High-pressure Hydrides Research Articles

Large impact of phonon lineshapes on the superconductivity of solid hydrogen

Phonon anharmonicity plays a crucial role in determining the stability and vibrational properties of high-pressure hydrides. Furthermore, strong anharmonicity can render phonon quasiparticle picture obsolete questioning standard approaches for modeling superconductivity in these material systems. In this work, we show the effects of non-Lorentzian phonon lineshapes on the superconductivity of high-pressure solid hydrogen. We calculate the superconducting critical temperature TC ab initio considering the full phonon spectral function and show that it overall enhances the TC estimate. The anharmonicity-induced phonon softening exhibited in spectral functions increases the estimate of the critical temperature, while the broadening of phonon lines due to phonon-phonon interaction decreases it. Our calculations also reveal that superconductivity emerges in hydrogen in the Cmca − 12 molecular phase VI at pressures between 450 and 500 GPa and explain the disagreement between the previous theoretical results and experiments.

Surface inducing high-temperature superconductivity in layered metal carborides Li2BC3 and LiBC by metallizing σ electrons.

Metallizing σ electrons provides a promising route to design high-temperature superconducting materials, such as MgB2 and high-pressure hydrides. Here, we focus on two MgB2-like layered carborides Li2BC3 and LiBC; their bulk does not have superconductivity because the B-C σ states are far away from the Fermi level (EF), however, based on first-principles calculations, we found that when their bulk systems are cleaved into surfaces with B-C termination, high Tc of ∼80 K could be observed in the exposed B-C layer on the surfaces. Detailed analysis reveals that surface symmetry reduction, due to lattice periodic breaking, not only introduces hole self-doping into surface B-C layers and shifts the σ-bonding states towards the EF - associated with emergent large electronic occupation, but also makes in-plane stretching modes on the surface layer experience significant softness. The enhanced σ states and softened phonon modes work to produce strong coupling, thus yielding high-Tc surface superconductivity, which distinctly differs from the superconducting features of the MgB2 film, which generates phonon stiffness accompanied by suppressed superconductivity. Our findings undoubtedly provide a novel platform to realize high-Tc surface superconductivity, and also clearly elucidate the microscopic mechanism of surface-enhanced superconductivity in favor of creating more high-Tc surface superconductors among MgB2-like layered materials.

Mapping superconductivity in high-pressure hydrides: The Superhydra project

The discovery of high-${T}_{c}$ conventional superconductivity in high-pressure hydrides has helped establish computational methods as a formidable tool to guide material discoveries in a field traditionally dominated by serendipitous experimental search. This paves the way to an ever-increasing use of data-driven approaches to the study and design of superconductors. In this work, we propose a new adaptive method to generate meaningful datasets of superconductors, based on element substitution into a small set of representative structural templates, generated by crystal structure prediction methods---adapted high-throughput approach. Our approach realizes an optimal compromise between structural variety and computational efficiency and can be easily generalized to other elements and compositions. As a first application, we apply it to binary hydrides at high pressure, realizing a database of 880 hypothetical structures, characterized with a set of electronic, vibrational, and chemical descriptors. In our Superhydra Database, 139 structures are superconducting according to the McMillan-Allen-Dynes approximation. Studying the distribution of ${T}_{c}$ and other properties across the database with advanced statistical and visualization techniques, we are able to obtain comprehensive material maps of the phase space of binary hydrides. The Superhydra database can be thought as a first step of a generalized effort to map conventional superconductivity.

Pressure-induced superconductivity of Ac-B-H hydrides.

The search for room-temperature superconductors among high-pressure hydrides is a hot research topic. In this study, the structures, stabilities and superconducting properties of ternary Ac-B-H hydrides were studied using a genetic algorithm (GA) combined with density functional theory (DFT) calculations. It was shown that the R3̄m-AcBH8 and I4/mmm-AcB2H8 structures were thermodynamically and dynamically stable above 70 and 125 GPa, respectively. In the R3̄m-AcBH8 structure, the BH6 unit and the dispersed H atoms were bonded to form a corrugated structure. The I4/mmm-AcB2H8 structure contained a cage and the Ac atom located at the cage center. The calculations of the electron-phonon coupling showed that the R3̄m-AcBH8 and I4/mmm-AcB2H8 structures had Tc values of 140 K (70 GPa) and 99 K (125 GPa), respectively. The analyses of the phonon dispersion curves revealed that electron-phonon coupling was closely related to the vibrations of the B-H bonds.

