Synthesis of clathrate cerium superhydride CeH9 at 80-100 GPa with atomic hydrogen sublattice

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.

Superior Superconducting Properties Realized in Quaternary La-Y-Ce Hydrides at Moderate Pressures

The recent revolution in the superconductivity field stems from hydride superconductors. Multicomponent hydrides provide a crucial platform for tracking high-temperature superconductors. Besides high superconducting transition temperature (Tc), achieving both giant upper critical magnetic field [μ0Hc2(0)] and high critical current density [Jc(0)] is also key to the latent potential of the application for hydride superconductors. In this work, we have successfully synthesized quaternary La-Y-Ce hydrides with excellent properties under moderate pressure by using the concept of "entropy engineering." The obtained temperature dependence of the resistance provides evidence for the superconductivity of Fm3m-(La,Y,Ce)H10, with the maximum Tc ∼ 190 K (at 112 GPa). Notably, Fm3m-(La,Y,Ce)H10 boasts exceptional properties: μ0Hc2(0) reaching 292 T and Jc(0) surpassing 4.61 × 107 A/cm2. Compared with the binary LaH10/YH10, we find that the Fm3m structure in (La,Y,Ce)H10 can be stable at relatively low pressures (112 GPa). These results indicate that multicomponent hydrides can significantly enhance the superconducting properties and regulate stabilizing pressure through the application of "entropy engineering." This work stimulates the experimental exploration of multihydride superconductors and also provides a reference for the search of room-temperature superconductors in more diversified hydride materials in the future.

Ternary superconducting hydrides stabilized via Th and Ce elements at mild pressures

The discovery of covalent H3S and clathrate structure LaH10 with excellent superconducting critical temperatures at high pressures has facilitated a multitude of research on compressed hydrides. However, their superconducting pressures are too high (generally above 150 GPa), thereby hindering their application. In addition, making room-temperature superconductivity close to ambient pressure in hydrogen-based superconductors is challenging. In this work, we calculated the chemically "pre-compressed" Be-H by heavy metals Th and Ce to stabilize the superconducting phase near ambient pressure. An unprecedented ThBeH8 (CeBeH8) with a "fluorite-type" structure was predicted to be thermodynamically stable above 69 GPa (76 GPa), yielding a T c of 113 K (28 K) decompressed to 7 GPa (13 GPa) by solving the anisotropic Migdal-Eliashberg equations. Be-H vibrations play a vital role in electron-phonon coupling and structural stability of these ternary hydrides. Our results will guide further experiments toward synthesizing ternary hydride superconductors at mild pressures.

Realization of Organocerium-Based Fullerene Molecular Materials Showing Mott Insulator-Type Behavior

Electron-rich organocerium complexes (C5Me4H)3Ce and [(C5Me5)2Ce(ortho-oxa)], with redox potentials E1/2 = -0.82 V and E1/2 = -0.86 V versus Fc/Fc+, respectively, were reacted with fullerene (C60) in different stoichiometries to obtain molecular materials. Structurally characterized cocrystals: [(C5Me4H)3Ce]2·C60 (1) and [(C5Me5)2Ce(ortho-oxa)]3·C60 (2) of C60 with cerium-based, molecular rare earth precursors are reported for the first time. The extent of charge transfer in 1 and 2 was evaluated using a series of physical measurements: FT-IR, Raman, solid-state UV-vis-NIR spectroscopy, X-ray absorption near-edge structure (XANES) spectroscopy, and magnetic susceptibility measurements. The physical measurements indicate that 1 and 2 comprise the cerium(III) oxidation state, with formally neutral C60 as a cocrystal in both cases. Pressure-dependent periodic density functional theory calculations were performed to study the electronic structure of 1. Inclusion of a Hubbard-U parameter removes Ce f states from the Fermi level, opens up a band gap, and stabilizes FM/AFM magnetic solutions that are isoenergetic because of the large distances between the Ce(III) cations. The electronic structure of this strongly correlated Mott insulator-type system is reminiscent of the well-studied Ce2O3.

Diverse high-pressure chemistry in Y-NH3BH3 and Y-paraffin oil systems

The yttrium-hydrogen system has gained attention because of near-ambient temperature superconductivity reports in yttrium hydrides at high pressures. We conducted a study using synchrotron single-crystal x-ray diffraction (SCXRD) at 87 to 171 GPa, resulting in the discovery of known (two YH3 phases) and five previously unknown yttrium hydrides. These were synthesized in diamond anvil cells by laser heating yttrium with hydrogen-rich precursors-ammonia borane or paraffin oil. The arrangements of yttrium atoms in the crystal structures of new phases were determined on the basis of SCXRD, and the hydrogen content estimations based on empirical relations and ab initio calculations revealed the following compounds: Y3H11, Y2H9, Y4H23, Y13H75, and Y4H25. The study also uncovered a carbide (YC2) and two yttrium allotropes. Complex phase diversity, variable hydrogen content in yttrium hydrides, and their metallic nature, as revealed by ab initio calculations, underline the challenges in identifying superconducting phases and understanding electronic transitions in high-pressure synthesized materials.

