General trend for pressurized superconducting hydrogen-dense materials

Prediction of high-Tcsuperconductivity in two corrugated graphene sheets with intercalated CeH9molecules

Recent discoveries involving high-temperature superconductivity in H3S and LaH10have sparked a renewed interest in exploring the potential for superconductivity within hydrides. These superconductors require extremely high-pressure condition (∼100GPa), rendering them virtually impractical for industrial applications. In this study, we verify the occurrence of a low pressure superconductivity phase transition in a system containing two graphene layers with sine form corrugations where CeH9doped molecules are intercalated between the layers. The lowest-order constrained variational method is applied to calculate the thermodynamic and electrical properties of the valence electrons. We examine 9900 different distributions of CeH9molecules separately for finding a second-order phase transition with maximized critical temperature. The novelty of the present work is the prediction of a superconductivity transition atTc=198.61 K for a specific distribution of CeH9molecules with applying no external pressure on the exterior surfaces of the graphene sheets. Notably, this critical temperature is approximately 65 K higher than that observed in cuprate materials (HgBa2Ca2Cu3O8+δ), which are known for their highTcvalues at room pressure. It is interesting that in this particular case, the distribution periodicity of CeH9molecules bears the closest resemblance to the periodicity of the graphene corrugations among all 9900 examined cases. Computing the energy gap of the valence electrons reveals that this critical behavior corresponds to an unconventional superconductivity phase transition exhibiting a high critical current density on the order of∼107A cm-2.

Data-driven Design of High Pressure Hydride Superconductors using DFT and Deep Learning

The observation of superconductivity in hydride-based materials under ultrahigh pressures (for example, H3S and LaH10) has fueled the interest in a more data-driven approach to discovering new high-pressure hydride superconductors. In this work, we performed density functional theory (DFT) calculations to predict the critical temperature (Tc) of over 900 hydride materials under a pressure range of (0 to 500) GPa, where we found 122 dynamically stable structures with a Tc above MgB2 (39 K). To accelerate screening, we trained a graph neural network (GNN) model to predict Tc and demonstrated that a universal machine learned force-field can be used to relax hydride structures under arbitrary pressures, with significantly reduced cost. By combining DFT and GNNs, we can establish a more complete map of hydrides under pressure.

First-principles study of the structures and superconductivity of H-S-La systems under high pressure

Recent experimental and theoretical studies have shown that a La-H system displays remarkable superconducting properties, and it is also possible to improve the superconducting state by introducing other elements into this system. In this study, we systematically investigated the crystal structures and physical properties of an H-S-La system by using first-principles calculations combined with the CALYPSO structure exploration technique. We predicted four stable stoichiometries containing H2SLa, H3SLa, H4Sla, and H6SLa. These compounds undergo a series of phase transitions under 50-300 GPa. The bonding characters and electronic properties were calculated. It was found that Cm-H2SLa, C2/c-H2SLa, and Cmcm-H6SLa exhibit good metallic nature, which stimulates us to further study their superconducting properties. The calculated superconducting transition temperatures (Tc) of Cm-H2SLa, C2/c-H2Sla, and Cmcm-H6SLa are 15.0 K at 200 GPa, 6.9 K at 300 GPa, and 23.6 K at 300 GPa, respectively.

Lattice dynamic stability and electronic structures of ternary hydrides La1−xYxH3via first-principles cluster expansion

Lanthanum hydride compounds LaH₃ become stabilized by yttrium substitution under the influence of moderate pressure. Novel materials with a wide range of changes in the structural properties as a function of hydrogen are investigated by means of the first-principles cluster expansion technique. Herein, the new compounds La_1−xY_xH₃, where 0 ≤ x ≤ 1, are determined to adopt tetragonal structures under high-pressure with the compositions La_0.8Y_0.2H₃, La_0.75Y_0.25H₃, and La_0.5Y_0.5H₃. The corresponding thermodynamic and dynamical stabilities of the predicted phases are confirmed by a series of calculations including, for example, phonon dispersion, electronic band structure, and other electronic characteristics. According to the band characteristics, all hydrides except that of I4₁/amd symmetry are semiconductors. The tetragonal La_0.5Y_0.5H₃ phase is found to become semi-metallic, as confirmed by adopting the modified Becke–Johnson exchange potential. The physical origins of the semiconductor properties in these stable hydrides are discussed in detail. Our findings provide a deeper insight into this class of rare-earth ternary hydrides.

