Identifying the atomic structure and properties of solid hydrogen under high pressures is a long-standing problem of high-pressure physics with far-reaching significance in planetary and materials science. Determining the pressure-temperature phase diagram of hydrogen is challenging for experiment and theory due to the extreme conditions and the required accuracy in the quantum mechanical treatment of the constituent electrons and nuclei, respectively. Here, we demonstrate explicitly that coupled cluster theory can serve as a computationally efficient theoretical tool to predict solid hydrogen phases with high accuracy. We present a first principles study of solid hydrogen phases at pressures ranging from 100 to 450 GPa. The computed static lattice enthalpies are compared to state-of-the-art diffusion Monte Carlo results and density functional theory calculations. Our coupled cluster theory results for the most stable phases including C2/c-24 and P21/c-24 are in good agreement with those obtained using diffusion Monte Carlo, with the exception of Cmca-4, which is predicted to be significantly less stable. We discuss the scope of the employed methods and how they can contribute as efficient and complementary theoretical tools to solve the long-standing puzzle of understanding solid hydrogen phases at high pressures.
It was predicted that solid metallic hydrogen can be obtained if solid molecular hydrogen is pressured to high pressure at low temperature about 80 years ago. Furthermore, the solid metallic hydrogen was theoretically predicted to show superconductivity at room temperature. In addition, surprising prediction was made that the solid metallic hydrogen is a metastable metal with a potential barrier of 1 eV. This prediction implies that the solid metallic hydrogen remains in the metallic solid phase state even after it is released from high pressure. Shock compression synthesized liquid metallic h…
The primary purpose of this paper is to stimulate theoretical predictions of how to retain metastably hydrogenous materials made at high pressure P on release to ambient. Ultracondensed metallic hydrogen has been made at 140 GPa at finite temperatures T in the fluid. The term metallic here means quantum mechanically degenerate. A single sample of ultracondensed hydrogen has been made at an estimated pressure of 495 GPa at 5.5 K. Whether that sample is solid or fluid remains to be determined. Those results imply the long quest for metallic hydrogen is likely to be concluded in the relatively ne…
Carbons are being widely investigated as hydrogen storage material owing to their light weight, fast hydrogen adsorption kinetics and cost effectiveness. However, these materials suffer from low hydrogen storage capacity, particularly at room temperature. The aim of the present study is to develop carbon-based material from natural bio-precursor which shows at least moderate hydrogen storage at room temperature. For this purpose, hydrogenation characteristics of carbon derived from solid endosperm of coconut is studied. Solid endosperm and its carbonized version have a native grown distributio…
Studying crystal structures of superprotonic phases of alkali metal hydrogen sulfates and selenates, a very unusual phenomenon has been revealed. Dynamically disordered hydrogen atoms with low position occupancies are clearly seen in corresponding electron density maps. To explain this effect, an idea about a new type of twinning, dynamic twinning, was proposed and discussed.
We report Raman scattering and visible to near-infrared absorption spectra of solid hydrogen under static pressure up to 285 GPa at 85-140 K. We obtain pressure dependences of vibron and phonon modes in agreement with 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.