Synthesis and superconductivity in yttrium-cerium hydrides at high pressures

Further increasing the critical temperature and/or decreasing the stabilized pressure are the general hopes for the hydride superconductors. Inspired by the low stabilized pressure associated with Ce 4f electrons in superconducting cerium superhydride and the high critical temperature in yttrium superhydride, we carry out seven independent runs to synthesize yttrium-cerium alloy hydrides. The synthetic process is examined by the Raman scattering and X-ray diffraction measurements. The superconductivity is obtained from the observed zero-resistance state with the detected onset critical temperatures in the range of 97-141 K. The upper critical field towards 0 K at pressure of 124 GPa is determined to be between 56 and 78 T by extrapolation of the results of the electrical transport measurements at applied magnetic fields. The analysis of the structural data and theoretical calculations suggest that the phase of Y0.5Ce0.5H9 in hexagonal structure with the space group of P63/mmc is stable in the studied pressure range. These results indicate that alloying superhydrides indeed can maintain relatively high critical temperature at relatively modest pressures accessible by laboratory conditions.

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.

Stoichiometric Ternary Superhydride LaBeH_{8} as a New Template for High-Temperature Superconductivity at 110 K under 80 GPa

The search for high-temperature superconducting superhydrides has recently moved into a new phase by going beyond extensively probed binary compounds and focusing on ternary ones with vastly expanded material types and configurations for property optimization. Theoretical and experimental works have revealed promising ternary compounds that superconduct at or above room temperature, but it remains a pressing challenge to synthesize stoichiometric ternary compounds with a well-resolved crystal structure that can host high-temperature superconductivity at submegabar pressures. Here, we report on the successful synthesis of ternary LaBeH_{8} obtained via compression in a diamond anvil cell under 110-130 GPa. X-ray diffraction unveils a rocksalt-like structure composing La and BeH_{8} units in the lattice. Transport measurements determined superconductivity with critical temperature T_{c} up to 110 K at 80 GPa, as evidenced by a sharp drop of resistivity to zero and a characteristic shift of T_{c} driven by a magnetic field. Our experiment establishes the first superconductive ternary compound with a resolved crystal structure. These findings raise the prospects of rational development of the class of high-T_{c} superhydrides among ternary compounds, opening greatly expanded and more diverse structural space for exploration and discovery of superhydrides with enhanced high-T_{c} superconductivity.

Magnetic and Electronic Properties of Sr Doped Infinite-Layer NdNiO2 Supercell: A Screened Hybrid Density Functional Study

To understand the influence of doping Sr atoms on the structural, magnetic, and electronic properties of the infinite-layer NdSrNiO₂, we carried out the screened hybrid density functional study on the Nd_9-nSr_nNi₉O₁₈ (n = 0-2) unit cells. Geometries, substitution energies, magnetic moments, spin densities, atom- and lm-projected partial density of states (PDOS), spin-polarized band structures, and the average Bader charges were studied. It showed that the total magnetic moments of the Nd₉Ni₉O₁₈ and Nd₈SrNi₉O₁₈ unit cells are 37.4 and 24.9 emu g^-1, respectively. They are decreased to 12.6 and 4.2 emu g^-1 for the Nd₇Sr₂Ni₉O₁₈-Dia and Nd₇Sr₂Ni₉O₁₈-Par unit cells. The spin density distributions demonstrated that magnetic disordering of the Ni atoms results in the magnetism decrease. The spin-polarized band structures indicated that the symmetry of the spin-up and spin-down energy bands around the Fermi levels also influence the total magnetic moments. Atom- and lm-projected PDOS as well as the band structures revealed that Ni(dx2-y2) is the main orbital intersecting the Fermi level. As a whole, electrons of Sr atoms tend to locate locally and hybridize weakly with the O atoms. They primarily help to build the infinite-layer structures, and influence the electronic structure near the Fermi level indirectly.

Enhancement of superconducting properties in the La-Ce-H system at moderate pressures

Ternary hydrides are regarded as an important platform for exploring high-temperature superconductivity at relatively low pressures. Here, we successfully synthesized the hcp-(La,Ce)H_9-10 at 113 GPa with the initial La/Ce ratio close to 3:1. The high-temperature superconductivity was strikingly observed at 176 K and 100 GPa with the extrapolated upper critical field H_c2(0) reaching 235 T. We also studied the binary La-H system for comparison, which exhibited a T_c of 103 K at 78 GPa. The T_c and H_c2(0) of the La-Ce-H are respectively enhanced by over 80 K and 100 T with respect to the binary La-H and Ce-H components. The experimental results and theoretical calculations indicate that the formation of the solid solution contributes not only to enhanced stability but also to superior superconducting properties. These results show how better superconductors can be engineered in the new hydrides by large addition of alloy-forming elements.

