High-temperature superconductivities and crucial factors influencing the stability of LaThH12 under moderate pressures.

The recent discovery of high-temperature superconductivity in compressed hydrides has reignited the long-standing quest for room-temperature superconductors. However, the synthesis of superconducting hydrides under moderate pressure and the identification of crucial factors that affect their stability remain challenges. Here, we predicted the ternary clathrate phases of LaThH12 with potential superconductivity under high pressures and specifically proposed a novel R3̄c-LaThH12 phase exhibiting a remarkable Tc of 54.95 K at only 30 GPa to address these confusions. Our first-principles studies show that the high-Tc value of Pm3̄m and Cmmm-LaThH12 phases was induced by the strong electron-phonon coupling driven by the synergy of the electron-phonon matrix element and phonon softening caused by Fermi surface nesting. Importantly, we demonstrate the dual effects of enhanced ionic bonding and expanded orbital hybridization between Th-6f and H-sp3 orbitals during depressurization are primary factors governing the dynamic stability of R3̄c-LaThH12 at low pressures. Our findings offer crucial insights into the underlying mechanisms governing low-pressure stability and provide guidance for experimental efforts aimed at realizing hydrogen-based superconductors with both low synthesis pressures and high-Tc.

First-principles study of high-pressure structural phase transition and superconductivity of YBeH8.

The theory-led prediction of LaBeH8, which has a high superconducting critical temperature (Tc) above liquid nitrogen under a pressure level below 1 Mbar, has been experimentally confirmed. YBeH8, which has a structural configuration similar to that of LaBeH8, has also been predicted to be a high-temperature superconductor at high pressure. In this study, we focus on the structural phase transition and superconductivity of YBeH8 under pressure by using first-principles calculations. Except for the known face-centered cubic phase of Fm3̄m, we found a monoclinic phase with P1̄ symmetry. Moreover, the P1̄ phase transforms to the Fm3̄m phase at ∼200 GPa with zero-point energy corrections. Interestingly, the P1̄ phase undergoes a complex electronic phase transition from semiconductor to metal and then to superconducting states with a low Tc of 40 K at 200 GPa. The Fm3̄m phase exhibits a high Tc of 201 K at 200 GPa, and its Tc does not change significantly with pressure. When we combine the method using two coupling constants, λopt and λac, with first-principles calculations, λopt is mainly supplied by the Be-H alloy backbone, which accounts for about 85% of total λ and makes the greatest contribution to the high Tc. These insights not only contribute to a deeper understanding of the superconducting behavior of this ternary hydride but may also guide the experimental synthesis of hydrogen-rich compounds.

Superconductivity of cubic MB6 (M = Na, K, Rb, Cs).

Previous studies have shown that NaB6, KB6, and RbB6 adopting Pm3̄m are superconductors with a relatively high Tc under ambient conditions. In this paper, we conducted systematic structural and related properties research on CsB6 through a genetic evolution algorithm and total energy calculations based on density functional theory between 0 and 20 GPa. Our results reveal a cubic Pm3̄m CsB6, which is dynamically stable under the pressures we studied. We systematically calculated the formation enthalpies, electronic properties, and superconducting properties of Pm3̄m MB6 (M = Na, K, Rb, Cs). They all exhibit metallic features, and boron has high contributions to band structures, density of states, and electron-phonon coupling (EPC). The calculated results about the Helmholtz free energy difference of Pm3̄m CsB6 at 0, 10, and 20 GPa indicate that it is stable upon chemical decomposition (decomposition to simple substances Cs and B) from 0 to 400 K. The phonon density of states indicates that boron atoms occupy the high frequency area. The EPC results show that Pm3̄m CsB6 is a superconductor with Tc = 11.7 K at 0 GPa, close to NaB6 (13.1 K), KB6 (11.7 K), and RbB6 (11.3 K) at 0 GPa in our work, which indicates that boron atoms play an essential role in superconductivity: vibrations of B6 regular octagons lead to the high Tc of Pm3̄m MB6. Our work about Pm3̄m hexaborides provides a supplementary study on the borides of the group IA elements (without Fr and Li) and has an important guiding significance for the experimental synthesis of CsB6.

