H2 Units Research Articles

Unlocking the Origin of High‐Temperature Superconductivity in Molecular Hydrides at Moderate Pressures

AbstractThe current pressing challenge in the field of superconducting hydride research is to lower the stable pressure of such materials for practical applications. Molecular hydrides are usually stable under moderate pressure, but the underlying metallization mechanism remains elusive. Here, the important role of chemical interactions in governing the structures and properties of molecular hydrides is demonstrated. A new mechanism is proposed for obtaining high‐temperature and even room‐temperature superconductivity in molecular hydrides and report that the ternary hydride NaKH12 hosts Tc values up to 245 K at moderate pressure of 60 GPa. Both the excellent stability and superconductivity of NaKH12 originate from the fact that the localized electrons in the interstitial region of the metal lattice occupying the crystal orbitals well matched with the hydrogen lattice and forming chemical templates to assist the assembly of H2 units. These localized electrons weaken the H─H covalent bonds and improve the charge connectivity between the H2 units, ensuring the strong coupling between electrons and hydrogen‐dominated optical phonons. The theory provides a key perspective for understanding the superconductivity of molecular hydrides with various structural motifs, opening the door to obtaining high‐temperature superconductors from molecular hydrides at moderate pressures.

Prediction of Room-Temperature Superconductivity in Quasi-Atomic H2-Type Hydrides at High Pressure.

Achieving superconductivity at room temperature (RT) is a holy grail in physics. Recent discoveries on high-Tc superconductivity in binary hydrides H3S and LaH10 at high pressure have directed the search for RT superconductors to compress hydrides with conventional electron-phonon mechanisms. Here, an exceptional family of superhydrides is predicated under high pressures, MH12 (M = Mg, Sc, Zr, Hf, Lu), all exhibiting RT superconductivity with calculated Tcs ranging from 313 to 398 K. In contrast to H3S and LaH10, the hydrogen sublattice in MH12 is arranged as quasi-atomic H2 units. This unique configuration is closely associated with high Tc, attributed to the high electronic density of states derived from H2 antibonding states at the Fermi level and the strong electron-phonon coupling related to the bending vibration of H2 and H-M-H. Notably, MgH12 and ScH12 remain dynamically stable even at pressure below 100 GPa. The findings offer crucial insights into achieving RT superconductivity and pave the way for innovative directions in experimental research.

Tuning chemical precompression: Theoretical design and crystal chemistry of novel hydrides in the quest for warm and light superconductivity at ambient pressures

Over the past decade, a combination of crystal structure prediction techniques and experimental synthetic work has thoroughly explored the phase diagrams of binary hydrides under pressure. The fruitfulness of this dual approach is demonstrated in the recent identification of several superconducting hydrides with Tcs approaching room temperature. We start with an overview of the computational procedures for predicting stable structures and estimating their propensity for superconductivity. A survey of phases with high Tc reveals some common structural features that appear conducive to the strong coupling of the electronic structure with atomic vibrations that leads to superconductivity. We discuss the stability and superconducting properties of phases containing two of these—molecular H2 units mixed with atomic H and hydrogenic clathrate-like cages—as well as more unique motifs. Finally, we argue that ternary hydride phases, whose exploration is still in its infancy, are a promising route to achieve simultaneous superconductivity at high temperatures and stability at low pressures. Several ternary hydrides arise from the addition of a third element to a known binary hydride structure through site mixing or onto a new site, and several more are based on altogether new structural motifs.

Predicted structures and superconductivity of LiYHn (n = 5-10) under high pressure.

The structures of LiYHn (n = 5-10) compounds in the pressure range of 0-300 GPa have been extensively explored using the CALYPSO structure prediction method based on the particle swarm optimization algorithm and first-principles calculation. Four stable structures (P21/m LiYH6, C2/c LiYH8, P1̄ LiYH9, R3̄m LiYH10) and three metastable phases (Pnma LiYH6, P1̄ LiYH8, Immm LiYH9) were predicted. They all exhibit metallic and superconducting behavior in their respective stable pressure ranges, and the predicted superconducting transition temperature Tc is within 22-109 K when the pressure is greater than 100 GPa. It was found that after doping Li into YHn (n = 6, 9, 10), the H2 units in the system increased, the electron-phonon coupling interaction weakened, and Tc decreased when the structural characteristics, electronic density of states distribution, and superconductivity of LiYHn and YHn (n = 6, 8, 9, 10) were compared. Systems that have a high density of H_s states and a low number of Y_d states at the Fermi level have stronger electron-phonon coupling (EPC) interactions and higher Tc.