Lanthanum hydride compound LaH₃ become stabilized by yttrium substitution under the influence of moderate pressure.

Superconducting ScH3 and LuH3 at Megabar Pressures

Rare-earth (RE) superhydrides have great potential as high-temperature superconductors, with recent discoveries almost achieving room-temperature superconductivity in compressed LaH₁₀ and YH₉. Here, we continue to study the rare-earth hydrides by focusing on the new hydrides that the lightest element Sc and the heaviest element Lu formed under pressure. Two new superconducting hydrides ScH₃ (T_c ∼ 18.5 K at 131 GPa) and LuH₃ (T_c ∼ 12.4 K at 122 GPa) have been identified both with cubic structure by combining X-ray diffraction and electrical resistance techniques. Among all of the REH₃, only the superconducting properties of ScH₃ and LuH₃ have been experimentally confirmed. Our current results may offer guidance to other REH₃, which were predicted to be superconductors but have not been experimentally confirmed.

Strong correlation between electronic bonding network and critical temperature in hydrogen-based superconductors

By analyzing structural and electronic properties of more than a hundred predicted hydrogen-based superconductors, we determine that the capacity of creating an electronic bonding network between localized units is key to enhance the critical temperature in hydrogen-based superconductors. We define a magnitude named as the networking value, which correlates with the predicted critical temperature better than any other descriptor analyzed thus far. By classifying the studied compounds according to their bonding nature, we observe that such correlation is bonding-type independent, showing a broad scope and generality. Furthermore, combining the networking value with the hydrogen fraction in the system and the hydrogen contribution to the density of states at the Fermi level, we can predict the critical temperature of hydrogen-based compounds with an accuracy of about 60 K. Such correlation is useful to screen new superconducting compounds and offers a deeper understating of the chemical and physical properties of hydrogen-based superconductors, while setting clear paths for chemically engineering their critical temperatures.

Electron- and hole-doping on ScH2and YH2: effects on superconductivity without applied pressure

We present the evolution of the structural, electronic, and lattice dynamical properties, as well as the electron-phonon (el-ph) coupling and superconducting critical temperature (T_c) of ScH₂and YH₂metal hydrides solid solutions, as a function of the electron- and hole-doping content. The study was performed within the density functional perturbation theory, taking into account the effect of zero-point energy through the quasi-harmonic approximation, and the solid solutions Sc_1-xM_xH₂(M = Ca, Ti) and Y_1-xM_xH₂(M = Sr, Zr) were modeled by the virtual crystal approximation. We have found that, under hole-doping (M = Ca, Sr), the ScH₂and YH₂hydrides do not improve their el-ph coupling properties, sensed byλ(x). Instead, by electron-doping (M = Ti, Zr), the systems reach a critical contentx≈ 0.5 where the latent coupling is triggered, increasingλas high as 70%, in comparison with itsλ(x= 0) value. Our results show thatT_cquickly decreases as a function ofxon the hole-doping region, fromx= 0.2 tox= 0.9, collapsing at the end. Alternatively, for electron-doping,T_cfirst decreases steadily untilx= 0.5, reaching its minimum, but forx> 0.5 it increases rapidly, reaching its maximum value of the entire range at the Sc_0.05Ti_0.95H₂and Y_0.2Zr_0.8H₂solid solutions, demonstrating that electron-doping can improve the superconducting properties of pristine metal hydrides, in the absence of applied pressure.