Pressure-induced stability and superconductivity in LuH12 polyhydrides

The phase stability and superconductivity of lutetium polyhydrides under pressure were systematically explored via particle swarm optimization. Several lutetium hydrides, such as LuH, LuH₃, LuH₄, LuH₆, LuH₈, and LuH₁₂, were found to be dynamically and thermodynamically stable. Combined with the electronic properties, there are a large number of H-s states and low density of Lu-f states at the Fermi level, leading to superconductivity. The phonon spectrum and electron-phonon coupling interaction are considered to calculate the superconducting critical temperature (T_c) of stable lutetium hydrides at high pressure. The new predicted cubic LuH₁₂ has the highest T_c value of 187.2 K at 400 GPa in all the stable LuH_n compounds, which was estimated by directly solving the Eliashberg equation. The calculated results provide insights into the design of new superconducting hydrides under pressure.

Giant enhancement of superconducting critical temperature in substitutional alloy (La,Ce)H9

A sharp focus of current research on superconducting superhydrides is to raise their critical temperature T_c at moderate pressures. Here, we report a discovery of giant enhancement of T_c in CeH₉ obtained via random substitution of half Ce by La, leading to equal-atomic (La,Ce)H₉ alloy stabilized by maximum configurational entropy, containing the LaH₉ unit that is unstable in pure compound form. The synthesized (La,Ce)H₉ alloy exhibits T_c of 148–178 K in the pressure range of 97–172 GPa, representing up to 80% enhancement of T_c compared to pure CeH₉ and showcasing the highest T_c at sub-megabar pressure among the known superhydrides. This work demonstrates substitutional alloying as a highly effective enabling tool for substantially enhancing T_c via atypical compositional modulation inside suitably selected host crystal. This optimal substitutional alloying approach opens a promising avenue for synthesis of high-entropy multinary superhydrides that may exhibit further increased T_c at even lower pressures.

Superconductivity was recently discovered in the clathrate hydride CeH₉ with superconducting temperature (T_c) of 57 K at pressures below 1 megabar. Here, the authors show that T_c can be increased to 148 K in the substitutional alloy (La,Ce)H₉, while maintaining a pressure below 1 megabar.

Prediction of Above-Room-Temperature Superconductivity in Lanthanide/Actinide Extreme Superhydrides

Achieving room-temperature superconductivity has been an enduring scientific pursuit driven by broad fundamental interest and enticing potential applications. The recent discovery of high-pressure clathrate superhydride LaH₁₀ with superconducting critical temperatures (T_c) of 250-260 K made it tantalizingly close to realizing this long-sought goal. Here, we report a remarkable finding based on an advanced crystal structure search method of a new class of extremely hydrogen-rich clathrate superhydride MH₁₈ (M: rare-earth/actinide atom) stoichiometric compounds stabilized at an experimentally accessible pressure of 350 GPa. These compounds are predicted to host T_c up to 330 K, which is well above room temperature. The bonding and electronic properties of these MH₁₈ clathrate superhydrides closely resemble those of atomic metallic hydrogen, giving rise to the highest T_c hitherto found in a thermodynamically stable hydride compound. An in-depth study of these extreme superhydrides offers insights for elucidating phonon-mediated superconductivity above room temperature in hydrogen-rich and other low-Z materials.

Machine learning accelerated random structure searching: Application to yttrium superhydrides

The search for new superhydrides, promising materials for both hydrogen storage and high temperature superconductivity, made great progress, thanks to atomistic simulations and Crystal Structure Prediction (CSP) algorithms. When they are combined with Density Functional Theory (DFT), these methods are highly reliable and often match a great part of the experimental results. However, systems of increasing complexity (number of atoms and chemical species) become rapidly challenging as the number of minima to explore grows exponentially with the number of degrees of freedom in the simulation cell. An efficient sampling strategy preserving a sustainable computational cost then remains to be found. We propose such a strategy based on an active-learning process where machine learning potentials and DFT simulations are jointly used, opening the way to the discovery of complex structures. As a proof of concept, this method is applied to the exploration of tin crystal structures under various pressures. We showed that the α phase, not included in the learning process, is correctly retrieved, despite its singular nature of bonding. Moreover, all the expected phases are correctly predicted under pressure (20 and 100 GPa), suggesting the high transferability of our approach. The method has then been applied to the search of yttrium superhydrides (YH x) crystal structures under pressure. The YH6 structure of space group Im-3m is successfully retrieved. However, the exploration of more complex systems leads to the appearance of a large number of structures. The selection of the relevant ones to be included in the active learning process is performed through the analysis of atomic environments and the clustering algorithm. Finally, a metric involving a distance based on x-ray spectra is introduced, which guides the structural search toward experimentally relevant structures. The global process (active-learning and new selection methods) is finally considered to explore more complex and unknown YH x phases, unreachable by former CSP algorithms. New complex phases are found, demonstrating the ability of our approach to push back the exponential wall of complexity related to CSP.