Theoretical Prediction of CHn Crystal Structures under High Pressures

CHn is the precursor unit for graphene synthesis. We have theoretically predicated a series of CHn structures with n = 1, 2, 4, 6, 8, 10, and 12 at elevated pressures (ambient pressure, 50, 100, 200, 300, 350, and 400 GPa) using evolutionary algorithms. The predicted CH and CH2 structures are graphane-type and polyethylene over the whole considered pressure range, respectively. The molecular crystalline methane is predicted for the stoichiometry of CH4. The combination of methane and H2 for CH6, CH8, CH10, and CH12 up to 300 GPa are obtained. At 400 GPa, the mixture of polymer and H2 for CH6, CH10, and CH12 comes into play. From the computed enthalpy, higher pressure and more hydrogen concentration contributed to the decomposition (to carbon and H2) of CHn systems. The total density of states for these CHn structures show that only the CH12 phase is metallic above 300 GPa. The rotational properties are traced in H2 and the CHn structures. The CH4 rotation is more sensitive to the pressure. The H2 units are nearly freely rotational. Other structures of CHn, including fcc-type and experimentally known structures, are not competitive with the structures predicted by evolutionary algorithms under high pressure region. Our results suggest that the CHn (n > 4) system is a potential candidate for hydrogen storage where H2 could be released by controlling the pressure.

Quantum anharmonic enhancement of superconductivity in P63/mmc ScH6 at high pressures: A first-principles study

Making use of first-principles calculations, we analyze the effect of quantum ionic fluctuations and lattice anharmonicity on the crystal structure and superconductivity of P63/mmc ScH6 in the 100–160 GPa pressure range within the stochastic self-consistent harmonic approximation. We predict a strong correction to the crystal structure, the phonon spectra, and the superconducting critical temperatures, which have been estimated in previous calculations without considering ionic fluctuations on the crystal structure and assuming the harmonic approximation for the lattice dynamics. Quantum ionic fluctuations have a large impact on the H2 molecular-like units present in the crystal by increasing the hydrogen–hydrogen distance about a 5%. According to our anharmonic phonon spectra, this structure will be dynamically stable at least above 85 GPa, which is 45 GPa lower than the pressure given by the harmonic approximation. Contrary to many superconducting hydrogen-rich compounds, where quantum ionic effects and the consequent anharmonicity tend to lower the superconducting critical temperature, our results show that it can be enhanced in P63/mmc ScH6 by approximately 15%. We attribute the enhancement of the critical temperature to the stretching of the H2 molecular-like units and the associated increase of the electron–phonon interaction. Our results suggest that quantum ionic effects increase the superconducting critical temperature in hydrogen-rich materials with H2 units by increasing the hydrogen–hydrogen distance and, consequently, the electron–phonon interaction.

Cover Feature: Electronic Structure and Superconductivity of Compressed Metal Tetrahydrides (Chem. Eur. J. 60/2021)

Exotic physical properties are often exhibited by compounds crystallizing in the ubiquitous ThCr2Si2 structure type illustrated in the picture. A plethora of metal tetrahydrides belonging to this family, with metal cations, H- and quasimolecular H2 units, have been synthesized in diamond anvil cells and/or predicted to be stable under pressure. A theoretical investigation unveils why some are superconducting, whereas others are not. More information can be found in the Full Paper by T. Bi and E. Zurek (DOI: 10.1002/chem.202102679).