High-pressure structures of yttrium hydrides

In this work, the crystal structures of YH₃ and YH₄ at high pressure (100-250 GPa) have been explored using a genetic algorithm combined with first-principles calculations. New structures of YH₃ with space group symmetries of P21/m and I4/mmm were predicted. The electronic structures and the phonon dispersion properties of various YH₃ and YH₄ structures at different temperatures and pressures were investigated. Among YH₃ phases, the P21/m structure of YH₃ was found to have a relatively high superconducting transformation temperature T _c of 19 K at 120 GPa, which is reduced to 9 K at 200 GPa. Other YH₃ structures have much lower T _cs. Compared with YH₃, the T _c of the YH₄ compound is much higher, i.e. 94 K at 120 GPa and 55 K at 200 GPa.

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.

Prediction of superconducting iron-bismuth intermetallic compounds at high pressure

We report the discovery of novel iron-bismuth compounds, FeBi₂ and FeBi₃, at high-pressure.

The synthesis of materials in high-pressure experiments has recently attracted increasing attention, especially since the discovery of record breaking superconducting temperatures in the sulfur–hydrogen and other hydrogen-rich systems. Commonly, the initial precursor in a high pressure experiment contains constituent elements that are known to form compounds at ambient conditions, however the discovery of high-pressure phases in systems immiscible under ambient conditions poses an additional materials design challenge. We performed an extensive multi component ab initio structural search in the immiscible Fe–Bi system at high pressure and report on the surprising discovery of two stable compounds at pressures above ≈36 GPa, FeBi₂ and FeBi₃. According to our predictions, FeBi₂ is a metal at the border of magnetism with a conventional electron–phonon mediated superconducting transition temperature of T_c = 1.3 K at 40 GPa.

Electronic structure and electron-phonon coupling in TiH2

Calculations using first principles methods and strong coupling theory are carried out to understand the electronic structure and superconductivity in cubic and tetragonal TiH₂. A large electronic density of states at the Fermi level in the cubic phase arises from Ti-t_2g states and leads to a structural instability towards tetragonal distortion at low temperatures. However, constraining the in-plane lattice constants diminishes the energy gain associated with the tetragonal distortion, allowing the cubic phase to be stable at low temperatures. Calculated phonon dispersions show decoupled acoustic and optic modes arising from Ti and H vibrations, respectively, and frequencies of optic modes to be rather high. The cubic phase has a large electron-phonon coupling parameter λ and critical temperature of several K. Contribution of the hydrogen sublattice to λ is found to be small in this material, which we understand from strong coupling theory to be due to the small H-s DOS at the Fermi level and high energy of hydrogen modes at the tetrahedral sites.

Stability and properties of the Ru–H system at high pressure

The calculated formation enthalpies of RuH_n (n = 1–8) with respect to Ru and H at different pressures.

Pressure induced superconductivity and electronic structure properties of scandium hydrides using first principles calculations

The electronic, vibrational and superconducting properties of scandium hydrides (ScH₂ and ScH₃) under pressure were studied using first-principles calculations.

Pressure-stabilized superconductive yttrium hydrides

The search for high-temperature superconductors has been focused on compounds containing a large fraction of hydrogen, such as SiH₄(H₂)₂, CaH₆ and KH₆. Through a systematic investigation of yttrium hydrides at different hydrogen contents using an structure prediction method based on the particle swarm optimization algorithm, we have predicted two new yttrium hydrides (YH₄ andYH₆), which are stable above 110 GPa. Three types of hydrogen species with increased H contents were found, monatomic H in YH₃, monatomic H+molecular “H₂” in YH₄ and hexagonal “H₆” unit in YH₆. Interestingly, H atoms in YH₆ form sodalite-like cage sublattice with centered Y atom. Electron-phonon calculations revealed the superconductive potential of YH₄ and YH₆ with estimated transition temperatures (T_c) of 84–95 K and 251–264 K at 120 GPa, respectively. These values are higher than the predicted maximal T_c of 40 K in YH₃.

