Potential high-Tc superconducting lanthanum and yttrium hydrides at high pressure

Pressure-induced superconductivity in ternary yttrium borohydride systems

Driven by the excitement surrounding high-temperature superconductivity in hydrides, we systematically investigated the structures, electronic properties, and superconductivity of ternary Y-B-H compounds using first-principles calculations combined with a genetic algorithm for structure search. Five stable phases (YBH, YBH2, YBH10, YB2H6 and YB2H10) were predicted at specified pressures. Several structural building units, such as boron rings, folded boron chains, BH4 tetrahedrons, and dumbbell-shaped B2H6, were observed in these stable phases. The metallic phase C2-YB2H6 with dumbbell-shaped B2H6 units was found to be dynamically stable at 50 GPa and thermodynamically stable at 100 GPa. Compared to the binary B-H superconducting system, the addition of the rare-earth element Y significantly reduces the stabilization pressure for the ternary phases. Electron-phonon coupling calculations showed that C2-YB2H6 and the metastable YBH5 (P3m1 and F4̄3m) and YB2H12 (P1) phases with tetrahedral BH4 units are superconductors at a pressure of 50 GPa. Among them, F4̄3m-YBH5 exhibits strong electron-phonon coupling, which drives the superconducting Tc up to ∼50 K at 50 GPa. This study provides useful guidance for expanding the search directions for conventional superconductors under lower-pressure conditions.

Conventional High-Temperature Superconductivity at Ambient Pressure in Zincblende-Like Light-Element Compounds

To validate the feasibility of high-temperature superconductivity in light-element compounds at ambient pressure, we designed a class of XY4Z4 (Y4Z4 = B4N4, Si4C4, B4P4) structures based on the zincblende configuration. Using high-throughput calculations based on density functional theory, we evaluate 201 compounds and identify 17 materials that are both dynamically and mechanically stable at ambient pressure. These materials demonstrate an insulator-to-metal transition achieved through carrier doping, including 12 superconductors, with 4 of them showing a Tc above 20 K. Notably, LiB4N4 and MgB4P4 stand out with predicted Tc values of 67 and 45 K, respectively, both surpassing the Tc of MgB2. A high electronic density of states at the Fermi level, combined with phonon softening in the low-frequency region, enhances electron-phonon coupling strength. The exploration of strong-bond and lightweight materials will pave the way for achieving high-temperature superconductivity at ambient pressure.

First-principles investigation of the phase diagram and superconducting properties of the Sc-Mg-H system under high pressure

Superconductors, known for their zero electrical resistance and perfect diamagnetism, hold great promise for broad applications. In this study, we employed particle swarm optimization combined with first-principles calculations to predict the structures of Sc-Mg-H compounds under elevated pressure. Our approach identified four stable configurations within the pressure range of 30 to 250 GPa: ScMgH8-P4/mmm (stable from 30 to 250 GPa), ScMgH12-Cmmm (stable from 80 to 250 GPa), Sc2MgH18-P3̄m1 (stable from 110 to 250 GPa), and ScMg2H18-P3̄m1 (stable from 200 to 250 GPa). Through enthalpy calculations, we constructed the pressure-composition phase diagram of the Sc-Mg-H system and explored the stability and superconductivity of these compounds. The superconducting transition temperature (Tc) of Sc2MgH18 reaches 112 K at 150 GPa, ScMgH8 (at 80 GPa) and ScMgH12 (at 100 GPa) have Tc values of 56 K and 87 K, respectively. As the hydrogen content increases, ScMg2H18 requires the highest stabilizing pressure of 200 GPa and has a Tc of 98 K. These findings offer valuable insights for future high-temperature superconductor research and provide theoretical guidance for synthesizing novel materials with superior properties.

Superconducting Lithium Hydride in a Chemical Capacitor Setup: A Theoretical Study

Metallization of the ionic hydride LiH has never been achieved experimentally, even under high external pressure. Herein, a novel "chemical capacitor" setup to facilitate its metallization under ambient pressure conditions is applied. The findings reveal that a single layer of this material can withstand doping levels up to an impressive 0.61 holes per H atom without structural collapse, as demonstrated in the ZrC | LiH | ZrC system. Additionally, the electron-phonon coupling strength (λ) reaches a remarkable value of 2.1 in the TiO | LiH | TiO system, indicative of the strong coupling regime. Superconductivity calculations further predict a maximum critical temperature (T c $T_{\text{c}}$ ) of 17.5 K for 0.31-hole-doped LiH with (LiBaF3)2 as surrounding support layers in the absence of external pressure.

Coumarin-naphthalene conjugate for rapid optical detection of OCl- and Y3+ in a cascade manner: combined experimental and theoretical studies

The coumarin-naphthalene conjugate (A3), an ESIPT-active probe, selectively recognized OCl- in a ratiometric manner in DMSO-water media. The recognition was associated with sky-blue emission (under UV light) as well as yellow emission (under visible light). The OCl- assisted inhibition of the ESIPT process via H-bonding resulted in an intense emission at 484 nm (λ ex = 365 nm). It allowed for the detection of OCl- as low as 18.42 nM with a strong association constant, K = 1.08 × 105 M-1, around physiological pH. Furthermore, the A3-OCl- adduct (Ad1) ratiometrically detected Y3+ via bright orange emission at 556 nm (λ ex = 440 nm) under both UV and visible light. Detection up to 98.51 nM was achieved with a binding constant, K = 1.38 × 105 M-1, at physiological pH. Density functional theory (DFT) and lifetime decay measurements substantiated the interactions. Real sample analysis were also achieved with the developed method.

Ductile copper hydride Eliashberg superconductors with Tc in the liquid-nitrogen temperature range and band topology at ambient pressure

The engineering demands for superconductors require not only a high transition temperature (Tc) but also eco-friendliness, mechanical workability, and abundance. Currently, superconductors exhibiting both mechanical ductility and Tc above the liquid-nitrogen temperature are still lacking. Considering that copper is one of the most important conductive materials for power transmission, we investigate the synthetic routes, band topology, electron-phonon coupling (EPC) and anharmonic superconductivity of copper hydrides using first-principles calculations. Cubic-Cu4H3 remains stable at ambient pressure after kinetic simulations from its experimentally synthesized pressure state. The incorporation of hydrogen impacts the ductility of Cu4H3 negligibly compared to copper, while enabling high-Tc superconductivity up to 77 K and non-trivial band topology at ambient pressure. The novel properties arise from the strong EPC, Fermi surface nesting and hydrogen-induced band inversion. This discovery may fill the gap in the lack of ductile superconductors in the liquid-nitrogen temperature range and pave a new way for realizing high-temperature topological superconductivity at atmospheric pressure.