Pressure-induced high-temperature superconductivity in ternary Y-Zr-H compounds.

Compressed hydrogen-rich compounds have received extensive attention as appealing contenders for superconductors. Here, we found several stable hydrides YZrH6, YZrH8, YZr3H16 and YZrH18, and a series of metastable clathrate hexahydrides in the systematic investigation of Y-Zr-H ternary hydrides under pressure. Electron-phonon coupling calculations indicate that they all exhibit high temperature superconductivity and perform better than the binary Zr-H system. YZrH6 can maintain dynamic stability down to ambient pressure and keep a critical temperature (Tc) of 16 K. The stable YZrH18 and metastable Y3ZrH24 with high hydrogen content exhibit high Tc of 156 K and 185 K at 200 GPa, respectively. Further analysis shows that the phonon modes associated with H atoms contribute significantly to the electron-phonon coupling. The hydrogen content and the stoichiometric ratio of Y and Zr closely affect the density of states at the Fermi level, thereby affecting the superconductivity. Our work presents an important step toward understanding the superconductivity and stability of transition metal ternary hydrides.

Phase diagrams and superconductivity of ternary Ca-Al-H compounds under high pressure.

The search for high-temperature superconductors in hydrides under high pressure has always been a research hotspot. Hydrogen-based superconductors offer an avenue to achieve the long-sought goal of superconductivity at room temperature. Here we systematically explored the high-pressure phase diagram, electronic properties, lattice dynamics and superconductivity of the ternary Ca-Al-H system using ab initio methods. At 80 GPa, CaAlH5 transforms from Cmcm to P21/m phase. Both of Cmcm-CaAlH5 and Pnnm-CaAl2H8 are semiconductors. At 200 GPa, P4/mmm-CaAlH7 and a metastable compound Immm-Ca2AlH12 were found. Furthermore, P4/mmm-CaAlH7 shows obvious softening of the high frequency vibration modes, which improves the strength of electron-phonon coupling. Therefore, a superconducting transition temperature Tc of 71 K is generated in P4/mmm-CaAlH7 at 50 GPa. In addition, the thermodynamic metastable Immm-Ca2AlH12 exhibits a superconducting transition temperature of 118 K at 250 GPa. These results are very useful for the experimental searching of new high-Tc superconductors in ternary hydrides. Our work may provide an opportunity to search for high Tc superconductors at lower pressure.

Revealing the Mechanism of Pressure-Induced Emission in Layered Silver-Bismuth Double Perovskites.

Pressure-induced emission (PIE) associated with self-trapping excitons (STEs) in low-dimensional halide perovskites has attracted great attention for better materials-by-design. Here, using 2D layered double perovskite (C6 H5 CH2 CH2 NH3 + )4 AgBiBr8 as a model system, we advance a fundamental physicochemical mechanism of the PIE from the perspective of carrier dynamics and excited-state behaviors of local lattice distortion. We observed a pressure-driven STE transformation from dark to bright states, corresponding a strong broadband Stokes-shifted emission. Further theoretical analysis demonstrated that the suppressed lattice distortion and enhanced electronic dimensionality in the excited-state play an important role in the formation of stabilized bright STEs, which could manipulate the self-trapping energy and lattice deformation energy to form an energy barrier between the potential energy curves of ground- and excited-state, and enhance the electron-hole orbital overlap, respectively.

Design Principles for High-Temperature Superconductors with a Hydrogen-Based Alloy Backbone at Moderate Pressure.

Hydrogen-based superconductors provide a route to the long-sought goal of room-temperature superconductivity, but the high pressures required to metallize these materials limit their immediate application. For example, carbonaceous sulfur hydride, the first room-temperature superconductor made in a laboratory, can reach a critical temperature (T_{c}) of 288 K only at the extreme pressure of 267 GPa. The next recognized challenge is the realization of room-temperature superconductivity at significantly lower pressures. Here, we propose a strategy for the rational design of high-temperature superconductors at low pressures by alloying small-radius elements and hydrogen to form ternary H-based superconductors with alloy backbones. We identify a "fluorite-type" backbone in compositions of the form AXH_{8}, which exhibit high-temperature superconductivity at moderate pressures compared with other reported hydrogen-based superconductors. The Fm3[over ¯]m phase of LaBeH_{8}, with a fluorite-type H-Be alloy backbone, is predicted to be thermodynamically stable above 98 GPa, and dynamically stable down to 20 GPa with a high T_{c}∼185 K. This is substantially lower than the synthesis pressure required by the geometrically similar clathrate hydride LaH_{10} (170 GPa). Our approach paves the way for finding high-T_{c} ternary H-based superconductors at conditions close to ambient pressures.