We present a comparative study of both the magnetic and magnetotransport properties for two members of the perovskite solid solution LaNixCo1-xO3 (x=0.2, 0.6) located on opposite sides of the chemically induced metal-to-insulator transition. LaNi0.6Co0.4O3 exhibits metallic behavior and small but negative magnetoresistance, whereas LaNi0.2Co0.8O3 exhibits semiconducting behavior and giant negative magnetoresistance at low temperatures. On the other hand, we observe pronounced similarities in the magnetic properties of both compounds. A consistent explanation regarding the origin of the magneto…
The high-pressure phases of solid hydrogen are of fundamental interest and relevant to the interior of giant planets; however, knowledge of these phases is far from complete. Particle swarm optimization (PSO) techniques were applied to a structural search, yielding hitherto unexpected high-pressure phases of solid hydrogen at pressures up to 5 TPa. An exotic quasi-molecular mC24 structure (space group C2/c, stable at 0.47-0.59 TPa) with two types of intramolecular bonds was predicted, providing a deeper understanding of molecular dissociation in solid hydrogen, which has been a mystery for dec…
An ideal material for on-board hydrogen storage must release hydrogen at practical temperature and pressure and also regenerate efficiently under similarly gentle conditions. Therefore, thermodynamically, the hydride material must lie within a narrow range near the hydrogenation/dehydrogenation phase boundary. Materials involving only conventional bonding mechanisms are unlikely to meet these requirements. In contrast, materials containing certain frustrated bonding are designed to be on the verge of frustration-induced phase transition, and they may be better suited for hydrogen storage. Here…
It has recently been established that two-dimensional massless graphene-like systems and three-dimensional line-node topological semimetals comprise a special class of centrosymmetric materials where edge/surface states of topological nature inevitably appear in some k-regions. We show that the phases of solid hydrogen produced at megabar pressures can be line-node topological semimetals. In such phases, the material exhibits topological surface states and therefore can be a poor bulk metal but good surface conductor. The results may help to explain discrepant high-pressure experimental data r…
The recent discovery of phase IV of solid hydrogen and deuterium consisting of two alternate layers of graphenelike three-molecule rings and unbound H2 molecules have generated great interests. However, vibrational nature of phase IV remains poorly understood. Here, we report a peculiar proton transfer and a simultaneous rotation of three molecule rings in graphenelike layers predicted by ab initio variable cell molecular dynamics simulations for phase IV of solid hydrogen and deuterium at pressure ranges of from 250 to 350 GPa and temperature range of from 300 to 500 K. This proton transfer i…
The experimental data on the thermal conductivity of ortho-parahydrogen solutions are analyzed on the basis of a relaxation-time model taking account of configurational relaxation of the ortho subsystem. The influence of configurational relaxation on the thermal conductivity is analyzed using resonance scattering of phonons by pair clusters of orthomolecules taking account of their rotational spectrum.
Metal hydrides are critical materials in numerous technologies including hydrogen storage, gas separation, and electrocatalysis. Here, using Pd-H as a model metal hydride, we perform electrochemical insertion studies of hydrogen via liquid and solid state electrolytes at 1 atm ambient pressure, and achieve H:Pd ratios near unity, the theoretical solubility limit. We show that the compositions achieved result from a dynamic balance between the rate of hydrogen insertion and evolution from the Pd lattice, the combined kinetics of which are sufficiently rapid that operando experiments are necessa…
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.
We examine the influence of the main approximations employed in density-functional theory descriptions of the solid phase of molecular hydrogen near dissociation. We consider the importance of nuclear quantum effects on equilibrium properties and find that they strongly influence intramolecular properties, such as bond fluctuations and stability. We demonstrate that the combination of both thermal and quantum effects make a drastic change to the predicted optical properties of the molecular solid, suggesting a limited value to dynamical, e.g., finite-temperature, predictions based on classical…
Systematic first-principles molecular dynamics (MD) simulations with long simulation times (7-13 ps) for phase IV of solid hydrogen using different supercell sizes of 96, 288, 576, and 768 atoms established that the diffusive proton motions process in the graphene-like layer is an intrinsic property and independent of the simulation cell sizes. The present study highlights an often overlook issue in first-principles calculations that long time MD is essential to achieve ergodicity, which is mandatory for a proper description of dynamics of a system. The present results contradict a recent work…
By performing constant-pressure variable-cell ab initio molecular dynamics simulations we find a quadrupolar orthorhombic structure, of $Pca2_1$ symmetry, for the broken symmetry phase (phase II) of solid H2 at T=0 and P =110 - 150 GPa. We present results for the equation of state, lattice parameters and vibronic frequencies, in very good agreement with experimental observations. Anharmonic quantum corrections to the vibrational frequencies are estimated using available data on H2 and D2. We assign the observed modes to specific symmetry representations.