Hydrogen-Vacancy-Induced Stable Superconducting Niobium Hydride at High Pressure

In recent years, the discovery of unconventional polyhydrides under high pressure, including notable instances like CaH6, YH9, and LaH10, with superconducting critical temperature (Tc) above 200 K, has ignited considerable interest in the quest for high-temperature superconductivity in hydrogen-based materials. Recent studies have suggested the highly probable existence of hydrogen vacancies in these high-Tc superconducting hydrides, although there is no conclusive evidence. In this study, taking niobium (Nb) hydride as a model, we showcase the observation of nonstoichiometric face-centered cubic (fcc) NbH4-δ (δ∼0.23-0.51) at pressures ranging from 113 to 175 GPa, employing in situ high-pressure X-ray diffraction experiments in conjunction with first-principles calculations. Remarkably, our further analyses indicate that the hydrogen vacancies, along with the resulting configurational entropy, play crucial roles in stabilizing this nonstoichiometric fcc NbH4-δ. Electrical transport measurements confirmed the superconductivity, as evidenced by zero resistance as well as suppression of Tc with applying magnetic fields, with a Tc reaching up to 34 K. Our current results not only confirm the presence of hydrogen vacancies in high-Tc hydrides, but also provide key insights into the understanding of hydrogen-vacancy-induced stability for nonstoichiometric hydrides under high pressure.

Mechanism of high-temperature superconductivity in compressed H2-molecular-type hydride

The discovery of compressed atomic-type hydrides offers a promising avenue toward achieving room-temperature superconductivity, but it necessitates extremely high pressures to completely dissociate hydrogen molecules to release free electrons. Here, we report a remarkable finding of compressed H2-molecular-type hydride CaH14 exhibiting an unusual transition temperature (Tc) of 204.0 kelvin. The peculiarity of its electronic structure lies in the pronounced emergence of near-free electrons, which manifest metallic bonding, but molecular hydrogen fragments persist. This finding indicates that the necessary condition for superconducting transition is forming the Fermi sea with Cooper pairs rather than the monatomic hydrogen. Notably, the formation mechanism of free electrons can be effectively explained by the finite-depth potential wells model. Intriguingly, this H2-molecular-type hydride can downgrade the required pressure to 80 gigapascal while maintaining a high Tc of 84 kelvin, well above the liquid-nitrogen temperature. Our study has established a high-temperature superconducting paradigm and opened the prospect for achieving high-Tc superconductors in H2-molecular-type hydrides at low pressure.

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.

Prediction of high-T c superconductivity in heavy rare earth metals compressed Be-H alloy backbone

As the lightest element, hydrogen has the potential to become a room temperature superconductor upon metallization, though achieving this remains a significant challenge. Hydrogen-rich compounds leveraging the hydrogen pre-compression effect may offer promising alternatives for high-temperature superconductivity. In this study, we incorporated heavy rare earth elements into a fluorite-type Be-H alloy framework, resulting in the formation of XBeH8 (where X = Tm, Yb, Lu). This approach led to enhanced critical temperatures (T c ) while maintaining stability at lower pressures. Specifically, TmBeH8 exhibits a T c of 41-48 K at 80 GPa, whereas YbBeH8, which stabilizes at 100 GPa, demonstrates a T c of 134-145 K. LuBeH8 achieves stabilization at 140 GPa and shows a remarkable T c of 228-245 K. Despite LuBeH8 having a much higher T c compared to LaBeH8, it shows lower stability at low pressures than LaBeH8. This research presents a viable pathway for designing high-T c hydride superconductors under relatively moderate pressure conditions.

LoreX: A Low-Energy Region Explorer Boosts Efficient Crystal Structure Prediction

Machine learning has boosted the remarkable development of crystal structure prediction (CSP), greatly accelerating modern materials design. However, slow location of the low-energy regions on the potential energy surface (PES) is still a key bottleneck for the overall search efficiency. Here, we develop a low-energy region explorer (LoreX) to rapidly locate low-energy regions on the PES. This achievement stems from graph-deep-learning-based PES slicing, which classifies structures into different prototypes to divide and conquer the PES. The accuracy and efficiency of LoreX are validated on 27 typical compounds, showing that it correctly locates low-energy regions with only 100 selected samples. The powerful capability of LoreX is demonstrated in solving two challenging problems: discovering new boron allotropes and identifying the puzzling crystal structures of the ordered vacancy compound CuIn5Se8. This study establishes a new method for rapid PES exploration and offers a highly efficient and generally applicable approach to accelerating CSP.

Stability and superconductivity of hexagonal prism-structured polyhydrides X2MgH18 (X = Li, Na, K, Rb, Cs) under moderate pressure

The ultra-high pressure required to maintain hydride superconductors currently limits their further development and practical application, making it urgent to explore stable high-temperature hydride superconductors that can operate at moderate pressures. Here, we conducted an extensive search for various structures with the chemical formula X2MgH18 (X = Li, Na, K, Rb, Cs) and ultimately identified a hexagonal prism hydrogen structure belonging to the space group C2/m. Then, we investigated the stability and superconductivity of these structures. K2MgH18, Rb2MgH18, and Cs2MgH18 were found to be unstable, whereas the structure of Li2MgH18 is dynamically stable and demonstrates superconductivity under pressures of 20 GPa (with a transition temperature of about 110 K) and 235 GPa (with a transition temperature of about 122 K). Additionally, we found that the Na2MgH18 structure, also in the C2/m space group, achieves a superconducting transition temperature of about 105 K at 20 GPa and about 147 K at 235 GPa. We have conducted a comprehensive calculation and analysis of these structures, revealing that the vibrations of hydrogen atoms primarily contribute to the superconducting behavior, and finding that the distortion of the hexagonal prism structure leads to the weakening of the H-H covalent bonds, the strengthening of the metallic bonds, and ultimately results in an increase in the superconducting transition temperature. This work significantly enriches our understanding of the chemistry and superconducting properties of high-pressure hydrides, providing a new idea for further exploration in the field of moderate-pressure hydrides.