New Insights into Phase Separation of Cerium Hydrides under Pressure

We report theoretical calculations of the static ground-state structures and pressure-induced phase transformations of four cerium hydrides: CeH, CeH₂, CeH_2.5, and CeH₃. Under pressure, the experimental CaF₂-type structure of CeH₂ is likely to disproportionate to face-centered cubic (fcc) Ce and a cubic Pm3̅n (β-UH₃ type) structure of CeH₃ above 6 GPa. At further increasing pressures, fcc Ce will transform to a tetragonal I4/mmm structure above 12 GPa, while CeH₃ moves through the following sequence of phases: Pm3®n (β-UH₃ type) → Pm3̅n (A15 type) → R3®m; the corresponding transition pressures are calculated to be 10 and 70 GPa, respectively. The tetragonal I4₁/amd structure of CeH_2.5 has the similar decomposition as that of CeH₂. Finding this previously unreported pressure-induced decomposition of CeH₂ will pave the way for investigations on the nature of hydrogen-metal interactions.

Structure and Chemical Bonding in Medium-Size Boron Clusters Doped with Praseodymium

After reports of unusually low oxidation states of lanthanide elements in Ln-B clusters and their inverse sandwich geometrical topologies, the interest shifted from boride clusters doped with transition metal (TM) elements to the boride clusters doped with lanthanide atoms. In this work, the results obtained by a combined approach consisting of CALYPSO structure predictions and density functional theory (DFT) calculations for the neutral and anionic PrB_n series, n = 7-16, are reported. A close agreement between our calculated vertical detachment energies and experimental data supports the accuracy of the results obtained. Contrary to the medium-size TM-doped medium boron clusters, which prefer three types of structural configurations, all lowest-energy states of the medium-size Pr-doped boron clusters have half-sandwich geometries. An interesting structural evolution pattern was found for both neutral and anionic PrB_n clusters at n = 7, 10, 13, and 16, which includes quasi-planar B₇ units half-sandwiching the Pr atom. Unusual oxidation numbers of +2 and +1 were found for the Pr atom in the PrB₇^- and PrB₈^- anions, respectively. Chemical bonding analysis for the neutral PrB₇ and PrB₁₃ clusters revealed that their high stability stems from interactions between Pr 5d and B 2p orbitals. A stable tubular-shaped PrB₃₀ cluster is proposed as a promising building block for boron-based nanotubes.

Stabilizing superconductivity of ternary metal pentahydride [Formula: see text] via electronic topological transitions under high pressure from first principles evolutionary algorithm

We explored the phase stability of ternary pentahydride [Formula: see text] based on the first principles evolutionary algorithm. Here, we successfully search for a candidate structure up to 500 GPa. As a consequence, the possible stable structure of [Formula: see text] is found be to a monoclinic structure with space group Pm at a pressure of 50 GPa. Moreover, the orthorhombic structure with a space group of Cmcm is found to be thermodynamically stable above 316 GPa. With this, the Kohn-Sham equation plays a crucial role in determining the structural stability and the electronic structure. Therefore, its structural stability is discussed in term of electronic band structure, Fermi surface topology, and dynamic stability. With these results, we propose that the superconducting transition temperature ([Formula: see text]) of Cmcm structure is estimated to be 50 K at 450 GPa. This could be implied that the proposed Cmcm structure may be emerging as a new class of superconductive ternary metal pentahydride. Our findings pave the way for further studies on an experimental observation that can be synthesized at high pressure.

Compressed superhydrides: the road to room temperature superconductivity

Room-temperature superconductivity has been a long-held dream and an area of intensive research. The discovery of H₃S and LaH₁₀under high pressure, with superconducting critical temperatures (T_c) above 200 K, sparked a race to find room temperature superconductors in compressed superhydrides. In recent groundbreaking work, room-temperature superconductivity of 288 K was achieved in carbonaceous sulfur hydride at 267 GPa. Here, we describe the important attempts of hydrides in the process of achieving room temperature superconductivity in decades, summarize the main characteristics of high-temperature hydrogen-based superconductors, such as hydrogen structural motifs, bonding features, electronic structure as well as electron-phonon coupling etc. This work aims to provide an up-to-date summary of several type hydrogen-based superconductors based on the hydrogen structural motifs, including covalent superhydrides, clathrate superhydrides, layered superhydrides, and hydrides containing isolated H atom, H₂and H₃molecular units.