The integrity of irradiated zirconium based nuclear fuel cladding is related to the precipitation of hydrides which is closely connected with the solubility of hydrogen. A review on the development of measurement technologies is given and the resulting terminal solid solubilities are reflected with respect to the established model of hydrogen solubility in zirconium alloys. The results often allow for a different interpretation than the established model. An alternative qualitative approach is proposed in which the fixed TSSp for the precipitation of zirconium hydrides is replaced by a kinetic…
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 energeti…
Hydrogen is the most abundant element in the universe, and its properties under conditions of high temperature and pressure are crucial to understand the interior of of large gaseous planets and other astrophysical bodies. At ultra high pressures solid hydrogen has been predicted to transform into a quantum fluid, because of its high zero point motion. Here we report first principles two phase coexistence and Z method determinations of the melting line of solid hydrogen in a pressure range spanning from 30 to 600 GPa. Our results suggest that the melting line of solid hydrogen, as derived from…
A Comparative Study of Molecular Dynamics Approaches for Simulating Ionic Conductivity in Solid Lithium Electrolytes
Université de Montréal（蒙特利尔大学）；Mila – Quebec AI Institute（米拉-魁北克人工智能研究所）；National Research Council Canada（加拿大国家研究委员会）；Institut Courtois, Université de Montréal（蒙特利尔大学库尔图瓦研究所）
Hydrogen embrittlement in 304L (18wt.% Cr, 8-10wt.% Ni) austenitic stainless steel (ASS) fabricated by laser powder-bed-fusion (LPBF) was investigated by tensile testing after electrochemical hydrogen pre-charging and compared to conventionally available 304L ASSs with two different processing histories, (i) casting plus annealing (CA) and (ii) CA plus thermomechanical treatment (TMT). It was revealed that hydrogen-charging led to a significant reduction in ductility for the CA sample, but only a small effect of hydrogen was observed for the LPBF and CA-TMT samples. Hydrogen-assisted cracking …
By employing first-principles metadynamics simulations, we explore the 300 K structures of solid hydrogen over the pressure range 150-300 GPa. At 200 GPa, we find the ambient-pressure disordered hexagonal close-packed (hcp) phase transited into an insulating partially ordered hcp phase (po-hcp), a mixture of ordered graphene-like H2 layers and the other layers of weakly coupled, disordered H2 molecules. Within this phase, hydrogen remains in paired states with creation of shorter intra-molecular bonds, which are responsible for the very high experimental Raman peak above 4000 cm-1. At 275 GPa,…
A theoretical study is reported of the molecular-to-atomic transition in solid hydrogen at high pressure. We use the diffusion quantum Monte Carlo method to calculate the static lattice energies of the competing phases and a density-functional-theory-based vibrational self-consistent field method to calculate anharmonic vibrational properties. We find a small but significant contribution to the vibrational energy from anharmonicity. A transition from the molecular Cmca-12 direct to the atomic I4_1/amd phase is found at 374 GPa. The vibrational contribution lowers the transition pressure by 91 …
Recently, Tao and Mo (TM) proposed an accurate all-purpose nonempirical meta-generalized gradient approximation (meta-GGA). The exchange part was derived from the density matrix approximation, while the correlation part is based on a modification of TPSS correlation in the low-density or strong-interaction limit. To further understand this density functional, we combine the TM exchange part with the original TPSS correlation and make a comprehensive assessment of this combination, which we call TMTPSS functional, on solids and solid surfaces. Our test includes 22 lattice constants and bulk mod…
We have studied solid hydrogen up to pressures of 300 GPa and temperatures of 350 K using density functional theory methods and have found "mixed structures" that are more stable than those predicted earlier. Mixed structures consist of alternate layers of strongly bonded molecules and weakly bonded graphene-like sheets. Quasi-harmonic vibrational calculations show that mixed structures are the most stable at room temperature over the pressure range 250-295 GPa. These structures are stabilized with respect to strongly-bonded molecular phases at room temperature by the presence of lower-frequen…