Superconductivity and high hardness in scandium-borides under pressure

Exploration of new superconducting or superhard transition-metal borides has attracted extensive interest in the past few decades. In this study, we conducted comprehensive theoretical investigations in the scandium-boron binary system by employing a structural search method based upon first-principles density functional theory. Among the six predicted superconducting scandium-borides, ScB14 (Pm3̄) has the highest superconducting transition temperature Tc = 12.3 K and a Vickers hardness of 12.6 GPa at ambient pressure. The superconductor ScB4 (C2/m) has Tc = 3.6 K and a high Vickers hardness of 25.5 GPa at ambient pressure. Further analysis indicates that the previously synthesized ScB15 (P41) is of superhardness. Our discoveries not only enrich the phase diagram of scandium-borides, but also pave the way for future experimental validations and potential applications of superconducting scandium-borides in industry.

Molecular Hydride Superconductor BiH4 with Tc up to 91 K at 170 GPa

In pursuit of high-Tc hydride superconductors, the molecular hydrides have attracted less attention because the hydrogen quasimolecules are usually inactive for superconductivity. Here, we report on the successful synthesis of a novel bismuth hydride superconductor C2/c-BiH4 at pressures around 170-180 GPa. Its structure comprises bismuth atoms and elongated hydrogen molecules with a H-H bond length of 0.81 Å at 170 GPa, characterizing it as a typical molecular hydride. Transport measurements revealed the occurrence of superconductivity with Tc up to 91 K at 170 GPa, as evidenced by a sharp drop of resistivity to zero and a characteristic downward shift of Tc under magnetic fields. Calculations by density functional theory elucidate that both midfrequency H-derived phonons and low-frequency vibrations from Bi atoms are important for the strong electron-phonon coupling in BiH4, differentiating it from most high-Tc superconducting hydrides. Our work not only places C2/c-BiH4 among the molecular hydride superconductors with the highest Tc but also offers new directions for designing and synthesizing more high-Tc hydride superconductors.

Design of High-Temperature Superconducting Ternary Hydride NaY3H20 at Moderate Pressure via Introducing Hydrogen Vacancies

Superconducting hydrides exhibiting a high critical temperature (Tc) under extreme pressures have garnered significant interest. However, the extremely high pressures required for their stability have limited their practical applications. The current challenge is to identify high-Tc superconducting hydrides that can be stabilized at lower or even ambient pressures. Here, we propose a strategy for designing high-Tc superconducting hydrides at low pressures by introducing defects into the hydrogen frameworks of clathrate hydrides. We present a type of hydrogen-vacancy structural type AB3H20 derived from type-I clathrate hydrides and identified a stable NaY3H20 through high-throughput calculations. Further calculations show that NaY3H20 is thermodynamically stable above 133 GPa and dynamically stable down to 20 GPa, with a predicted high Tc of approximately 115 K. It significantly reduces the pressure required for stability compared to that of type-I clathrate hydrides with high Tc. Our results provide a foundation for further exploration of high-Tc superconducting hydrides at lower pressures or even ambient conditions.

Prediction of high-Tc superconductivity in H6SX below megabar pressure

The discovery of high critical temperature in cubic H3S under high pressures attracts significant attention. Of particular recent interest is to search for superconductors stabilized at low pressure in order to approach practical applications. To examine the doping effect on the stability and superconductivity of H3S, we construct H6SX by replacing half of the S atoms with an X atom (X = Si, Ge, Sn, P, As, Sb, Te, Cl, Br and I). In addition to previously reported compounds, we identified seven new superconductors that are dynamically stable below 200 GPa, e.g., Fd3̄m H6SAs, H6SSb, H6SI, H6SSn, H6STe, Cmmm H6SCl, H6SBr. Among these, H6SSb exhibits Tc of 91 K at 200 GPa, and increases to 115 K as pressure decreases to 40 GPa. Electron-phonon analysis indicates that the coupling between the electron of Sb atoms and the associated low-frequency phonons is responsible for the enhancement of the superconductivity. Additionally, the superconductivity of H6SSX is closely related to the electronegativity of X in the same main group, where X with a larger electronegativity results in a strong H-X bond, resulting in a higher superconductivity.

Possible Superconductivity Transition in Nitrogen-Doped Lutetium Hydride Observed at Megabar Pressure

The pursuit of room-temperature superconductivity at an accessible synthetic pressure has been a long-held dream for both theoretical and experimental physicists. Recently, a controversial report by Dasenbrock-Gammon et al. claims that the nitrogen-doped lutetium trihydride exhibits room-temperature superconductivity at near-ambient pressure. However, many researchers have failed to independently reproduce these results, which has sparked intense skepticism on this report. In this work, a LuH2±xNy sample is fabricated using high-pressure and high-temperature methods. The composition and structural characterization are the same as the aforementioned near-ambient superconductor. In situ X-ray diffraction investigations indicate that a high-pressure phase transition toward Fm3 ¯ $\bar{3}$ m-LuH3±xNy occurred in the sample at 59 GPa. The temperature-dependent resistance measurements reveal that two possible superconductivity transition are observed at 95 GPa, with Tc1 ≈6.5 K for high-Tc phase and Tc2 ≈2.1 K for low-Tc phase, arising from the disparate phases in the sample. Resistivity measurements in the Fm3 ¯ $\bar{3}$ m-LuH3±xNy phase under varying magnetic fields exhibited characteristics consistent with superconductivity, with an upper critical field μ0Hc2(0) of 3.3 T measured at 163 GPa. This work is expected to shed some light on the controversy surrounding superconductivity in the nitrogen-doped lutetium hydride system.

TcESTIME: predicting high-temperature hydrogen-based superconductors

Superconductivity can be considered among the most exciting discoveries in material science of the 20th century. However, the hard conditions for the synthesis and the difficult characterization, make the statement of new high critical temperature (T c) complex from the experimental viewpoint and have recently led to several hot controversies in the literature. In this panorama, theory has become a trustworthy diagnosis. Nevertheless, this comes at an extremely high computational cost. A faster alternative would be to find cheap footprints of superconductivity from the electronic structure. Some of the authors have recently shown that a correlation exists between T c, the networking value [Nature Communications, 12, 5381 (2021)], and the molecularity index [arXiv:2403.07584v1 (2024)]. The networking value reflects the metallicity of the parent compound as a measure of its electron delocalization channels, by means of the Electron Localization Function topology (its bifurcation trees). Instead, the molecularity index quantifies the presence of H2 molecules within the system. All in all, these two quantities characterize bonding features that are related to high T c: high metallicity and low molecularity boost high T c states. However, the quantification or these bonding characteristics was initially made by a visual approach, which is not scalable for high throughput screening. We have developed a new code, TcESTIME, which allows to determine the networking value for a given hydrogen-based compound. In this contribution, we present such code and the underlying periodic algorithms we have developed. As a reference, the estimation of T c for LaH10 thanks to this new code amounts to 10 CPU minutes in a computer cluster equipped with Intel Xeon 2.4 GHz processor. Given the new potential for screening, we have applied it to a larger set including ternary hydrogen based superconductors, and have proposed new fits to estimate T c, leading to errors of ca. 33 K. We believe that this contribution settles the bases for an automatic high-throughput screening of hydrogen-based superconductors.