C60Con complexes as hydrogen adsorbing materials

An active line of contemporary research is dedicated to the adsorption and storage of hydrogen on metal-doped carbon materials. Using density functional theory and van der Waals corrections we have studied molecular and dissociative adsorption of H2 on neutral and cationic C60Con complexes with n = 1–8. The Co atoms form compact clusters on the surface of the fullerene. Dissociative chemisorption of one H2 molecule is more stable than molecular adsorption on neutral C60Con, with the only exception of C60Co. When C60Con is ionized, the electronic charge deficit remains localized in the cobalt cluster. The molecular and dissociative adsorption features on cationic C60Con+ and neutral C60Con complexes are in general similar, but some differences can be highlighted. Molecular and dissociative H2 adsorption on C60Co2+ are competitive; in fact, molecular adsorption is slightly more stable as a result of the localized electronic charge deficit on the Co dimer. Another difference is that the dissociative adsorption energies on some of the neutral C60Con complexes are substantially larger than the dissociative adsorption energies on the corresponding C60Con+ cationic complexes by amounts between 0.2 and 0.5 eV. Activation barriers for dissociation of the adsorbed H2 molecule strongly depend on the charge state and cluster size. These barriers help to interpret the adsorption state (molecular or dissociated) of experimentally produced hydrogenated C60Con+ complexes. Hydrogen saturation has been studied in two cases. C60Co adsorbs three H2 units in molecular form, and C60Con adsorbs up to thirteen H2 units, four dissociated and nine in molecular form.

The superconductivity of N–Si–H compounds at high pressure

Recently, the study of superconductive hydrides with high critical temperature (Tc) has attracted considerable attention in high-pressure community due to the finding of 240 K in YH9 at ~200 GPa and 250–260 K in LaH10 at 170–190 GPa. In this work, we performed structure searches on the exploration of the crystal structure and the superconductivity in ternary N–Si–H system at high pressure via a particle swarm optimization structure prediction methodology in combination with the first-principles electronic structure framework. As a result, our first-principles calculations show that a metastable phase NSiH11 is a potential superconductor Tc of 98–110 K at 300 GPa. This high superconductivity is associated with the H–H interaction among H2 units.

Heritage conservation hazard in archaeological sites in Santa María Valley (NW Argentina): A geoarchaeological approach

AbstractAlluvial fans are typical geomorphic features of arid and semiarid mountain areas. Most archaeological sites in Santa María valley (NW Argentina) are located on this kind of landform. The aims of this paper are to describe the geomorphological context and assess the state of conservation of four archaeological sites on the piedmont of Sierra de Quilmes (NW Argentina) from a geoarchaeological perspective; to diagnose the geomorphological processes affecting the sites over the last 50 years; to assess their vulnerability and conservation hazard; and to propose some corrective measures. By mapping with remote sensors (e.g., drones, aerial photographs, and satellite images) and conducting field surveys, we found that most archaeological sites are set on the H1 and H2 units of the alluvial fans. and that their geomorphological dynamic has increased in the last 50 years. The main active processes are debris flows, overflows, and mudflows, accompanied by the development of rills, sheetflood, and aeolian deflation. Human impact is also severe. The four sites need mitigation measures and a structured management plan. To date, no studies of this kind have been done rin the region, although one of the sites has been partially reconstructed for touristic purposes.

A Boosted Critical Temperature of 166 K in Superconducting D3 S Synthesized from Elemental Sulfur and Hydrogen.

The discovery of superconductivity in H3S at 203 K marked an advance towards room‐temperature superconductivity and demonstrated the potential of H‐dominated compounds to possess a high critical temperature (Tc). There have been numerous reports of the H‐S system over the last five years, but important questions remain unanswered. It is crucial to verify whether the Tc was determined correctly for samples prepared from compressed H2S, since they are inevitably contaminated with H‐depleted byproducts. Here, we prepare stoichiometric H3S by direct in situ synthesis from elemental S and excess H2. The Im3‾ m phase of D3S samples exhibits a Tc significantly higher than previously reported values (ca. 150 K), reaching a maximum Tc of 166 K at 157 GPa. Furthermore, we confirm that the sharp decrease in Tc below 150 GPa is accompanied by continuous rhombohedral structural distortions and demonstrate that the Cccm phase is non‐metallic, with molecular H2 units in the crystal structure.

Exotic Hydrogen Bonding in Compressed Ammonia Hydrides.

Hydrogen-rich compounds attract significant fundamental and practical interest for their ability to accommodate diverse hydrogen bonding patterns and their promise as superior energy storage materials. Here, we report on an intriguing discovery of exotic hydrogen bonding in compressed ammonia hydrides and identify two novel ionic phases in an unusual stoichiometry NH7. The first is a hexagonal R3̅ m phase containing NH3-H+-NH3, H-, and H2 structural units stabilized above 25 GPa. The exotic NH3-H+-NH3 unit comprises two NH3 molecules bound to a proton donated from a H2 molecule. Above 60 GPa, the structure transforms to a tetragonal P41212 phase comprising NH4+, H-, and H2 units. At elevated temperatures, fascinating superionic phases of NH7 with part-solid and part-liquid structural forms are identified. The present findings advance fundamental knowledge about ammonia hydrides at high pressure with broad implications for studying planetary interiors and superior hydrogen storage materials.