Balancing Thermodynamics, Kinetics, and Reversibility in Ti-Doped MgB2H8: A First-Principles Assessment of a Practical Solid-State Hydrogen Storage Material
New Technologies Research Center, University of West Bohemia（新技研究中心，西波希米亚大学）；Department of Physics, Bacha Khan University（巴哈汗大学物理系）
A hexagonal structure of solid molecular hydrogen with $P6_122$ symmetry is calculated to be more stable below about 200 GPa than the monoclinic $C2/c$ structure identified previously as the best candidate for phase III. We find that the effects of nuclear quantum and thermal vibrations play a central role in the stabilization of $P6_122$. The $P6_122$ and $C2/c$ structures are very similar and their Raman and infra-red data are in good agreement with experiment. However, our calculations show that the hexagonal $P6_122$ structure provides better agreement with the available x-ray diffraction …
We present an attempt to build up a new two-body effective potential for hydrogen fluoride, fitted to theoretical and experimental data relevant not only to the gas and liquid phases, but also to the crystal. The model is simple enough to be used in Molecular Dynamics and Monte Carlo simulations. The potential consists of: a) an intra-molecular contribution, allowing for variations of the molecular length, plus b) an inter-molecular part, with three charged sites on each monomer and a Buckingham "exp-6" interaction between fluorines. The model is able to reproduce a significant number of obser…
We present results predicting experimentally measurable structural quantities from molecular dynamics studies of hydrogen. In doing this, we propose a paradigm shift for experimentalists -- that the predictions from such calculations should be seen as the most likely hypotheses. Specifically, the experimental results should be aiming to distinguish between the candidate low-energy structures, rather than aiming to solve the simplest structure consistent with the data. We show that the room temperature X-ray diffraction patterns for hydrogen phases I, III, IV and V are very similar, with only s…
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 IR peak corresponds to a collection of many such molecular vibrations, which have a distribution of freque…
We have studied solid hydrogen under pressure at low temperatures. With increasing pressure we observe changes in the sample, going from transparent, to black, to a reflective metal, the latter studied at a pressure of 495 GPa. We have measured the reflectance as a function of wavelength in the visible spectrum finding values as high as 0.90 from the metallic hydrogen. We have fit the reflectance using a Drude free electron model to determine the plasma frequency of 30.1 eV at T= 5.5 K, with a corresponding electron carrier density of 6.7x1023 particles/cm3, consistent with theoretical estimat…
When metal surfaces are exposed to hydrogen ion irradiation, the light ions are expected to penetrate deep into the material and dissolve in the matrix. However, these atoms are seen to cause significant modification of surfaces, indicating that they accumulate in vicinity of the surface. The process known as blistering may reduces the vacuum dielectric strength above the metal surface, which shows a dense population of surface blisters. In this paper, we investigate how a bubble can grow under the pressure exerted by hydrogen atoms on the walls of the bubble and how this affect to the surface…
Hydrogen has been the essential element in the development of atomic and molecular physics1). Moving to the properties of dense hydrogen has appeared a good deal more complex than originally thought by Wigner and Hungtinton in their seminal paper predicting metal hydrogen2): the electrons and the protons are strongly coupled to each other and ultimately must be treated equally3)4). The determination of how and when molecular solid hydrogen will transform into a metal is the stepping stone towards a full understanding of the quantum-many body properties of dense hydrogen. The quest for metal hy…
Four basic processes are envisioned, among them migration (diffusion), local rotation (reorientation), isothermic chemical reactions and nucleation. All of them are unified by a common approach to the barrier currents that has been suggested as far back as 1961 by John Bardeen and then extended by Stefan Christov some ten years later. By introducing the respective radial potentials, we incorporate Schroedinger's equation and thereby a quantum insight into the phenomena. Numerical calculations of the obtained statistical transition rates are reported.
