, known to be a Kondo insulator decades ago, by theoretical calculations and many experimental measurements creates new interest for the investigation of the RB6. This significant progress encourages people to revisit the RB6 with an attempt to establish a new physics that links the SCES and the unusual metallic surface state which is a common feature of a topological insulator (TI). It is well known that pressure has the capability of tuning the electronic structure and modifying the ground state of solids, or even inducing a quantum phase transition which is one of the kernel issues in studies of SCESs. In this brief review, we will describe the progress in high pressure studies on the RB6 based on our knowledge and research interests, mainly focusing on the pressure-induced phenomena in YbB6 and SmB6, especially on the quantum phase transitions and their connections with the valence state of the rare earth ions. Moreover, some related high-pressure results obtained from CeB6 and EuB6 are also included. Finally, a summary is given in the conclusions and perspectives section.

Optical Properties of Fluid Hydrogen at the Transition to a Conducting State

We use fast transient transmission and emission spectroscopies in the pulse laser heated diamond anvil cell to probe the energy-dependent optical properties of hydrogen at pressures of 10-150 GPa and temperatures up to 6000 K. Hydrogen is absorptive at visible to near-infrared wavelengths above a threshold temperature that decreases from 3000 K at 18 GPa to 1700 K at 110 GPa. Transmission spectra at 2400 K and 141 GPa indicate that the absorptive hydrogen is semiconducting or semimetallic in character, definitively ruling out a first-order insulator-metal transition in the studied pressure range.

Liquid-liquid phase transition in hydrogen by coupled electron-ion Monte Carlo simulations

The phase diagram of high-pressure hydrogen is of great interest for fundamental research, planetary physics, and energy applications. A first-order phase transition in the fluid phase between a molecular insulating fluid and a monoatomic metallic fluid has been predicted. The existence and precise location of the transition line is relevant for planetary models. Recent experiments reported contrasting results about the location of the transition. Theoretical results based on density functional theory are also very scattered. We report highly accurate coupled electron-ion Monte Carlo calculations of this transition, finding results that lie between the two experimental predictions, close to that measured in diamond anvil cell experiments but at 25-30 GPa higher pressure. The transition along an isotherm is signaled by a discontinuity in the specific volume, a sudden dissociation of the molecules, a jump in electrical conductivity, and loss of electron localization.

X-ray scattering measurements of dissociation-induced metallization of dynamically compressed deuterium

Hydrogen, the simplest element in the universe, has a surprisingly complex phase diagram. Because of applications to planetary science, inertial confinement fusion and fundamental physics, its high-pressure properties have been the subject of intense study over the past two decades. While sophisticated static experiments have probed hydrogen's structure at ever higher pressures, studies examining the higher-temperature regime using dynamic compression have mostly been limited to optical measurement techniques. Here we present spectrally resolved x-ray scattering measurements from plasmons in dynamically compressed deuterium. Combined with Compton scattering, and velocity interferometry to determine shock pressure and mass density, this allows us to extract ionization state as a function of compression. The onset of ionization occurs close in pressure to where density functional theory-molecular dynamics (DFT-MD) simulations show molecular dissociation, suggesting hydrogen transitions from a molecular and insulating fluid to a conducting state without passing through an intermediate atomic phase.

New Phases and Dissociation-Recombination of Hydrogen Deuteride to 3.4 Mbar

We present infrared absorption studies of solid hydrogen deuteride to pressures as high as 340 GPa (100  GPa=1  Mbar) in a diamond anvil cell and temperatures in the range 5-295 K. Above 198 GPa the HD sample transforms to a mixture of HD, H_{2}, and D_{2}, interpreted as a process of dissociation and recombination. Three new phase lines are observed, two of which differ remarkably from those of the high-pressure homonuclear species, but none are metallic. The time-dependent spectral changes are analyzed to determine the molecular concentrations as a function of time; the nucleon exchange achieves steady state concentrations in ∼20  h at ∼200  GPa.