Theoretical Insights into High-T c Superconductivity of Structurally Ordered YThH18: A First-Principles Study

There has been a marked increase in interest in high-temperature superconductors over the past few years, sparked by their potential to revolutionize multiple fields, including energy generation and transportation. A particularly promising avenue of exploration has emerged in the form of ternary superhydrides, compounds composed of hydrogen along with two other rare-earth elements. Our investigation focuses on the search for Y-Th-H ternary compounds; employing an evolutionary search methodology complemented by electron-phonon calculations reveals a stable superhydride, P6̅m2-YThH18, capable of exhibiting a critical temperature (T c) as high as 222 K at 200 GPa along a few low-T c novel hydrides. Our analysis explores the possibility of alloyed structure formation from the disordered condition of Th-doped YH9 and establishes that the P6̅m2-YThH18 is indeed a structurally ordered structure. This opens up an exciting avenue for research on multinary superhydrides, which could facilitate experimental synthesis and provides potential implications for high-temperature superconductivity research.

Data-assimilated crystal growth simulation for multiple crystalline phases

To determine crystal structures from a powder x-ray diffraction (PXRD) pattern containing multiple unknown phases, a data-assimilated crystal growth (DACG) simulation method has been developed. The PXRD penalty function selectively stabilizes the structures in the experimental data, promoting their grain growth during simulated annealing. Since the PXRD pattern is calculated as the Fourier transform of the pair distribution function, the DACG simulation can be performed without prior determination of the lattice parameters. We applied it to carbon (graphite and diamond) and SiO2 (low-quartz, low-cristobalite, and coesite) systems, demonstrating that the DACG simulation successfully reproduced multiple crystal structures.

Ultrafast dynamics evidence of strong coupling superconductivity in LaH10±δ

Recently, tremendous research interest has been aroused in clathrate superhydrides. However, their microscopic properties, especially the superconducting (SC) gap and electron-phonon coupling (EPC) strength, are largely unexplored experimentally. Here, we investigate the time-resolved ultrafast spectroscopy of a superconductor LaH10±δ under an ultrahigh pressure of 165 GPa. By analyzing the ultrafast dynamics of the quasiparticles, we experimentally obtain the SC gap Δ(0) = 53 ± 5 meV, revealing a gap ratio 2Δ(0)/kBTc = 5.6 and a gap parameter ϑ = 1.95. Significantly, we experimentally estimate λ ⟨ Ω 2 ⟩  = (2.4 ± 0.1) × 104 (meV)2, which corresponds to an EPC strength λ = 2.58 ± 0.11. These results together provide direct experimental evidence that strong EPC is responsible for the near-room-temperature superconductivity in clathrate superhydrides. Our investigation significantly advances the experimental exploration of superhydrides, and contributes to the ultrafast dynamics investigations of quantum materials under high pressures.

Introducing electron correlation in solid-state calculations for superconducting states

Analyzing the electronic localization of superconductors has been recently shown to be relevant for understanding their critical temperature [Nature Communications, 12, 5381, (2021)]. However, these relationships have only been shown at the Kohn-Sham density functional theory (DFT) level, where the onset of strong correlation linked to the superconducting state is missing. In this contribution, we approximate the superconducting gap in order to reconstruct the superconducting the one-reduced density matrix (1RDM) from a DFT calculation. This allows us to analyse the electron density and localization in the strong correlation regime. The method is applied to two well-known superconductors. Electron localization features along the electron-phonon coupling directions and hydrogen cluster formations are observed for different solids. However, in both cases we see that the overall localization channels are not affected by the onset of superconductivity, explaining the ability of DFT localization channels to characterize the superconducting ones.

Designing multicomponent hydrides with potential high Tc superconductivity

While hydrogen-rich materials have been demonstrated to exhibit high Tc superconductivity at high pressures, there is an ongoing search for ternary, quaternary, and more chemically complex hydrides that achieve such high critical temperatures at much lower pressures. First-principles searches are impeded by the computational complexity of solving the Eliashberg equations for large, complex crystal structures. Here, we adopt a simplified approach using electronic indicators previously established to be correlated with superconductivity in hydrides. This is used to study complex hydride structures, which are predicted to exhibit promisingly high critical temperatures for superconductivity. In particular, we propose three classes of hydrides inspired by the Fm[Formula: see text]m RH[Formula: see text] structures that exhibit strong hydrogen network connectivity, as defined through the electron localization function. The first class [RH[Formula: see text]X[Formula: see text]Y] is based on a Pm[Formula: see text]m structure showing moderately high Tc, where the Tc estimate from electronic properties is compared with direct Eliashberg calculations and found to be surprisingly accurate. The second class of structures [(RH[Formula: see text])[Formula: see text]X[Formula: see text]YZ] improves on this with promisingly high density of states with dominant hydrogen character at the Fermi energy, typically enhancing Tc. The third class [(R[Formula: see text]H[Formula: see text])(R[Formula: see text]H[Formula: see text])X[Formula: see text]YZ] improves the strong hydrogen network connectivity by introducing anisotropy in the hydrogen network through a specific doping pattern. These design principles and associated model structures provide flexibility to optimize both Tc and the structural stability of complex hydrides.