High-pressure synthesis of seven lanthanum hydrides with a significant variability of hydrogen content

The lanthanum-hydrogen system has attracted significant attention following the report of superconductivity in LaH10 at near-ambient temperatures and high pressures. Phases other than LaH10 are suspected to be synthesized based on both powder X-ray diffraction and resistivity data, although they have not yet been identified. Here, we present the results of our single-crystal X-ray diffraction studies on this system, supported by density functional theory calculations, which reveal an unexpected chemical and structural diversity of lanthanum hydrides synthesized in the range of 50 to 180 GPa. Seven lanthanum hydrides were produced, LaH3, LaH~4, LaH4+δ, La4H23, LaH6+δ, LaH9+δ, and LaH10+δ, and the atomic coordinates of lanthanum in their structures determined. The regularities in rare-earth element hydrides unveiled here provide clues to guide the search for other synthesizable hydrides and candidate high-temperature superconductors. The hydrogen content variability in lanthanum hydrides and the samples’ phase heterogeneity underline the challenges related to assessing potentially superconducting phases and the nature of electronic transitions in high-pressure hydrides.

Tracing the Anharmonicity and Superionic Phase Transition of Hydrous FeO2H

The weak x-ray scattering of hydrogen (H) has brought major challenges to the characterization of superionic transitions in high-pressure ice, hydrides, and hydroxides. Combining first-principles molecular dynamics and simulated nuclear magnetic resonance (NMR) spectroscopy, we investigated the behavior of the hydroxyl bonding and structural transitions in the hydrous FeO2H between 300 and 2750 K and up to 130 GPa. Evidence show that an intermediate plastic state with regional H diffusion and anharmonic O-H vibration exists in between the ordinary solid and the superionic phase. The intermediate state features asymmetric hydrogen bonds and anharmonic vibrations, which are readily distinguished from the high-temperature superionic phase. Our work shows NMR is a more sensitive probe to detect H diffusion in superionic solids even in the extreme conditions of Earth’s deep interiors.

Lattice dynamics of high-pressure hydrides studied by inelastic neutron scattering

Due to the small mass and anomalously large neutron scattering cross-section of proton (about 80 barns compared to a few barns for other nuclei), inelastic neutron scattering is considered as one of the most effective tools in studying optical vibrations of hydrogen atoms in metal hydrides. The current review is focused on the binary hydrides of 3d- and 4d-metals of groups VI–VIII, which were produced at high hydrogen pressures of several gigapascals in relatively large quantities of hundreds of mg, quenched to low temperature and studied by INS ex situ at ambient pressure with high statistical accuracy. One of the unusual effects revealed by INS is a strong increase in the strength of the metal-hydrogen interactions with decreasing atomic number of the d-metal accompanied by an increase in the Me-H distance. Based on the available experimental results, the spectra g(E) of the phonon density of states and temperature dependencies CV(T) of the heat capacity at constant volume at T up to 1000 K have been derived in this paper and presented both in the figures and in digital form. This provides the reference data for the theoretical investigations of the crystal structures and compositions of new practically important hydrides giving the opportunity to validate calculation methods by comparing the calculated g(E) and CV(T) with the accurate experimental dependencies for the binary hydrides. Recent INS studies showed [R.A. Klein et al., J. Alloy. Compd. 894 (2022) 162381] that the fingerprints of anomalously short H-H separations of 1.6 Å violating the “2 Å rule” can be easily and unambiguously identified in the complex INS spectra of quaternary hydrides (La,Ce)NiInH1+x. This makes neutron spectroscopy an attractive means for obtaining valuable data in the search for novel hydrides with a record high hydrogen capacity.