New Calcium Hydrides with Mixed Atomic and Molecular Hydrogen

Two new polyhydrides of calcium have been synthesized at high pressures and high temperatures and characterized by Raman spectroscopy, infrared spectroscopy, and synchrotron X-ray diffraction. Above 20 GPa and 700 K, we synthesize a phase having a monoclinic (C2/m) structure with Ca2H5 composition, which is characterized by a distinctive vibration at 3789 cm–1 at 25 GPa. The observed Raman spectrum is in close agreement with first-principles calculations of a Ca2H5 structure characterized by a lattice containing a central layer of H2 molecules oriented along the (100) direction. At higher pressures (e.g., 116 GPa and 1600 K), we synthesize another phase, which has the composition of CaH4 and a denser body-centered tetragonal structure. This weakly metallic phase also contains molecular-like H2 units, and its spectroscopic as well as diffraction signatures match closely with those predicted from first-principles calculations. This phase is observed to persist on decompression to 60 GPa at room temperature. The elongation of the H–H bond in these hydrides is a result of the Ca–H2 interaction, analogous to what occurs in molecular compounds, where H2 binds side-on to a d-element, such as in Kubas complex.

Tellurium Hydrides at High Pressures: High-Temperature Superconductors.

Observation of high-temperature superconductivity in compressed sulfur hydrides has generated an irresistible wave of searches for new hydrogen-containing superconductors. We herein report the prediction of high-T_{c} superconductivity in tellurium hydrides stabilized at megabar pressures identified by first-principles calculations in combination with a swarm structure search. Although tellurium is isoelectronic to sulfur or selenium, its heavier atomic mass and weaker electronegativity makes tellurium hydrides fundamentally distinct from sulfur or selenium hydrides in stoichiometries, structures, and chemical bondings. We identify three metallic stoichiometries of H_{4}Te, H_{5}Te_{2}, and HTe_{3}, which are not predicted or known stable structures for sulfur or selenium hydrides. The two hydrogen-rich H_{4}Te and H_{5}Te_{2} phases are primarily ionic and contain exotic quasimolecular H_{2} and linear H_{3} units, respectively. Their high-T_{c} (e.g., 104K for H_{4}Te at 170GPa) superconductivity originates from the strong electron-phonon couplings associated with intermediate-frequency H-derived wagging and bending modes, a superconducting mechanism which differs substantially with those in sulfur or selenium hydrides where the high-frequency H-stretching vibrations make considerable contributions.

Pressure-driven formation and stabilization of superconductive chromium hydrides.

Chromium hydride is a prototype stoichiometric transition metal hydride. The phase diagram of Cr-H system at high pressures remains largely unexplored due to the challenges in dealing with the high activation barriers and complications in handing hydrogen under pressure. We have performed an extensive structural study on Cr-H system at pressure range 0 ∼ 300 GPa using an unbiased structure prediction method based on evolutionary algorithm. Upon compression, a number of hydrides are predicted to become stable in the excess hydrogen environment and these have compositions of Cr2Hn (n = 2–4, 6, 8, 16). Cr2H3, CrH2 and Cr2H5 structures are versions of the perfect anti-NiAs-type CrH with ordered tetrahedral interstitial sites filled by H atoms. CrH3 and CrH4 exhibit host-guest structural characteristics. In CrH8, H2 units are also identified. Our study unravels that CrH is a superconductor at atmospheric pressure with an estimated transition temperature (T c) of 10.6 K, and superconductivity in CrH3 is enhanced by the metallic hydrogen sublattice with T c of 37.1 K at 81 GPa, very similar to the extensively studied MgB2.

Pressure-Induced Structures and Properties in Indium Hydrides.

The structures, electron properties, and potential superconductivity of indium hydrides are systematically studied under high pressure by first-principles density functional calculations. Upon compression, two stable stoichiometries (InH3 and InH5) are predicted to be thermodynamically stable. Particularly, in the two compounds, all hydrogen atoms exist in the form of H2 or H3 units. The stable phases present metallic features with the overlap between the conduction and the valence bands. The Bader analysis indicates that charges transfer from In atoms to H atoms. Electron-phonon calculations show that the estimated transition temperatures (Tc) of InH3 and InH5 are 34.1-40.5 and 22.4-27.1 K at 200 and 150 GPa, respectively.