Pressure Induced Clathrate Hydrogen-Rich Superconductors KH20 and KH30.

Hydrogen-rich compounds have long been considered as one of the hotspot materials for achieving room-temperature superconductivity. We systematically investigate the high-pressure phase diagram of the K-H system and identified two unreported clathrate extreme superhydrides KH20 and KH30, hosting high superconducting transition temperatures (Tc) of 283 and 243 K at 500 GPa, respectively. The extremely high hydrogen content significantly increases H-derived electronic density of states at the Fermi level, constituting the main contributor to participate in electron-phonon coupling thus producing high-Tc. The large electron localizations in the interstitial region of the metal lattice under high pressure effectively assist the dissociation of hydrogen molecular units, forming unique H36 cages. These results offer key insights into the stability and potential high-Tc superconductivity of compressed extreme superhydrides and will further stimulate related research.

Reply to the 'Comment on "High-temperature superconductivity in transition metallic hydrides MH11 (M = Mo, W, Nb, and Ta) under high pressure"' by X. Zheng and J. Zheng, Phys. Chem. Chem. Phys., 2022, 24, DOI: 10.1039/D1CP01474A.

Our paper is concerned with the specific hydrogen compound MoH11. The authors of the Comment advocate investigating the role of umklapp processes (UP). For the hydrogen compounds, the main contribution to the strength of the pairing interaction is provided not by acoustic, but by optical phonons. This key factor leads to a diminishing role of the UP for the compound of interest.

Structural diversity and hydrogen storage properties in the system K-Si-H.

KSiH3 exhibits 4.1 wt% experimental hydrogen storage capacity and shows reversibility under moderate conditions, which provides fresh impetus to the search for other complex hydrides in the K-Si-H system. Here, we reproduce the stable Fm3̄m phase of K2SiH6 and uncover two denser phases, space groups P3̄m1 and P63mc at ambient pressure, by means of first-principles structure searches. We note that P3̄m1-K2SiH6 has a high hydrogen content of 5.4 wt% and a volumetric density of 88.3 g L-1. Further calculations suggest a favorable dehydrogenation temperature Tdes of -20.1/55.8 °C with decomposition into KSi + K + H2. The higher hydrogen density and appropriate dehydrogenation temperature indicate that K2SiH6 is a promising hydrogen storage material, and our results provide helpful and clear guidance for further experimental studies. We found three further potential hydrogen storage materials stable at high pressure: K2SiH8, KSiH7 and KSiH8. These results suggest the need for further investigations into hydrogen storage materials among such ternary hydrides at high pressure.

Multistep Dissociation of Fluorine Molecules under Extreme Compression.

All elements that form diatomic molecules, such as H_{2}, N_{2}, O_{2}, Cl_{2}, Br_{2}, and I_{2}, are destined to become atomic solids under sufficiently high pressure. However, as revealed by many experimental and theoretical studies, these elements show very different propensity and transition paths due to the balance of reduced volume, lone pair electrons, and interatomic bonds. The study of F under pressure can illuminate this intricate behavior since F, owing to its unique position on the periodic table, can be compared with H, with N and O, and also with other halogens. Nevertheless, F remains the only element whose solid structure evolution under pressure has not been thoroughly studied. Using a large-scale crystal structure search method based on first principles calculations, we find that, before reaching an atomic phase, F solid transforms first into a structure consisting of F_{2} molecules and F polymer chains and then into a structure consisting of F polymer chains and F atoms, a distinctive evolution with pressure that has not been seen in any other elements. Both intermediate structures are found to be metallic and become superconducting, a result that adds F to the elemental superconductors.

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.

Proposed Superconducting Electride Li_{6}C by sp-Hybridized Cage States at Moderate Pressures.