This paper investigates some of the successes and failures of density functional theory in the study of high-pressure solid hydrogen at low temperature. We calculate the phase diagram, metallization pressure, phonon spectrum, and proton zero-point energy using three popular exchange-correlation functionals: the local density approximation (LDA), the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation, and the semi-local Becke-Lee-Yang-Parr (BLYP) functional. We focus on the solid molecular P$6_3$/m, C2/c, Cmca-12, and Cmca structures in the pressure range from $100<P<500$ GPa over …
Through a series of Raman spectroscopy studies, we investigate the behaviour of hydrogen-helium and hydrogen-nitrogen mixtures at high pressure across wide ranging concentrations. We find that there is no evidence of chemical association, miscibility, nor any demixing of hydrogen and helium in the solid state up to pressures of 250 GPa at 300 K. In contrast, we observe the formation of concentration-dependent N$_2$-H$_2$ van der Waals solids, which react to form N-H bonded compounds above 50 GPa. Through this combined study, we can demonstrate that the recently claimed chemical association of …
Hydrogen-induced pitting corrosion of metallic is a common phenomenon that damages the integrity and durability of the materials. Its numerical simulation is still a challenge due to many complex mechanisms, especially solid-solid phase transformation and mechanical interaction, leading to the anisotropic growth of hydride and inducing some bulges on the metal surface. In our work, we propose a phase-field model and numerical technique for simulation of hydrogen-induced pitting corrosion, and apply it to the system of $α$-Uranium. In our model, the elastic strain energy is introduced to approx…
Generally hydrogen will adopt the +1, 0 or -1 oxidation state in solids depending upon the chemical environment it occupy. Typically, there are some exceptional cases in which hydrogen exhibits both anionic and cationic behavior in the same structural frame works. In this study we briefly explore an amphoteric behavior of hydrogen present in ammine bimetallic borohydrides with the chemical formula M1M2 (BH4)3 (NH3)2 (M1= Na; M2 = Zn, Mg) using the state-of-the-art density functional calculations. In order to establish the amphoteric behavior of hydrogen in M1M2 (BH4)3 (NH3)2, we have made deta…
We study the effect of temperature up to 1000K on the structure of dense molecular para-hydrogen and ortho-deuterium, using the path-integral Monte Carlo method. We find a structural phase transition from orientationally disordered hexagonal close packed (hcp) to an orthorhombic structure of Cmca symmetry before melting. The transition is basically induced by thermal fluctuations, but quantum fluctuations of protons (deuterons) are important in determining the transition temperature through effectively hardening the intermolecular interaction. We estimate the phase line between hcp and Cmca ph…
We use the diffusion quantum Monte Carlo (DMC) method to calculate the ground state phase diagram of solid molecular hydrogen and examine the stability of the most important insulating phases relative to metallic crystalline molecular hydrogen. We develop a new method to account for finite-size errors by combining the use of twist-averaged boundary conditions with corrections obtained using the Kwee-Zhang-Krakauer (KZK) functional in density functional theory. To study band-gap closure and find the metallization pressure, we perform accurate quasi-particle many-body calculations using the $GW$…
We present results of lattice dynamics calculations of the $P-V$ equation of state and the pressure dependence of the Raman-active $E_{2g}$ lattice phonon for $p-$H$_2$ and $o-$D$_2$ in a wide pressure range up to $\sim$2 Mbar using our recently developed semi-empirical many-body potential, and density-functional theory. Comparison with existing body of experimental and theoretical results showed that the employed many-body potential is a reliable basis for high-precision calculations for phases I, II, and III of solid hydrogens.