High-Temperature Superconductivity in Perovskite Hydride Below 10 GPa

Hydrogen and hydride materials have long been considered promising materials for high-temperature superconductivity. However, the extreme pressures required for the metallization of hydrogen-based superconductors limit their applications. Here, a series of high-temperature perovskite hydrides is designed that can be stable within 10 GPa. The research covered 182 ternary systems and ultimately determined that eight new compounds are stable within 20 GPa, of which five exhibited superconducting transition temperatures exceeding 120 K within 10 GPa, including KGaH3 (146 K at 10 GPa), RbInH3 (130 K at 6 GPa), CsInH3 (153 K at 9 GPa), RbTlH3 (170 K at 4 GPa) and CsTlH3 (163 K at 7 GPa). Excitingly, KGaH3 and RbGaH3 are thermodynamically stable at 50 GPa. Among these perovskite hydrides, alkali metals are responsible for providing a fixed amount of charge and supporting alloy framework composed of hydrogen and IIIA group elements to maintain stable crystal structure, while the cubic hydrogen alloy framework formed by IIIA group elements and hydrogen is crucial for high-temperature superconductivity. This work will inspire further experimental exploration and take an important step in the exploration of low-pressure stable high-temperature superconductors.

Hydrogen-Doped c-BN as a Promising Path to High-Temperature Superconductivity Above 120 K at Ambient Pressure

Finding high-temperature superconductivity in light-weight element containing compounds at atmosphere pressure is currently a research hotspot but has not been reached yet. Here it is proposed that hard or superhard materials can be promising candidates to possess the desirable high-temperature superconductivity. By studying the electronic structures and superconducting properties of H and Li doped c-BN within the framework of the first-principles, it is demonstrated that the doped c-BN are indeed good superconductors at ambient pressure after undergoing the phase transition from the insulating to metallic behavior, though holding different nature of metallization. Li doped c-BN is predicted to exhibit the superconducting transition temperature of ≈58 K, while H doped c-BN has stronger electron-phonon interaction and possesses a higher transition temperature of 122 K. These results and findings thus point out a new direction for exploring the ambient-pressure higher-temperature superconductivity in hard or superhard materials.

Superconducting Electride Li9S with a Transition Temperature above the McMillan Limit

An electride, characterized by unique interstitial anionic electrons (IAEs), offers promising avenues for modulating its superconductivity. The pressure-dependent coupling between IAEs and orbital electrons significantly affects the superconducting transition temperature (Tc). However, existing research has predominantly concentrated on pressures within 300 GPa. To advance the understanding, we propose investigating the Li-S system under ultrahigh pressure to unveil novel electride superconductors. Five stable Li-rich electrides with diverse IAE topologies, including one Li7S, three Li9S, and one Li12S phases, are identified through structural search calculations. Among the Li9S phases, in the C2/c phase (600 GPa), the IAEs are connected to the S atomic extra-nuclear electrons with the unconventional d orbital attribute due to the extreme pressure, while two low-pressure R-3 (25 GPa) and C2/m (400 GPa) phases have interconnected IAEs. Due to its unique IAE attributes, C2/c Li9S exhibits the highest Tc of 53.29 K at 600 GPa. Its superconductivity results from the coupling of the S d, Li p electrons, and IAEs with the low-frequency phonons associated with the attraction between IAEs and the Li-S framework. Our work enhances insights into IAEs within electrides and their role in facilitating superconductivity.

Computational electron-phonon superconductivity: from theoretical physics to material science

The search for room-temperature superconductors is a major challenge in modern physics. The discovery of copper-oxide superconductors in 1986 brought hope but also revealed complex mechanisms that are difficult to analyze and compute. In contrast, the traditional electron-phonon coupling (EPC) mechanism facilitated the practical realization of superconductivity (SC) in metallic hydrogen. Since 2015, the discovery of new hydrogen compounds has shown that EPC can enable room-temperature SC under high pressures, driving extensive research. Advances in computational capabilities, especially exascale computing, now allow for the exploration of millions of materials. This paper reviews newly predicted superconducting systems in 2023-2024, focusing on hydrides, boron-carbon systems, and compounds with nitrogen, carbon, and pure metals. Although many computationally predicted high-Tcsuperconductors were not experimentally confirmed, some low-temperature superconductors were successfully synthesized. This paper provides a review of these developments and future research directions.

Superconductivity in Diamond-Like BC15

Advancing the compositional space of a compound class can result in intriguing superconductors, such as LaH10. Herein, we performed a comprehensive first-principles structural search on a binary B-C system with various chemical compositions. The identified diamond-like BC15, named d-BC15, is thermodynamically superior to the synthesized BC3 and BC5. Interestingly, d-BC15 shows anisotropic superconductivity resulting from three distinct Fermi surfaces. Its predicted critical temperature (Tc) is 43.6 K at ambient pressure, beyond the McMillan limit. d-BC15 reaches a maximum of around 75 K at 0.43% hole doping due to the substantially enhanced density of states at the Fermi level. Additionally, d-BC15 demonstrates superhard characteristics with a Vickers hardness of 75 GPa. The calculated tensile and shear stresses are 72 and 73 GPa, respectively. The combination of high superconductivity and superhardness in d-BC15 offers new insights into the design of multifunctional materials.

Ultrafast Yttrium Hydride Chemistry at High Pressures via Non-equilibrium States Induced by an X-ray Free Electron Laser

Controlling the formation and stoichiometric content of the desired phases of materials has become of central interest for a variety of fields. The possibility of accessing metastable states by initiating reactions by X-ray-triggered mechanisms over ultrashort time scales has been enabled by the development of X-ray free electron lasers (XFELs). Utilizing the exceptionally high-brilliance X-ray pulses from the EuXFEL, we report the synthesis of a previously unobserved yttrium hydride under high pressure, along with nonstoichiometric changes in hydrogen content as probed at a repetition rate of 4.5 MHz using time-resolved X-ray diffraction. Exploiting non-equilibrium pathways, we synthesize and characterize a hydride in a Weaire-Phelan structure type at pressures as low as 125 GPa, predicted using a crystal structure search, with a hydrogen content of 4.0-5.75 hydrogens per cation, that is enthalpically metastable on the convex hull.

Structural Features, Chemical Diversity, and Physical Properties of Microporous Sodalite-Type Materials: A Review

This review contains data on a wide class of microporous materials with frameworks belonging to the sodalite topological type. Various methods for the synthesis of these materials, their structural and crystal chemical features, as well as physical and chemical properties are discussed. Specific properties of sodalite-related materials make it possible to consider they as thermally stable ionic conductors, catalysts and catalyst carriers, sorbents, ion exchangers for water purification, matrices for the immobilization of radionuclides and heavy metals, hydrogen and methane storage, and stabilization of chromophores and phosphors. It has been shown that the diversity of properties of sodalite-type materials is associated with the chemical diversity of their frameworks and extra-framework components, as well as with the high elasticity of the framework.