Beyond the Fröhlich Hamiltonian: Path-integral treatment of large polarons in anharmonic solids

The properties of an electron in a typical solid are modified by the interaction with the crystal ions, leading to the formation of a quasiparticle: the polaron. Such polarons are often described using the Fr\"ohlich Hamiltonian, which assumes the underlying lattice phonons to be harmonic. However, this approximation is invalid in several interesting materials, including the recently discovered high-pressure hydrides which superconduct at temperatures above $200$K. In this paper, we show that Fr\"ohlich theory can be extended to eliminate this problem. We derive four additional terms in the Fr\"ohlich Hamiltonian to account for anharmonicity up to third order. We calculate the energy and effective mass of the new polaron, using both perturbation theory and Feynman's path integral formalism. It is shown that the anharmonic terms lead to significant additional trapping of the electron. The derived Hamiltonian is well-suited for analytical calculations, due to its simplicity and since the number of model parameters is low. Since it is a direct extension of the Fr\"ohlich Hamiltonian, it can readily be used to investigate the effect of anharmonicity on other polaron properties, such as the optical conductivity and the formation of bipolarons.

Distinguishing the Structures of High-Pressure Hydrides with Nuclear Magnetic Resonance Spectroscopy.

The structural characterization of high-pressure hydrides has encountered many difficulties mainly due to the weak X-ray scattering of hydrogen. Herein, we investigate the prospect of detecting the H3S and LaH10 structures with nuclear magnetic resonance (NMR) spectroscopy. Our calculations demonstrate that the different candidate structures of H3S (or LaH10) exhibit significant differences in the electric field gradient (EFG) tensor of the 33S (or 139La) sites, indicating that the NMR spectroscopy can well capture the structural differences, even the small changes in the atomic position, and hence can be used to effectively probe the structures and the phase transitions of H3S and LaH10. Our results clarify the relationship between the structures and the EFG tensor parameters and provide a potential means to detect the structures of high-pressure hydrides.

Detecting Superconductivity in the High Pressure Hydrides and Metallic Hydrogen from Optical Properties

We present a new technique for measuring the critical temperature T_{c} in the high pressure, high T_{c} electron-phonon-driven superconducting hydrides. This technique does not require connecting leads to the sample. In the region of the absorption spectrum above the sum of the optical gap and maximum phonon energy, the reflectance mirrors the temperature variation of the superconducting order parameter. For an appropriately chosen value of fixed photon energy, the temperature dependence of the reflectance varies much more rapidly below T=T_{c} than above. It increases with increasing temperature in the superconducting state while it decreases in the normal state. Examining the temperature dependence of the reflectance at a fixed photon energy, there is a cusp at T=T_{c} which provides a measurement of the critical temperature. We discuss these issues within the context of the recently reported atomic metallic phase of hydrogen, but our proposed technique should prove useful for other hydrides with large coupling to high energy phonons.

Prediction of high- Tc conventional superconductivity in the ternary lithium borohydride system

We investigate the superconducting ternary lithium borohydride phase diagram at pressures of 0 and 200$\,$GPa using methods for evolutionary crystal structure prediction and linear-response calculations for the electron-phonon coupling. Our calculations show that the ground state phase at ambient pressure, LiBH$_4$, stays in the $Pnma$ space group and remains a wide band-gap insulator at all pressures investigated. Other phases along the 1:1:$x$ Li:B:H line are also insulating. However, a full search of the ternary phase diagram at 200$\,$GPa revealed a metallic Li$_2$BH$_6$ phase, which is thermodynamically stable down to 100$\,$GPa. This {\em superhydride} phase, crystallizing in a $Fm\bar{3}m$ space group, is characterized by six-fold hydrogen-coordinated boron atoms occupying the $fcc$ sites of the unit cell. Due to strong hydrogen-boron bonding this phase displays a critical temperature of $\sim$ 100$\,$K between 100 and 200$\,$GPa. Our investigations confirm that ternary compounds used in hydrogen-storage applications are a suitable choice for observing high-$T_\text{c}$ conventional superconductivity in diamond anvil cell experiments, and suggest a viable route to optimize the critical temperature of high-pressure hydrides.