Ionic liquids (ILs) gain much interest as possible electrolytes in the next generation of mixed-solid Li-ion batteries. However, such properties of ionic liquids as melting transition, diffusion, strength and structure of hydrogen bond network remain poorly investigated. Here 2H NMR study of a prototypical ionic liquid [TEA][NTf2] has been performed. We show that the dynamical melting occurs through a dynamically heterogeneous phase stable between 223-277 K and the transition process is characterized by two standard molar enthalpy changes indicating to the multistage nature of the melting. The…
We compare dewetting characteristics of a thin nonwetting solid film in the absence of stress, for two models of a wetting potential: the exponential and the algebraic. The exponential model is a one-parameter (r) model, and the algebraic model is a two-parameter (r,m) model, where r is the ratio of the characteristic wetting length to the height of the unperturbed film, and m is the exponent of h (film height) in a smooth function that interpolates the system's surface energy above and below the film-substrate interface at z=0. The exponential model gives monotonically decreasing (with h) wet…
Using ab initio thermodynamics, the stability of a wide range of hydrocarbon adsorbates under various chemical vapor deposition (CVD) conditions (temperature, methane and hydrogen pressures) used in experimental graphene growth protocols at solid and liquid Cu surfaces has been explored. At the employed high growth temperatures around the melting point of Cu, we find that commonly used thermodynamic models such as the harmonic oscillator model may no longer be accurate. Instead, we account for the translational and rotational mobility of adsorbates using a recently developed hindered translato…
Recent experimental developments in hydrogen-rich materials in high pressures have put this class of materials above others in the race toward room temperature superconductivity. As it is the basis of all the materials in this class, the efforts to determine the properties of pure solid hydrogen at high pressures remain intense. Most notably, a recent experimental study of the metallization of hydrogen identified the crystal phase of the solid as the C2/c-24 molecular phase up to ~425 GPa. It is possible that the observed metallization is caused by band structure effects and not a structural p…
The many experimental puzzles of solid hydrogen, that seem to defy the generally accepted view of condensed matter, are shown to be a rather straightforward consequence of a new approach to condensed matter physics, that takes into account the coherent electrodynamic processes, predicted by Quantum ElectroDynamics (QED) that have been so far overlooked. As a consequence, a revision of the present view, based on electrostatic forces only, is called for.
Gas-solid reactions are cornerstones of many catalytic and redox processes that will underpin the energy and sustainability transition. The specific case of hydrogen-based iron oxide reduction is the foundation to render the global steel industry fossil-free, an essential target as iron production is the largest single industrial emitter of carbon dioxide. Our perception of gas-solid reactions has not only been limited by the availability of state-of-the-art techniques which can delve into the reacted solids in great structural and chemical detail, but we continue to miss an important reaction…
Electronic structure calculations for compressed molecular hydrogen are performed to provide more insight into the diversity of phenomena recently observed experimentally. We perform full-potential LAPW calculations and analyze them in terms of a molecular tight-binding model. We show that $σ_g$ and $σ_u$ bands overlap occurs at rather low pressure, while an insulating state persists at specific orientations up to 200 GPa, in accord with previous work, and is due to opening of hybridization gaps at the Fermi level. We also present calculations of electronic properties such as plasma frequencie…
We present an accurate study of the static-nucleus electronic energy band gap of solid molecular hydrogen at high pressure. The excitonic and quasiparticle gaps of the $C2/c$, $Pc$, $Pbcn$, and $P6_3/m$ structures at pressures of 250, 300, and 350~GPa are calculated using the fixed-node diffusion quantum Monte Carlo (DMC) method. The difference between the mean-field and many-body band gaps at the same density is found to be almost independent of system size and can therefore be applied as a scissor correction to the mean-field gap of an infinite system to obtain an estimate of the many-body g…
Metallic solid-state hydrogen storage crystals achieved through chemical precompression under ambient conditions
Key Laboratory of Material Simulation Methods & Software of Ministry of Education, College of Physics, Jilin University（吉林大学物理学院教育部材料模拟方法与软件重点实验室）；Institute of Atomic and Molecular Physics, Jilin University（吉林大学原子与分子物理研究所）；Institute of Theoretical Chemistry, College of Chemistry, Jilin University（吉林大学化学学院理论化学研究所）
The ab-initio phase diagram of dense hydrogen is very sensitive to errors in the treatment of electronic correlation. Recently, it has been shown that the choice of the density functional has a large effect on the predicted location of both the liquid-liquid phase transition and the solid insulator-to-metal transition in dense hydrogen. To identify the most accurate functional for dense hydrogen applications, we systematically benchmark some of the most commonly used functionals using Quantum Monte Carlo. By considering several measures of functional accuracy, we conclude that the van der Waal…