A perspective on reducing stabilizing pressure for high-temperature superconductivity in hydrides

The theoretical predictions and experimental syntheses of hydrogen sulfide (H3S) have ignited a surge of research interest in hydride superconductors. Over the past two decades, extensive investigations have been conducted on hydrides with the ultimate goal of achieving room-temperature superconductivity under ambient conditions. In this review, we present a comprehensive summary of the current strategies and progress towards this goal in hydride materials. We conclude their electronic characteristics, hydrogen atom aggregation forms, stability mechanisms, and more. While providing a real-time snapshot of the research landscape, our aim is to offer deeper insights into reducing the stabilizing pressure for high-temperature superconductors in hydrides. This involves defining key long-term theoretical and experimental opportunities and challenges. Although achieving high critical temperatures for hydrogen-based superconductors still requires high pressure, we remain confident in the potential of hydrides as candidates for room-temperature superconductors at ambient pressure.

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.

Molecular hydrogen in the N-doped LuH3 system as a possible path to superconductivity

The discovery of ambient superconductivity would mark an epochal breakthrough long-awaited for over a century, potentially ushering in unprecedented scientific and technological advancements. The recent findings on high-temperature superconducting phases in various hydrides under high pressure have ignited optimism, suggesting that the realization of near-ambient superconductivity might be on the horizon. However, the preparation of hydride samples tends to promote the emergence of various metastable phases, marked by a low level of experimental reproducibility. Identifying these phases through theoretical and computational methods entails formidable challenges, often resulting in controversial outcomes. In this paper, we consider N-doped LuH3 as a prototypical complex hydride: By means of machine-learning-accelerated force-field molecular dynamics, we have identified the formation of H2 molecules stabilized at ambient pressure by nitrogen impurities. Importantly, we demonstrate that this molecular phase plays a pivotal role in the emergence of a dynamically stable, low-temperature, experimental-ambient-pressure superconductivity. The potential to stabilize hydrogen in molecular form through chemical doping opens up a novel avenue for investigating disordered phases in hydrides and their transport properties under near-ambient conditions.

Emerging superconductivity rules in rare-earth and alkaline-earth metal hydrides

Hydrides of alkaline-earth and rare-earth metals have garnered significant interest in high-temperature superconductor research due to their excellent electron-phonon coupling and high T c upon pressurization. This study explores the electronic structures and electron-phonon coupling of metal hydrides XHn (n = 4,6), where X includes Ca, Mg, Sc, and Y. The involvement of d-orbital electrons alters the Fermi surface, leading to saddle-point nesting and a charge density wave (CDW) phase transition, which opens the superconducting gap. For instance, in YH6, the exchange coupling between Y-4d and H-1s holes in the phonon softening region results in T c values up to 230 K. The study suggests that factors, such as the origin of the CDW order, hydrogen concentration, and d-orbital contributions are crucial to superconductivity. This work proposes a new rule for high T c superconductors, emphasizing the importance of double gaps and electron-phonon interactions at exchange coupling sites, and predicts potential high-quality superconductors among rare-earth hydrides.

Discovery of novel materials through machine learning

Experimental exploration of new materials relies heavily on a laborious trial-and-error approach. In addition to substantial time and resource requirements, traditional experiments and computational modelling are typically limited in finding target materials within the enormous chemical space. Therefore, creating innovative techniques to expedite material discovery becomes essential. Recently, machine learning (ML) has emerged as a valuable tool for material discovery, garnering significant attention due to its remarkable advancements in prediction accuracy and time efficiency. This rapidly developing computational technique accelerates the search and optimization process and enables the prediction of material properties at a minimal computational cost, thereby facilitating the discovery of novel materials. We provide a comprehensive overview of recent studies on discovering new materials by predicting materials and their properties using ML techniques. Beginning with an introduction of the fundamental principles of ML methods, we subsequently examine the current research landscape on the applications of ML in predicting material properties that lead to the discovery of novel materials. Finally, we discuss challenges in employing ML within materials science, propose potential solutions, and outline future research directions.

Superconductivity in CaH 6 and ThH 10 through fully ab initio Eliashberg method and self-consistent Green's function

Pressurized hydrogen-based superconductors are phonon-mediated superconductors that exhibit high phonon frequencies. In these superconductors, in addition to the density of states (DOS) at the Fermi energy ( E F ), the energy dependence of the DOS around E F becomes important for evaluating their transition temperature ( T c ). Systems with peak structures in the DOS around E F , such as I m 3 ¯ m H3 S and F m 3 ¯ m LaH10 , highlight this point. We use the fully ab initio Eliashberg method to investigate this phenomenon in I m 3 ¯ m CaH6 and F m 3 ¯ m ThH10 with a dip structure in their DOS around E F . Our calculated T c values (225-235 K for CaH6 at 200 GPa and 156-158 K for ThH10 at 170 GPa) are quantitatively consistent with the experimental results. Remarkably, our results from the self-consistent treatment of the electron Green's function contrasts with those cases with a peak structure in the DOS. This finding unifies the understanding of how DOS structures influence the evaluation of T c .

A new perspective on ductile high-Tcsuperconductors under ambient pressure: few-hydrogen metal-bonded hydrides

Superconducting materials have garnered widespread attention due to their zero-resistance characteristic and complete diamagnetism. After more than 100 years of exploration, various high-temperature superconducting materials including cuprates, nickelates, iron-based compounds, and ultra-high pressure multi-hydrides have been discovered. However, the practical application of these materials is severely hindered by their poor ductility and/or the need for high-pressure conditions to maintain structural stability. To address these challenges, we first provide a new thought to build high-temperature superconducting materials based on few-hydrogen metal-bonded hydrides under ambient pressure. We then review the related research efforts in this article. Moreover, based on the bonding type of atoms, we classify the existing important superconducting materials and propose the new concepts of pseudo-metal and quasi-metal superconductivity, which are expected to be helpful for the design of new high-temperature superconducting materials in the future.