High-pressure hydrogen sulfide from first principles: a strongly anharmonic phonon-mediated superconductor

We use first-principles calculations to study structural, vibrational, and superconducting properties of H_{2}S at pressures P≥200  GPa. The inclusion of zero-point energy leads to two different possible dissociations of H2S, namely 3H2S→2H3S+S and 5H2S→3H3S+HS2, where both H3S and HS2 are metallic. For H3S, we perform nonperturbative calculations of anharmonic effects within the self-consistent harmonic approximation and show that the harmonic approximation strongly overestimates the electron-phonon interaction (λ≈2.64 at 200 GPa) and Tc. Anharmonicity hardens H─S bond-stretching modes and softens H─S bond-bending modes. As a result, the electron-phonon coupling is suppressed by 30% (λ≈1.84 at 200 GPa). Moreover, while at the harmonic level Tc decreases with increasing pressure, the inclusion of anharmonicity leads to a Tc that is almost independent of pressure. High-pressure hydrogen sulfide is a strongly anharmonic superconductor.

The hcp to fcc transformation path of scandium trihydride under high pressure

We used density functional theory to calculate the phase stability of the hcp (hexagonal close packed) and the fcc (face centered cubic) structures of ScH3. The hcp form is stable up to 22 GPa according to the generalized gradient approximation calculation. Then the fcc form becomes energetically more stable. In order to provide insight into the phase transition mechanism, we modeled the hcp to fcc transition by sliding the hcp basal planes, i.e. (001)h planes, in such a way that the ABABAB sequence of the hcp form is altered into the ABCABC sequence of the fcc form. This sliding was suggested by the experiment. The configurations of these sliding steps are our proposed intermediate configurations, whose symmetry group is the Cm group. By using the Cm crystallography, we can match the d-spacings from the lattice planes of the hcp and fcc forms and the intermediate planes measured from the experiment. We also calculated the enthalpy per step, from which the energy barrier between the two phases at various pressures was derived. The barrier at 35 GPa is 0.370 eV per formula or 0.093 eV/atom. The movements of the hydrogen atoms during the hcp to intermediate phase transition are consistent with the result from the Raman spectra.

First-principles theory of anharmonicity and the inverse isotope effect in superconducting palladium-hydride compounds

Palladium hydrides display the largest isotope effect anomaly known in the literature. Replacement of hydrogen with the heavier isotopes leads to higher superconducting temperatures, a behavior inconsistent with harmonic theory. Solving the self-consistent harmonic approximation by a stochastic approach, we obtain the anharmonic free energy, the thermal expansion, and the superconducting properties fully ab initio. We find that the phonon spectra are strongly renormalized by anharmonicity far beyond the perturbative regime. Superconductivity is phonon mediated, but the harmonic approximation largely overestimates the superconducting critical temperatures. We explain the inverse isotope effect, obtaining a -0.38 value for the isotope coefficient in good agreement with experiments, hydrogen anharmonicity being mainly responsible for the isotope anomaly.

Compressed cesium polyhydrides: Cs+ sublattices and H3(-) three-connected nets

The cesium polyhydrides (CsH(n), n > 1) are predicted to become stable, with respect to decomposition into CsH and H2, at pressures as low as 2 GPa. The CsH3 stoichiometry is found to have the lowest enthalpy of formation from CsH and H2 between 30 and 200 GPa. Evolutionary algorithms predict five distinct, mechanically stable, nearly isoenthalpic CsH3 phases consisting of H3(–) molecules and Cs+ atoms. The H3(–) sublattices in two of these adopt a hexagonal three-connected net; in the other three the net is twisted, like the silicon sublattice in the α-ThSi2 structure. The former emerge as being metallic below 100 GPa in our screened hybrid density functional theory calculations, whereas the latter remain insulating up to pressures greater than 250 GPa. The Cs+ cations in the most-stable I4(1)/amd CsH3 phase adopt the positions of the Cs atoms in Cs-IV, and the H3(–) molecules are found in the (interstitial) regions which display a maximum in the electron density.