Materials by design at high pressures

Pressure, a fundamental thermodynamic variable, can generate two essential effects on materials. First, pressure can create new high-pressure phases via modification of the potential energy surface. Second, pressure can produce new compounds with unconventional stoichiometries via modification of the compositional landscape. These new phases or compounds often exhibit exotic physical and chemical properties that are inaccessible at ambient pressure. Recent studies have established a broad scope for developing materials with specific desired properties under high pressure. Crystal structure prediction methods and first-principles calculations can be used to design materials and thus guide subsequent synthesis plans prior to any experimental work. A key example is the recent theory-initiated discovery of the record-breaking high-temperature superhydride superconductors H3S and LaH10 with critical temperatures of 200 K and 260 K, respectively. This work summarizes and discusses recent progress in the theory-oriented discovery of new materials under high pressure, including hydrogen-rich superconductors, high-energy-density materials, inorganic electrides, and noble gas compounds. The discovery of the considered compounds involved substantial theoretical contributions. We address future challenges facing the design of materials at high pressure and provide perspectives on research directions with significant potential for future discoveries.

Theoretical Prediction of Graphene-like 2D Uranyl Material with p-Orbital Antiferromagnetism

Versatile graphene-like two-dimensional materials with s-, p- and d-block elements have aroused significant interests because of their extensive applications while there is a lack of f-block one. Herein we report...

Future Study of Dense Superconducting Hydrides at High Pressure

The discovery of a record high superconducting transition temperature (Tc) of 288 K in a pressurized hydride inspires new hope to realize ambient-condition superconductivity. Here, we give a perspective on the theoretical and experimental studies of hydride superconductivity. Predictions based on the BCS-Eliashberg-Midgal theory with the aid of density functional theory have been playing a leading role in the research and guiding the experimental realizations. To date, about twenty hydrides experiments have been reported to exhibit high-Tc superconductivity and their Tc agree well with the predicted values. However, there are still some controversies existing between the predictions and experiments, such as no significant transition temperature broadening observed in the magnetic field, the experimental electron-phonon coupling beyond the Eliashberg-Midgal limit, and the energy dependence of density of states around the Fermi level. To investigate these controversies and the origin of the highest Tc in hydrides, key experiments are required to determine the structure, bonding, and vibrational properties associated with H atoms in these hydrides.

Electronic Structure and Superconductivity of Compressed Metal Tetrahydrides

Tetrahydrides crystallizing in the ThCr₂ Si₂ structure type have been predicted to become stable for a plethora of metals under pressure, and some have recently been synthesized. Through detailed first-principles investigations we show that the metal atoms within these I 4 / m m m symmetry MH₄ compounds may be divalent, trivalent or tetravalent. The valence of the metal atom and its radius govern the bonding and electronic structure of these phases, and their evolution under pressure. The factors important for enhancing superconductivity include a large number of hydrogenic states at the Fermi level, and the presence of quasi-molecular H 2 δ - units whose bonds have been stretched and weakened (but not broken) via electron transfer from the electropositive metal, and via a Kubas-like interaction with the metal. Analysis of the microscopic mechanism of superconductivity in MgH₄ , ScH₄ and ZrH₄ reveals that phonon modes involving a coupled libration and stretch of the H 2 δ - units leading to the formation of more complex hydrogenic motifs are important contributors towards the electron phonon coupling mechanism. In the divalent hydride MgH₄ , modes associated with motions of the hydridic hydrogen atoms are also key contributors, and soften substantially at lower pressures.

High-Temperature Superconducting Phases in Cerium Superhydride with a T_{c} up to 115 K below a Pressure of 1 Megabar

The discoveries of high-temperature superconductivity in H_{3}S and LaH_{10} have excited the search for superconductivity in compressed hydrides, finally leading to the first discovery of a room-temperature superconductor in a carbonaceous sulfur hydride. In contrast to rapidly expanding theoretical studies, high-pressure experiments on hydride superconductors are expensive and technically challenging. Here, we experimentally discovered superconductivity in two new phases, Fm3[over ¯]m-CeH_{10} (SC-I phase) and P6_{3}/mmc-CeH_{9} (SC-II phase) at pressures that are much lower (<100  GPa) than those needed to stabilize other polyhydride superconductors. Superconductivity was evidenced by a sharp drop of the electrical resistance to zero and decreased critical temperature in deuterated samples and in external magnetic field. SC-I has T_{c}=115  K at 95 GPa, showing an expected decrease in further compression due to the decrease of the electron-phonon coupling (EPC) coefficient λ (from 2.0 at 100 GPa to 0.8 at 200 GPa). SC-II has T_{c}=57  K at 88 GPa, rapidly increasing to a maximum T_{c}∼100  K at 130 GPa, and then decreasing in further compression. According to the theoretical calculation, this is due to a maximum of λ at the phase transition from P6_{3}/mmc-CeH_{9} into a symmetry-broken modification C2/c-CeH_{9}. The pressure-temperature conditions of synthesis affect the actual hydrogen content and the actual value of T_{c}. Anomalously low pressures of stability of cerium superhydrides make them appealing for studies of superhydrides and for designing new superhydrides with stability at even lower pressures.