Prediction of potential high-temperature superconductivity in ternary Y-Hf-H compounds under high pressure

Compressed ternary alloy superhydrides are currently considered to be the most promising competitors for high-temperature superconducting materials. Here, the stable stoichiometries in the Y-Hf-H ternary system under pressure are comprehensively explored in theory and four fresh phases are predicted: Pmna-YHfH6 and P4/mmm-YHfH7 at 200 GPa, P4/mmm-YHfH8 at 300 GPa and P-6m2-YHfH18 at 400 GPa. The four Y-Hf-H ternary phases are thermodynamically and dynamically stable at corresponding pressure. In addition, structural features, bonding characteristics, electronic properties, and superconductivity of the four ternary Y-Hf-H phases are systematically calculated and discussed. As the hydrogen content and the density of states of H atoms at the Fermi level increase, the superconducting transition temperatures (Tc) of Y-Hf-H system are significantly enhanced. The P-6m2-YHfH18 with high hydrogen content exhibits a high calculated Tc value of 130 K at 400 GPa.

High-Tcsuperconductivity in doped molecular superconductors ofK4B8-xMxH32(M = C, N) under high pressure

Stabilized and metallic light elements hydrides have provided a potential route to achieve the goal of room-temperature superconductors at moderate or ambient pressures. Here, we have performed systematic DFT theoretical calculations to examine the effects of different light elements C and N atoms doped in cubic K4B8H32hydrides on the superconductivity at low pressures. As a result of various atoms substituting, we have found that metallic K4B_{8-x}MxH32(M = C, N) hydrides are dynamically stable at 50 GPa, band structures and density of states (DOS) indicate that sizeableTccorrelates with a high B-H DOS at the Fermi level. With the increasing of B atoms in K4B_{8-x}MxH32hydrides, the DOS values at Fermi level have been improved due to the delocalized electrons in B-H bonds, which result in strong electron-phonon coupling (EPC) interaction and increase theTcfrom 19.04 to 77.07 K for KC2H8and KB2H8at 50 GPa. The NH4unit in stable K4B7NH32hydrides has weakened the EPC and led to lowTcvalue of 21.47 K. Our results suggest the light elements hydrides KB2H8and K4B7CH32could estimate highTcvalues at 50 GPa, and the boron hydrides would be potential candidates to design or modulate hydrides superconductors with highTcat moderate or ambient pressures.

Novel ground state structures of N-doped LuH3

Ab-initiocrystal structure searches have played a pivotal role in recent discoveries of high-Tc hydride superconductors under high pressure. Using evolutionary crystal searches, we predict novel ground state structures of N-doped LuH3at ambient conditions. We find an insulating ground state structure for LuN0.125H2.875(∼1.0 wt.% N), contrary to earlier studies where assumed structures were all metallic. This insulating behavior of ground state was found to persist up to ∼45 GPa. However our crystal structure searches revealed a metallic state for an H-deficient variant of LuN0.125H2.875. We study bonding characteristics of important structures by calculating electronic density of states, electronic-localization functions and Bader charges. Our Bader charge analysis shows that insulators have both H+and H-ions whereas metals have only H-ions. We find that H+ions are bonded to N atomsviaa very short covalent bond. Thus we identify a clear relationship between formation of N-H bonds and insulating behavior of materials. Besides this, we perform crystal structure searches for three more compositions with higher N-content (>1.0 wt.%). Analysis of electronic properties shows that the ground states of these compositions are insulator.

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.

Anharmonic and quantum effects in Pm3̄ AlM(M = Hf, Zr)H6 under high pressure: A first-principles study

First-principles calculations were employed to investigate the impact of quantum ionic fluctuations and lattice anharmonicity on the crystal structure and superconductivity of Pm3̄ AlM(M = Hf, Zr)H6 at pressures of 0.3-21.2 GPa (AlHfH6) and 4.7-39.5 GPa (AlZrH6) within the stochastic self-consistent harmonic approximation. A correction is predicted for the crystal lattice parameters, phonon spectra, and superconducting critical temperatures, previously estimated without considering ionic fluctuations on the crystal structure and assuming the harmonic approximation for lattice dynamics. The findings suggest that quantum ionic fluctuations have a significant impact on the crystal lattice parameters, phonon spectra, and superconducting critical temperatures. Based on our anharmonic phonon spectra, the structures will be dynamically stable at 0.3 GPa for AlHfH6 and 6.2 GPa for AlZrH6, ∼6 and 7 GPa lower than pressures given by the harmonic approximation, respectively. Due to the anharmonic correction of their frequencies, the electron-phonon coupling constants (λ) are suppressed by 28% at 11 GPa for AlHfH6 and 22% at 30 GPa for AlZrH6, respectively. The decrease in λ causes Tc to be overestimated by ∼12 K at 11 GPa for AlHfH6 and 30 GPa for AlZrH6. Even if the anharmonic and quantum effects are not as strong as those of Pm3̄n-AlH3, our results also indicate that metal hydrides with hydrogen atoms in interstitial sites are subject to anharmonic effects. Our results will inevitably stimulate future high-pressure experiments on synthesis, structural, and conductivity measurements.

Symmetry-Shaped Singularities in High-Temperature Superconductor H3S

The superconducting critical temperature of H3S ranks among the highest measured, at 203 K. This impressive value stems from a singularity in the electronic density-of-states, induced by a flat-band region that consists of saddle points. The peak sits right at the Fermi level, so that it gives rise to a giant electron-phonon coupling constant. In this work, we show how atomic orbital interactions and space group symmetry work in concert to shape the singularity. The body-centered cubic Brillouin Zone offers a unique 2D hypersurface in reciprocal space: fully connecting squares with two different high-symmetry points at the corners, Γ and H, and a third one in the center, N. Orbital mixing leads to the collapse of fully connected 1D saddle point lines around the square centers, due to a symmetry-enforced s-p energy inversion between Γ and H. The saddle-point states are invariably nonbonding, which explains the unconventionally weak response of the superconductor's critical temperature to pressure. Although H3S appears to be a unique case, the theory shows how it is possible to engineer flat bands and singularities in 3D lattices through symmetry considerations.

Superconductivity with T c of 116 K discovered in antimony polyhydrides

Superconductivity (SC) was experimentally observed for the first time in antimony polyhydride. The diamond anvil cell combined with a laser heating system was used to synthesize the antimony polyhydride sample at high pressure and high temperature. In-situ high pressure transport measurements as a function of temperature with an applied magnetic field were performed to study the SC properties. It was found that the antimony polyhydride samples show superconducting transition with critical temperature T c 116 K at 184 GPa. The investigation of SC at magnetic field revealed the superconducting coherent length of ∼40 Å based on the Ginzburg Landau (GL) equation. Antimony polyhydride superconductor has the second highest T c in addition to sulfur hydride among the polyhydrides of elements from main groups IIIA to VIIA in the periodic table.