Conventional/unconventional superconductivity in high-pressure hydrides and beyond: insights from theory and perspectives

The observation of a superconducting critical temperature $$T_\mathrm{c}$$ exceeding 200K in ultra-dense hydrogen sulfide has demonstrated that high- $$T_\mathrm{c}$$ superconductivity can be achieved also in compounds where the superconducting pairing is mediated by phonons. This poses interesting challenges and opportunities. In particular, in this paper, we present a theoretical overview of the following points:

Anharmonic effects in atomic hydrogen: Superconductivity and lattice dynamical stability

We present first-principles calculations of metallic atomic hydrogen in the 400-600 GPa pressure range in a tetragonal structure with space group $I4_1/amd$, which is predicted to be its first atomic phase. Our calculations show a band structure close to the free-electron-like limit due to the high electronic kinetic energy induced by pressure. Bands are properly described even in the independent electron approximation fully neglecting the electron-electron interaction. Linear-response harmonic calculations show a dynamically stable phonon spectrum with marked Kohn anomalies. Even if the electron-electron interaction has a minor role in the electronic bands, the inclusion of electronic ex- change and correlation in the density response is essential to obtain a dynamically stable structure. Anharmonic effects, which are calculated within the stochastic self-consistent harmonic approxima- tion, harden high-energy optical modes and soften transverse acoustic modes up to a 20% in energy. Despite a large impact of anharmonicity has been predicted in several high-pressure hydrides, here the superconducting critical temperature is barely affected by anharmonicity, as it is lowered from its harmonic 318 K value only to 300 K at 500 GPa. We atribute the small impact of anharmoncity on superconductivity to the absence of softened optical modes and the fairly uniform distribution of the electron-phonon coupling among the vibrational modes.

Mg-based compounds for hydrogen and energy storage

Magnesium-based alloys attract significant interest as cost-efficient hydrogen storage materials allowing the combination of high gravimetric storage capacity of hydrogen with fast rates of hydrogen uptake and release and pronounced destabilization of the metal–hydrogen bonding in comparison with binary Mg–H systems. In this review, various groups of magnesium compounds are considered, including (1) RE–Mg–Ni hydrides (RE = La, Pr, Nd); (2) Mg alloys with p-elements (X = Si, Ge, Sn, and Al); and (3) magnesium alloys with d-elements (Ti, Fe, Co, Ni, Cu, Zn, Pd). The hydrogenation–disproportionation–desorption–recombination process in the Mg-based alloys (LaMg12, LaMg11Ni) and unusually high-pressure hydrides synthesized at pressures exceeding 100 MPa (MgNi2H3) and stabilized by Ni–H bonding are also discussed. The paper reviews interrelations between the properties of the Mg-based hydrides and p–T conditions of the metal–hydrogen interactions, chemical composition of the initial alloys, their crystal structures, and microstructural state.

Influence of bonding on superconductivity in high-pressure hydrides

The recent reports on high-temperature superconductivity above 190 K in hydrogen sulfide at 200 GPa pressure, exceeding all previously discovered superconductors, has greatly invigorated the interest in dense hydrogen-rich solids. In this paper, we investigate a possible way to optimize the critical temperature in these compounds using first-principles linear-response calculations. We construct hypothetical alchemical atoms to smoothly interpolate between elements of the chalcogen group and study their bonding and superconducting properties. Our results show that the already remarkable critical temperatures of H$_3$S could be improved even further by increasing the ionic character of the relevant bonds, i.e. replacing sulfur partially with more electronegative elements.

Electron band structure of the high pressure cubic phase of AlH3

The electronic band structure of the cubic Pm3n phase of AlH3 stable above 100 GPa is examined with semi-local, Tran-Blaha modified Becke-Johnson local density approximation (TB-mBJLDA), screened hybrid density functionals and GW methods. The shift of the conduction band to higher energy with increasing pressure is predicted by all methods. However, there are significant differences in detail band structure. In the pressure range from 90 to160 GPa, semi-local, hybrid functional and TB-mBJLDA calculations predicted that AlH3 is a poor metal. In comparison, GW calculations show a gap opening at 160 GPa and AlH3 becomes a small gap semi-conductor. From the trends of the calculated band shifts, it can be concluded that the favourable conditions leading to the nesting of Fermi surfaces predicted by semi-local calculation have disappeared if the exchange term is included. The results highlight the importance of the correction to the exchange energy on the band structure of hydrogen dominant dense metal hydrides at high pressure hydrides and may help to rationalize the absence of superconductivity in AlH3 from experimental measurements.