High-temperature superconductor of sodalite-like clathrate hafnium hexahydride

Hafnium hydrogen compounds have recently become the vibrant materials for structural prediction at high pressure, from their high potential candidate for high-temperature superconductors. In this work, we predict \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {HfH}_{6}$$\end{document}HfH6 by exploiting the evolutionary searching. A high-pressure phase adopts a sodalite-like clathrate structure, showing the body-centered cubic structure with a space group of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Im\bar{3}m$$\end{document}Im3¯m. The first-principles calculations have been used, including the zero-point energy, to investigate the probable structures up to 600 GPa, and find that the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Im\bar{3}m$$\end{document}Im3¯m structure is thermodynamically and dynamically stable. This remarkable result of the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$Im\bar{3}m$$\end{document}Im3¯m structure shows the van Hove singularity at the Fermi level by determining the density of states. We calculate a superconducting transition temperature (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{c}$$\end{document}Tc) using Allen-Dynes equation and demonstrated that it exhibits superconductivity under high pressure with relatively high-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_{c}$$\end{document}Tc of 132 K.

Formation Mechanism of Chemically Precompressed Hydrogen Clathrates in Metal Superhydrides

Recently, the experimental discovery of high-T_c superconductivity in compressed hydrides H₃S and LaH₁₀ at megabar pressures has triggered searches for various superconducting superhydrides. It was experimentally observed that thorium superhydrides, ThH₁₀ and ThH₉, are stabilized at much lower pressures than LaH₁₀. Based on first-principles density functional theory calculations, we reveal that the isolated Th frameworks of ThH₁₀ and ThH₉ have relatively more excess electrons in interstitial regions than the La framework of LaH₁₀. Such interstitial excess electrons easily participate in the formation of the anionic H cage surrounding the metal atom. The resulting Coulomb attraction between cationic Th atoms and anionic H cages is estimated to be stronger than the corresponding one of LaH₁₀, thereby giving rise to larger chemical precompressions in ThH₁₀ and ThH₉. Such a formation mechanism of H clathrates can also be applied to other superhydrides such as CeH₉, PrH₉, and NdH₉. Our findings demonstrate that interstitial excess electrons in the isolated metal frameworks of high-pressure superhydrides play an important role in generating the chemical precompression of H clathrates.

Anomalous High-Temperature Superconductivity in YH6

Pressure-stabilized hydrides are a new rapidly growing class of high-temperature superconductors, which is believed to be described within the conventional phonon-mediated mechanism of coupling. Here, the synthesis of one of the best-known high-T_C superconductors-yttrium hexahydride I m 3 ¯ m -YH₆ is reported, which displays a superconducting transition at ≈224 K at 166 GPa. The extrapolated upper critical magnetic field B_c2 (0) of YH₆ is surprisingly high: 116-158 T, which is 2-2.5 times larger than the calculated value. A pronounced shift of T_C in yttrium deuteride YD₆ with the isotope coefficient 0.4 supports the phonon-assisted superconductivity. Current-voltage measurements show that the critical current I_C and its density J_C may exceed 1.75 A and 3500 A mm^-2 at 4 K, respectively, which is higher than that of the commercial superconductors, such as NbTi and YBCO. The results of superconducting density functional theory (SCDFT) and anharmonic calculations, together with anomalously high critical magnetic field, suggest notable departures of the superconducting properties from the conventional Migdal-Eliashberg and Bardeen-Cooper-Schrieffer theories, and presence of an additional mechanism of superconductivity.

Stabilization and electronic topological transition of hydrogen-rich metal Li5MoH11 under high pressures from first-principles predictions

Regarded as doped binary hydrides, ternary hydrides have recently become the subject of investigation since they are deemed to be metallic under pressure and possibly potentially high-temperature superconductors. Herein, the candidate structure of Li₅MoH₁₁ is predicted by exploiting the evolutionary searching. Its high-pressure phase adopts a hexagonal structure with P6₃/mcm space group. We used first-principles calculations including the zero-point energy to investigate the structures up to 200 GPa and found that the P6₃cm structure transforms into the P6₃/mcm structure at 48 GPa. Phonon calculations confirm that the P6₃/mcm structure is dynamically stable. Its stability is mainly attributed to the isostructural second-order phase transition. Our calculations reveal the electronic topological transition displaying an isostructural second-order phase transition at 160 GPa as well as the topology of its Fermi surfaces. We used the projected crystal orbital Hamilton population (pCOHP) to examine the nature of the chemical bonding and demonstrated that the results obtained from the pCOHP calculation are associated with the electronic band structure and electronic localized function.