The combination of electride state and superconductivity within the same compound, e.g., [Ca_{24}Al_{28}O_{6}]^{4+}(4e^{-}), opens up a new category of conventional superconductors. However, neither the underlying causations to explain superconducting behaviors nor effects of interstitial quasiatoms (ISQs) on superconductivity remain unclear. Here we have designed an efficient and resource-saving method to identify superconducting electrides only by chemical compositions and bonding characteristics. A representative superconducting electride Li_{6}C with a noteworthy T_{c} of 10 K below 1 Mbar among the known binary electrides has been revealed. Our first-principles studies unveil that the anomalous sp-hybridized cage-state ISQs, as a guest in Li_{6}C, exhibit unexpected ionic and covalent bonds, which act as a chemical precompression to lower dynamically stable pressure. More importantly, we uncover that, contrary to common expectations, the high T_{c} is attributed to the strong electron-phonon coupling derived from the synergy of interatomic coupling effect, phonon softening caused by Fermi surface nesting, and phonon-coupled bands, which are mainly dominated by host sp-hybridized electrons, rather than the ISQs. Our present results elucidate a new superconducting mechanism of electrides and shed light on the way for seeking a high-T_{c} superconductor at lower pressures in cage-state electrides.

Pressure-induced superconducting CS2H10 with an H3S framework.

The discovery of the high-temperature superconducting state in the compounds of hydrogen, carbon and sulfur with a critical temperature (Tc) of 288 K at high pressure is an important milestone towards room-temperature superconductors. Here, we have extensively investigated the high-pressure phases of CS2H10, and found four phases Cmc21, P3m1, P3̄m1 and Pm. Among them, P3m1 can be dynamically stable at a pressure as low as 50 GPa, and Cmc21 has a high Tc of 155 K at 150 GPa. Both Cmc21 and P3m1 are host-guest hydrides, in which CH4 molecules are inserted into Im3̄m-H3S and R3m-H3S sublattices, respectively. Their Tc is dominated by the H3S lattice inside. The insertion of CH4 molecules greatly reduces the pressure required for the stability of the original H3S lattice, but it has a negative impact on superconductivity which cannot be ignored. By studying the effect of CH4 insertion in the H3S lattice, we can design hydrides with a Tc close to that of H3S and a greatly reduced pressure required for stability.

High-temperature superconductivity in transition metallic hydrides MH11 (M = Mo, W, Nb, and Ta) under high pressure.

The discovery of H3S and LaH10 is an important step towards the development of room temperature superconductors which fuels the enthusiasm for finding promising superconductors among hydrides at high pressure. In the present study, three new and stable stoichiometric MoH5, MoH6 and MoH11 compounds were found in the pressure range of 100-300 GPa. The highly hydrogen-rich phase of Cmmm-MoH11 has a layered structure that contains various forms of hydrogen: H, H2- and H3- units. It is a high-Tc material with an estimated Tc value in the range of 165-182 K at 250 GPa. The same structures are also found in NbH11, TaH11, and WH11, each material showing Tc ranging from 117 to 168 K. By combining the method of using two coupling constants λopt and λac, and two characteristic frequencies (optical and acoustic) with first-principle calculations, we found that the high values of Tc are mainly caused by the presence of high frequency optical modes, but the acoustic modes also play a noticeable role.

High-Pressure Synthesis of Magnetic Neodymium Polyhydrides.

Ongoing search for room-temperature superconductivity is inspired by the unique properties of the electron-phonon interaction in metal superhydrides. Encouraged by the recently found highest-TC superconductor fcc-LaH10, here we discover several superhydrides of another lanthanoid, neodymium. We identify three novel metallic Nd-H phases at pressures ranging from 85 to 135 GPa: I4/mmm-NdH4, C2/c-NdH7, and P63/mmc-NdH9, synthesized by laser-heating metal samples in NH3BH3 media for in situ generation of hydrogen. A lower trihydride Fm3̅m-NdH3 is found at pressures from 2 to 52 GPa. I4/mmm-NdH4 and C2/c-NdH7 are stable from 135 to 85 GPa, and P63/mmc-NdH9 is stable from 110 to 130 GPa. Measurements of the electrical resistance of NdH9 demonstrate a possible superconducting transition at ∼4.5 K in P63/mmc-NdH9. Our theoretical calculations predict that all of the neodymium hydrides have antiferromagnetic order at pressures below 150 GPa and represent one of the first discovered examples of strongly correlated superhydrides with large exchange spin-splitting in the electronic band structure (>450 meV). The critical Néel temperatures for new neodymium hydrides are estimated using the mean-field approximation to be about 4 K (NdH4), 251 K (NdH7), and 136 K (NdH9).