Superconductive sodalite-like clathrate calcium hydride at high pressures

Hydrogen-rich compounds hold promise as high-temperature superconductors under high pressures. Recent theoretical hydride structures on achieving high-pressure superconductivity are composed mainly of H(2) fragments. Through a systematic investigation of Ca hydrides with different hydrogen contents using particle-swam optimization structural search, we show that in the stoichiometry CaH(6) a body-centered cubic structure with hydrogen that forms unusual "sodalite" cages containing enclathrated Ca stabilizes above pressure 150 GPa. The stability of this structure is derived from the acceptance by two H(2) of electrons donated by Ca forming an "H(4)" unit as the building block in the construction of the three-dimensional sodalite cage. This unique structure has a partial occupation of the degenerated orbitals at the zone center. The resultant dynamic Jahn-Teller effect helps to enhance electron-phonon coupling and leads to superconductivity of CaH(6). A superconducting critical temperature (T(c)) of 220-235 K at 150 GPa obtained from the solution of the Eliashberg equations is the highest among all hydrides studied thus far.

Phase transitions and electron-phonon coupling in platinum hydride

Structural phase transitions and superconducting properties of platinum hydride under pressure are explored through the first-principles calculations based on the density functional theory. Three new low-pressure phases (Pm3m, Cmmm and P4/nmm) are predicted, and all of them are metallic and stable relative to decomposed cases. The superconducting critical temperature of two high-pressure phases correlates with the electron-phonon coupling. The presence of soft modes induced by Kohn anomalies and the hybridization between H and Pt atoms result in the strong electron-phonon coupling. Our results have major implications for other transition metal hydrides under pressure.

Rhodium dihydride (RhH2) with high volumetric hydrogen density

Materials with very high hydrogen density have attracted considerable interest due to a range of motivations, including the search for chemically precompressed metallic hydrogen and hydrogen storage applications. Using high-pressure synchrotron X-ray diffraction technique and theoretical calculations, we have discovered a new rhodium dihydride (RhH(2)) with high volumetric hydrogen density (163.7 g/L). Compressing rhodium in fluid hydrogen at ambient temperature, the fcc rhodium metal absorbs hydrogen and expands unit-cell volume by two discrete steps to form NaCl-typed fcc rhodium monohydride at 4 GPa and fluorite-typed fcc RhH(2) at 8 GPa. RhH(2) is the first dihydride discovered in the platinum group metals under high pressure. Our low-temperature experiments show that RhH(2) is recoverable after releasing pressure cryogenically to 1 bar and is capable of retaining hydrogen up to 150 K for minutes and 77 K for an indefinite length of time.

Cryogenic implementation of charging diamond anvil cells with H2 and D2

A cryogenic loading system for introducing H(2) and D(2) into the diamond anvil cell has been designed and constructed. The integration of pressure loading mechanism, ruby fluorescence spectrometer, and microscope camera allows for in situ tuning and calibrating the pressure. The performance of the system has been demonstrated by successful synthesis of hydride and deuteride of transition metal and rare earth metal. Our cryogenic methodology features facile start-over of loading and in situ electrical resistance measurement of as-synthesized sample.

Predicted formation of superconducting platinum-hydride crystals under pressure in the presence of molecular hydrogen

Noble metals adopt close-packed structures at ambient pressure and rarely undergo structural transformation at high pressures. Platinum (Pt) is normally considered to be unreactive and is therefore not expected to form hydrides under pressure. We predict that platinum hydride (PtH) has a lower enthalpy than its constituents solid Pt and molecular hydrogen at pressures above 21.5 GPa. PtH transforms to a hexagonal close-packed or face-centered cubic (fcc) structure between 70 and 80 GPa. Linear response calculations indicate that PtH is a superconductor at these pressures with a critical temperature of about 10-25 K. These findings help to shed light on recent observations of pressure-induced metallization and superconductivity in hydrogen-rich materials. We show that the formation of fcc noble metal hydrides under pressure is common and examine the possibility of superconductivity in these materials.