Novel Strongly Correlated Europium Superhydrides

We conducted a joint experimental-theoretical investigation of the high-pressure chemistry of europium polyhydrides at pressures of 86-130 GPa. We discovered several novel magnetic Eu superhydrides stabilized by anharmonic effects: cubic EuH₉, hexagonal EuH₉, and an unexpected cubic (Pm3n) clathrate phase, Eu₈H₄₆. Monte Carlo simulations indicate that cubic EuH₉ has antiferromagnetic ordering with T_N of up to 24 K, whereas hexagonal EuH₉ and Pm3n-Eu₈H₄₆ possess ferromagnetic ordering with T_C = 137 and 336 K, respectively. The electron-phonon interaction is weak in all studied europium hydrides, and their magnetic ordering excludes s-wave superconductivity, except, perhaps, for distorted pseudohexagonal EuH₉. The equations of state predicted within the DFT+U approach (U - J were found within linear response theory) are in close agreement with the experimental data. This work shows the great influence of the atomic radius on symmetry-breaking distortions of the crystal structures of superhydrides and on their thermodynamic stability.

Computationally Directed Discovery of MoBi2

Incorporating bismuth, the heaviest element stable to radioactive decay, into new materials enables the creation of emergent properties such as permanent magnetism, superconductivity, and nontrivial topology. Understanding the factors that drive Bi reactivity is critical for the realization of these properties. Using pressure as a tunable synthetic vector, we can access unexplored regions of phase space to foster reactivity between elements that do not react under ambient conditions. Furthermore, combining computational and experimental methods for materials discovery at high-pressures provides broader insight into the thermodynamic landscape than can be achieved through experiment alone, informing our understanding of the dominant chemical factors governing structure formation. Herein, we report our combined computational and experimental exploration of the Mo-Bi system, for which no binary intermetallic structures were previously known. Using the ab initio random structure searching (AIRSS) approach, we identified multiple synthetic targets between 0-50 GPa. High-pressure in situ powder X-ray diffraction experiments performed in diamond anvil cells confirmed that Mo-Bi mixtures exhibit rich chemistry upon the application of pressure, including experimental realization of the computationally predicted CuAl₂-type MoBi₂ structure at 35.8(5) GPa. Electronic structure and phonon dispersion calculations on MoBi₂ revealed a correlation between valence electron count and bonding in high-pressure transition metal-Bi structures as well as identified two dynamically stable ambient pressure polymorphs. Our study demonstrates the power of the combined computational-experimental approach in capturing high-pressure reactivity for efficient materials discovery.

Origin of enhanced chemical precompression in cerium hydride [Formula: see text]

The rare-earth metal hydrides with clathrate structures have been highly attractive because of their promising high-[Formula: see text] superconductivity at high pressure. Recently, cerium hydride [Formula: see text] composed of Ce-encapsulated clathrate H cages was synthesized at much lower pressures of 80-100 GPa, compared to other experimentally synthesized rare-earth hydrides such as [Formula: see text] and [Formula: see text]. Based on density-functional theory calculations, we find that the Ce 5p semicore and 4f/5d valence states strongly hybridize with the H 1s state, while a transfer of electrons occurs from Ce to H atoms. Further, we reveal that the delocalized nature of Ce 4f electrons plays an important role in the chemical precompression of clathrate H cages. Our findings not only suggest that the bonding nature between the Ce atoms and H cages is characterized as a mixture of ionic and covalent, but also have important implications for understanding the origin of enhanced chemical precompression that results in the lower pressures required for the synthesis of [Formula: see text].

Prediction and Synthesis of Dysprosium Hydride Phases at High Pressure

Crystal structure prediction (CSP) methods recently proposed a series of new rare-earth (RE) hydrides at high pressures with novel crystal structures, unusual stoichiometries, and intriguing features such as high-T_c superconductivity. RE trihydrides (REH₃) generally undergo a phase transition from ambient P6₃/mmc or P3̅c1 to Fm3̅m at high pressure. This cubic REH₃ (Fm3̅m) was considered to be a precursor to further synthesize RE polyhydrides such as YH₄, YH₆, YH₉, and CeH₉ with higher hydrogen contents at higher pressures. However, the structural stability and equation of state (EOS) of any of the REH₃ have not been fully investigated at sufficiently high pressures. This work presents high-pressure X-ray diffraction (XRD) measurements in a laser-heated diamond anvil cell up to 100 GPa and ab initio evolutionary CSP of stable phases of DyH₃ up to 220 GPa. Experiments observed the Fm3̅m phase of DyH₃ to be stable at pressures from 17 to 100 GPa and temperatures up to ∼2000 K. After complete decompression, the P3̅c1 and Fm3̅m phases of DyH₃ recovered under ambient conditions. Our calculations predicted a series of phases for DyH₃ at high pressures with the structural phase transition sequence P3̅c1 → Imm2 → Fm3̅m → Pnma → P6₃/mmc at 11, 35, 135, and 194 GPa, respectively. The predicted P3̅c1 and Fm3̅m phases are consistent with experimental observations. Furthermore, electronic band structure calculations were carried out for the predicted phases of DyH₃, including the 4f states, within the DFT+U approach. The inclusion of 4f states shows significant changes in electronic properties, as more Dy d states cross the Fermi level and overlap with H 1s states. The structural phase transition from P3̅c1 to Fm3̅m observed in DyH₃ is systematically compared with other REH₃ compounds at high pressures. The phase transition pressure in REH₃ shows an inverse relation with the ionic radius of RE atoms.