Hydriding and structural characteristics of thermally cycled and cold-worked V–0.5 at.%C alloy

High pressure hydrides of V 0.995C 0.005 were thermally cycled between β 2- and γ-phases hydrides for potential use in cryocoolers/heat pumps for space applications. The effect of addition of carbon to vanadium, on the plateau enthalpies of the high pressure β 2 + γ region is minimal. This is in contrast to the calculated plateau enthalpies for low pressure (α + β 1) mixed phases which showed a noticeable lowering of the values. Thermal cycling between β 2-and γ-phase hydrides increased the absorption pressures but desorption pressure did not change significantly and the free energy loss due to hysteresis also increased. Hydriding of the alloy with prior cold-work increased the pressure hysteresis significantly and lowered the hydrogen capacity. In contrast to the alloy without any prior straining (as-cast), desorption pressure of the alloy with prior cold-work also decreased significantly. Microstrains, 〈 ɛ 2〉 1/2, in the β 2-phase lattice of the thermally cycled hydrides decreased after 778 cycles and the domain sizes increased. However, in the γ-phase, both the microstrains and the domain sizes decreased after thermal cycling indicating no particle size effect. The dehydrogenated α-phase after 778 thermal cycles also showed residual microstrains in the lattice, similar to those observed in intermetallic hydrides. The effect of thermal cycling (up to 4000 cycles between β 2- and γ-phases) and cold working on absorption/desorption pressures, hydrogen storage capacity, microstrains, long-range strains, and domain sizes of β 2- and γ-phase hydrides of V 0.995C 0.005 alloys are presented.

High-pressure hydrides of iron and its alloys

Hydrides of iron and iron-based alloys are thermodynamically stable only athydrogen pressures in the gigapascal range and rapidly lose hydrogen underambient conditions. At low temperatures, however, these hydrides can be retainedin a metastable state at atmospheric pressure after being cooled under highpressure to liquid nitrogen temperature. This review will discuss the current stateof studies on phase transformations in the Fe–H and related systems and also onthe composition, crystal structure and physical properties of the hydrides, bothunder high hydrogen pressures and in the ‘quenched’ metastable state at ambientpressure. The studies at ambient pressure include magnetization measurements,x-ray and neutron diffraction, Mössbauer spectroscopy and inelastic neutronscattering. In the sections on Mössbauer and structural investigations of hydridesof Fe–Cr and Ni–Fe alloys new experimental results will be presented.

Lattice dynamics of high-pressure hydrides of the group VI–VIII transition metals

Monohydrides of the group VI–VIII transition metals Cr, Mn, Fe, Co, Ni, Mo, Rh and Pd are synthesised under high hydrogen pressures and elevated temperatures and studied by inelastic neutron scattering (INS) at ambient pressure and low temperatures. In the measured INS spectra, the energy of the main optical H peak exhibits a strong monotonic increase as a function of the nearest hydrogen–metal distance in the hydrides of both 3d- and 4d-metals, respectively. The spectra for FCC NiH and PdH appear strongly anisotropic at energy transfers above the fundamental band of optical hydrogen vibrations, while those for FCC PdD do not show a significant directional dependence at energies of the second optical D band.

Phase transformations, crystal and magnetic structures of high-pressure hydrides of d-metals

This paper will briefly discuss the high-pressure-hydrogen techniques used at the Institute of Solid State Physics, Russian Academy of Sciences, the T– P diagrams of the studied binary metal–hydrogen systems and the crystal and magnetic structures of high-pressure hydrides formed in those systems. A compilation of the available experimental data and a list of relevant publications are provided for reference purposes.