Hydrogen segregation and its roles in structural stability and metallization: silane under pressure.

We present results from first-principles calculations on silane (SiH4) under pressure. We find that a three dimensional P-3 structure becomes the most stable phase above 241 GPa. A prominent structural feature, which separates the P-3 structure from previously observed/predicted SiH4 structures, is that a fraction of hydrogen leaves the Si-H bonding environment and forms segregated H2 units. The H2 units are sparsely populated in the system and intercalated with a polymeric Si-H framework. Calculations of enthalpy of formation suggest that the P-3 structure is against the decomposition into Si-H binaries and/or the elemental crystals. Structural stability of the P-3 structure is attributed to the electron-deficient multicenter Si-H-Si interactions when neighboring silicon atoms are linked together through a common hydrogen atom. Within the multicenter bonds, electrons are delocalized and this leads to a metallic state, possibly also a superconducting state, for SiH4. An interesting outcome of the present study is that the enthalpy sum of SiH4 (P-3 structure) and Si (fcc structure) appears to be lower than the enthalpy of disilane (Si2H6) between 200 and 300 GPa (for all previously predicted crystalline forms of Si2H6), which calls for a revisit of the stability of Si2H6 under high pressure.

Structures and Properties of Osmium Hydrides under Pressure from First Principle Calculation

The pressure-induced new structures and properties of osmium hydrides were systematically explored in a wide pressure range 0–300 GPa using ab initio methods. Three stable stoichiometries, that is, OsH, OsH3, and OsH6, are predicted above 50 GPa. The above hydrides exhibit metallic character with the notable band structures exception of OsH6. It is interesting to note that the phase P21/c of hydrogen-rich OsH6 adopts intriguing structures with H2 units. The electron–phonon coupling calculations indicate that the superconducting critical temperature (Tc) values of Fm-3m-OsH is 2.1 K at 100 GPa. Comparing to pure Os, the addition of hydrogen is in favor of improving the superconducting temperature.

Synthesis of lithium polyhydrides above 130 GPa at 300 K

The prediction of novel lithium hydrides with nontraditional stoichiometries at high pressure has been seminal for highlighting a promising line of research on hydrogen-dense materials. Here, we report the evidences of the disproportionation of LiH above 130 GPa to form lithium hydrides containing H2 units. Measurements have been performed using the nonperturbing technique of synchrotron infrared absorption. The observed vibron frequencies match the predictions for LiH2 and LiH6. These polyhydrides remain insulating up to 215 GPa. A disproportionation mechanism based on the diffusion of lithium into the diamond anvil and a stratification of the sample into LiH6/LiH2/LiH layers is proposed. Polyhydrides containing an H2 sublattice do exist and could be ubiquitously stable at high pressure.

H2 saturation on palladium clusters.

The interaction of PdN clusters (N = 2, 3, 4, 7, and 13) with multiple H2 adsorbate molecules is investigated using density functional theory with the hybrid PBE0 functional. The optimal structure for each PdNH2(L) complex is determined systematically via a sequential addition of H2 units. The adsorption energy for each successive H2 addition is computed to determine the maximum number of molecules that can be stably added to a PdN at T = 0 K. The Gibbs free energy is then used to determine the saturation coverage at finite temperature. For N = 2, 3, and 4, a single H2 is found to dissociate, and up to two additional molecular H2 units per Pd atom can bind stably to the clusters at 0 K. At 300 K, one H2 unit dissociates, and only one additional H2 molecular unit per Pd atom is stably bound. For N = 7 and T = 0 K, two H2 units dissociate, and 11 additional H2 units bind molecularly. At 300 K, two units dissociate, and eight are bound molecularly. For N = 3, 4, and 7, we find that an additional H2 unit may dissociate if the underlying cluster structure rearranges. Eight H2 units dissociate on Pd13 at 0 K. At least one additional H2 binds molecularly at 0 K, but none bind at 300 K. This suggests that only dissociated H2 units will stably bind to larger Pd particles at room temperature. The influence of molecularly adsorbed H2 units on the migration of dissociated H atoms is investigated in a preliminary way. Both barrier heights and the relative stability of local minima of Pd4H2(L) are found to be affected by the degree of molecular H2 coverage.