Chemically Tuning Stability and Superconductivity of P-H Compounds

Experimental evidence has revealed superconductivity with a critical temperature, T_c, around 100 K in compressed solid phosphine, but theoretical studies have hitherto found no stable structure in any binary P-H system, leaving the characterization of the new superconductor unsettled. Here we present the findings of an advanced structure search and first-principles calculations unveiling the effect of Li as an electron donor that stabilizes the crystal structure and produces robust phonon-mediated superconductivity in the resulting Li-P-H compounds in wide ranges of stoichiometry and pressure. We showcase a trigonal LiP₂H₁₄ phase that reaches T_c of 169 K at 230 GPa and then decreases with rising pressure, which can be remedied by substituting Li with Be or Na, which considerably enhances T_c. These findings highlight the intricate and effective chemical tuning of stabilizing the crystal structure and enhancing the superconductivity in a distinct class of ternary hydrides, opening new avenues for designing and optimizing new high-T_c hydride superconductors.

Superconducting praseodymium superhydrides

Superhydrides have complex hydrogenic sublattices and are important prototypes for studying metallic hydrogen and high-temperature superconductors. Previous results for LaH10 suggest that the Pr-H system may be especially worth studying because of the magnetism and valence-band f-electrons in the element Pr. Here, we successfully synthesized praseodymium superhydrides (PrH9) in laser-heated diamond anvil cells. Synchrotron x-ray diffraction analysis demonstrated the presence of previously predicted F4 ¯ 3m-PrH9 and unexpected P63/mmc-PrH9 phases. Experimental studies of electrical resistance in the PrH9 sample showed the emergence of a possible superconducting transition (T c) below 9 K and T c dependent on the applied magnetic field. Theoretical calculations indicate that magnetic order and likely superconductivity coexist in a narrow range of pressures in the PrH9 sample, which may contribute to its low superconducting temperature. Our results highlight the intimate connections between hydrogenic sublattices, density of states, magnetism, and superconductivity in Pr-based superhydrides.

High pressure: a feasible tool for the synthesis of unprecedented inorganic compounds

After a simple classification of inorganic materials synthesized at high-temperature and high-pressure, this tutorial reviews the important research results in the field of high-temperature and high-pressure inorganic synthesis in the past 5 years.

Metallization and superconductivity in methane doped by beryllium at low pressure

As one of the simplest hydrocarbons, methane (CH₄) has great potential in the research of superconductors. However, the metallization of CH₄ has been an issue for a long time. Here, we report the structure, metallization, and superconductivity of CH₄ doped by Be at low pressures, based on first-principles calculations. The result shows that the thermodynamically stable BeCH₄ with P1[combining macron] space-group can transform into a metal at ambient pressure. This ternary hydride BeCH₄ exhibits a superconductivity of ∼6 K below 25.6 GPa. Interestingly, the superconducting critical temperature of BeCH₄ can reach ∼30 K at 80 GPa in the form of an a-P1 space-group phase. The charge transfer from Be to CH₄ molecules plays an important role in the superconductivity. Our results present a novel way to realize the metallization of methane at relative pressures and indicate that the doped methane is a potential candidate for seeking high temperature and low pressure superconductivity.

High-Temperature Superconductivity in a Th-H System under Pressure Conditions

New stable phase thorium decahydride Fm3̅ m-ThH₁₀, a high-temperature superconductor with T_C up to 241 K (-32 °C), critical field H_C up to 71 T, and superconducting gap Δ₀ of 52 meV at 80-100 GPa, is predicted by evolutionary algorithm USPEX. Another phase, P2₁/ c-ThH₇, is found to be a superconductor with T_C of 62 K. Analysis of the superconducting state was performed within Eliashberg formalism, and H_C( T), Δ( T), and T_C( P) functions with a jump in the specific heat at critical temperature were calculated. Several other new thorium hydrides were predicted to be stable under pressure, including ThH₃, Th₃H₁₀, ThH₄, and ThH₆. Thorium (which has s² d² electronic configuration) forms high- T_C polyhydrides similar to those formed by s² d¹ metals (Y-La-Ac). Thorium belongs to the Mg-Ca-Sc-Y-La-Ac family of elements forming high- T_C superconducting hydrides.