Structural Phase Transition and Decomposition of XeF2 under High Pressure and Its Formation of Xe-Xe Covalent Bonds

Noble gases with inert chemical properties have rich bonding modes under high pressure. Interestingly, Xe and Xe form covalent bonds, originating from the theoretical simulation of the pressure-induced decomposition of XeF2, which has yet to be experimentally confirmed. Moreover, the structural phase transition and metallization of XeF2 under high pressure have always been controversial. Therefore, we conducted extensive experiments using a laser-heated diamond anvil cell technique to investigate the above issues of XeF2. We propose that XeF2 undergoes a structural phase transition and decomposition above 84.1 GPa after laser heating, and the decomposed product Xe2F contains Xe-Xe covalent bonds. Neither the pressure nor temperature alone could bring about these changes in XeF2. With our UV-vis absorption experiment, I4/mmm-XeF2 was metalized at 159 GPa. This work confirms the existence of Xe-Xe covalent bonds and provides insights into the controversy surrounding XeF2, enriching the research on noble gas chemistry under high pressure.

Pressure-induced hydrogen-dominant high-temperature superconductors

The century-old pursuit of room temperature superconductivity has finally been reached in highly compressed hydrogen-dominant compounds. Future efforts will be focused on understanding the high-pressure hydrogen physics and ambient-pressure applications.

Clathrate metal superhydrides under high-pressure conditions: enroute to room-temperature superconductivity

Room-temperature superconductivity has been a long-held dream of mankind and a focus of considerable interest in the research field of superconductivity. Significant progress has recently been achieved in hydrogen-based superconductors found in superhydrides (hydrides with unexpectedly high hydrogen contents) that are stabilized under high-pressure conditions and are not capturable at ambient conditions. Of particular interest is the discovery of a class of best-ever-known superconductors in clathrate metal superhydrides that hold the record for high superconductivity (e.g. T c = 250-260 K for LaH10) among known superconductors and have great promise to be those that realize the long-sought room-temperature superconductivity. In these peculiar clathrate superhydrides, hydrogen forms unusual 'clathrate' cages containing encaged metal atoms, of which such a kind was first reported in a calcium hexa-superhydride (CaH6) showing a measured high T c of 215 K under a pressure of 170 GPa. In this review, we aim to offer an overview of the current status of research progress on the clathrate metal superhydride superconductors, discuss the superconducting mechanism and highlight the key features (e.g. structure motifs, bonding features, electronic structure, etc.) that govern the high-temperature superconductivity. Future research direction along this line to find room-temperature superconductors will be discussed.

Superconducting ternary hydrides: progress and challenges

Since the discovery of the high-temperature superconductors H3S and LaH10 under high pressure, compressed hydrides have received extensive attention as promising candidates for room-temperature superconductors. As a result of current high-pressure theoretical and experimental studies, it is now known that almost all the binary hydrides with a high superconducting transition temperature (T c) require extremely high pressure to remain stable, hindering any practical application. In order to further lower the stable pressure and improve superconductivity, researchers have started exploring ternary hydrides and had many achievements in recent years. Here, we discuss recent progress in ternary hydrides, aiming to deepen the understanding of the key factors regulating the structural stability and superconductivity of ternary hydrides, such as structural motifs, bonding features, electronic structures, electron-phonon coupling, etc. Furthermore, the current issues and challenges of superconducting ternary hydrides are presented, together with the prospects and opportunities for future research.

Predicted hot superconductivity in LaSc2H24 under pressure

The recent theory-driven discovery of a class of clathrate hydrides (e.g., CaH6, YH6, YH9, and LaH10) with superconducting critical temperatures (Tc) well above 200 K has opened the prospects for "hot" superconductivity above room temperature under pressure. Recent efforts focus on the search for superconductors among ternary hydrides that accommodate more diverse material types and configurations compared to binary hydrides. Through extensive computational searches, we report the prediction of a unique class of thermodynamically stable clathrate hydrides structures consisting of two previously unreported H24 and H30 hydrogen clathrate cages at megabar pressures. Among these phases, LaSc2H24 shows potential hot superconductivity at the thermodynamically stable pressure range of 167 to 300 GPa, with calculated Tcs up to 331 K at 250 GPa and 316 K at 167 GPa when the important effects of anharmonicity are included. The very high critical temperatures are attributed to an unusually large hydrogen-derived density of states at the Fermi level arising from the newly reported peculiar H30 as well as H24 cages in the structure. Our predicted introduction of Sc in the La-H system is expected to facilitate future design and realization of hot superconductors in ternary clathrate superhydrides.

Structural and electronic properties of clathrate-like hydride: MH6 and MH9 (M = Sc, Y, La)

CONTEXT

The addition of central metal atoms to hydrogen clathrate structures is thought to provide a certain amount of "internal chemical pressure" to offset some of the external physical pressure required for compound stability. The size and valence of the central atoms significantly affect the minimum pressure required for the stabilization of hydrogen-rich compounds and their superconducting transition temperature. In recent years, many studies have calculated the minimum stable pressure and superconducting transition temperature of compounds with H24, H29, and H32 hydrogen clathrates, with centrally occupied metal atoms. In order to investigate the stability and physical properties of compounds with H cages in which the central atoms change in the same third group B, herein, based on first-principles calculations, we systematically investigated the lattice parameters, crystal volume, band structures, density of states, Mulliken analysis, charge density, charge density difference, and electronic localization function in I m 3 ¯ m -MH6 and P63/mmc-MH9 systems with different centered rare earth atoms M (M = Sc, Y, La) under a series of pressures. We find that for MH9, the pressure mainly changes the crystal lattice parameters along the c-axis, and the contributions of the different H atoms in MH9 to the Fermi level are H3 > H1 > H2. The density of states at the Fermi level of MH6 is mainly provided by H 1 s. Moreover, the size of the central atom M is particularly important for the stability of the crystal. By observing a series of properties of the structures with H24 and H29 cages wrapping the same family of central atoms under a series of pressures, our theoretical study is helpful for further understanding the formation mechanism of high-temperature superconductors and provides a reference for future research and design of high-temperature superconductors.

METHODS

The first principles based on the density functional theory and density functional perturbation theory were employed to execute all calculations by using the CASTEP code in this work.