Evidence for a new phase of dense hydrogen above 325 gigapascals

Almost 80 years ago it was predicted that, under sufficient compression, the H-H bond in molecular hydrogen (H2) would break, forming a new, atomic, metallic, solid state of hydrogen. Reaching this predicted state experimentally has been one of the principal goals in high-pressure research for the past 30 years. Here, using in situ high-pressure Raman spectroscopy, we present evidence that at pressures greater than 325 gigapascals at 300 kelvin, H2 and hydrogen deuteride (HD) transform to a new phase--phase V. This new phase of hydrogen is characterized by substantial weakening of the vibrational Raman activity, a change in pressure dependence of the fundamental vibrational frequency and partial loss of the low-frequency excitations. We map out the domain in pressure-temperature space of the suggested phase V in H2 and HD up to 388 gigapascals at 300 kelvin, and up to 465 kelvin at 350 gigapascals; we do not observe phase V in deuterium (D2). However, we show that the transformation to phase IV' in D2 occurs above 310 gigapascals and 300 kelvin. These values represent the largest known isotropic shift in pressure, and hence the largest possible pressure difference between the H2 and D2 phases, which implies that the appearance of phase V of D2 must occur at a pressure of above 380 gigapascals. These experimental data provide a glimpse of the physical properties of dense hydrogen above 325 gigapascals and constrain the pressure and temperature conditions at which the new phase exists. We speculate that phase V may be the precursor to the non-molecular (atomic and metallic) state of hydrogen that was predicted 80 years ago.

Chemical bonding in hydrogen and lithium under pressure

Though hydrogen and lithium have been assigned a common column of the periodic table, their crystalline states under common conditions are drastically different: the former at temperatures where it is crystalline is a molecular insulator, whereas the latter is a metal that takes on simple structures. On compression, however, the two come to share some structural and other similarities associated with the insulator-to-metal and metal-to-insulator transitions, respectively. To gain a deeper understanding of differences and parallels in the behaviors of compressed hydrogen and lithium, we performed an ab initio comparative study of these systems in selected identical structures. Both elements undergo a continuous pressure-induced s-p electronic transition, though this is at a much earlier stage of development for H. The valence charge density accumulates in interstitial regions in Li but not in H in structures examined over the same range of compression. Moreover, the valence charge density distributions or electron localization functions for the same arrangement of atoms mirror each other as one proceeds from one element to the other. Application of the virial theorem shows that the kinetic and potential energies jump across the first-order phase transitions in H and Li are opposite in sign because of non-local effects in the Li pseudopotential. Finally, the common tendency of compressed H and Li to adopt three-fold coordinated structures as found is explained by the fact that such structures are capable of yielding a profound pseudogap in the electronic densities of states at the Fermi level, thereby reducing the kinetic energy. These results have implications for the phase diagrams of these elements and also for the search for new structures with novel properties.

Quantum Monte Carlo study of the phase diagram of solid molecular hydrogen at extreme pressures

Establishing the phase diagram of hydrogen is a major challenge for experimental and theoretical physics. Experiment alone cannot establish the atomic structure of solid hydrogen at high pressure, because hydrogen scatters X-rays only weakly. Instead, our understanding of the atomic structure is largely based on density functional theory (DFT). By comparing Raman spectra for low-energy structures found in DFT searches with experimental spectra, candidate atomic structures have been identified for each experimentally observed phase. Unfortunately, DFT predicts a metallic structure to be energetically favoured at a broad range of pressures up to 400 GPa, where it is known experimentally that hydrogen is non-metallic. Here we show that more advanced theoretical methods (diffusion quantum Monte Carlo calculations) find the metallic structure to be uncompetitive, and predict a phase diagram in reasonable agreement with experiment. This greatly strengthens the claim that the candidate atomic structures accurately model the experimentally observed phases.