Hydrogen Pentagraphenelike Structure Stabilized by Hafnium: A High-Temperature Conventional Superconductor.

The recent discovery of H_{3}S and LaH_{10} superconductors with record high superconducting transition temperatures T_{c} at high pressure has fueled the search for room-temperature superconductivity in the compressed superhydrides. Here we introduce a new class of high T_{c} hydrides with a novel structure and unusual properties. We predict the existence of an unprecedented hexagonal HfH_{10}, with remarkably high value of T_{c} (around 213-234 K) at 250 GPa. As concerns the novel structure, the H ions in HfH_{10} are arranged in clusters to form a planar "pentagraphenelike" sublattice. The layered arrangement of these planar units is entirely different from the covalent sixfold cubic structure in H_{3}S and clathratelike structure in LaH_{10}. The Hf atom acts as a precompressor and electron donor to the hydrogen sublattice. This pentagraphenelike H_{10} structure is also found in ZrH_{10}, ScH_{10}, and LuH_{10} at high pressure, each material showing a high T_{c} ranging from 134 to 220 K. Our study of dense superhydrides with pentagraphenelike layered structures opens the door to the exploration of a new class of high T_{c} superconductors.

Moderate Pressure Stabilized Pentazolate Cyclo-N5- Anion in Zn(N5)2 Salt.

Stabilization of the pentazole anion only by acidic circumstances entrapment impedes the realization of a full-nitrogen substance; however, compression of nitrogen-rich nitrides has been recommend as an alternative way that has more controllable advantages to acquire the atomic nitrogen states. Through the structure searches are in conjunction with first-principle calculations, moderate pressure stabilized nitrogen-rich zinc nitrides with abundant extended nitrogen structures, e.g., cyclo-N5, infinite -(N4)n- chains, three-point stars N(N3), and N2 dumbbells, are predicted. The resonance between alternating σ bonds and π bonds in poly nitrogen sublattices takes charge of the coexistence of single and double bonds. The Zn(N5)2 salt has a noteworthy energy density (6.57 kJ/g) among the reported binary metal nitrides and synthesized pentazolate hydrates. An excellent Vicker's hardness (34 GPa) and detonation performance is unraveled. Although Zn(N5)2 salt is not expected to be recoverable at ambient conditions, it is worth noting that Zn(N5)2 is found to be stable at a very low pressure of ∼30 GPa, which is only half of those pressures required to synthesize CsN5. We clarified that the metal-centering octahedral pentazolate framework was entrapped by dual ionic-covalent bonds. More importantly, the covalent bonding can effectively enhance the chemical insensitivity and thermal stability, further preventing the autodecomposition of monatomic solid N5- anions into dinitrogen. Meanwhile, a unique topological pseudogap that attached to a metastable phase of ZnN4 salt is exposed for the first time, due to the dual effects of strong covalent sp2 hybridization interaction and the origin of ionic states.

Unique Phase Diagram and Superconductivity of Calcium Hydrides at High Pressures.

Structure prediction studies on Ca-H binary systems under high pressures were carried out, and the structures of calcium hydrides in earlier works were reproduced. The previously unreported composition of CaH9 was found to be stable and experienced the phase transition series Cm → P21/ m → C2/ m from 100 to 400 GPa. To the best of our knowledge, CaH9 may be the only alkaline earth hydride with an odd H content. At 400 GPa, the metastable R3̅ m-CaH10 phase shares the same space group with the R3̅ m-SrH10 phase with puckered honeycomb H layers. The C2/ m phase of CaH9 and the R3̅ m phase of CaH10 are excellent superconductors with Tc values of about 240-266 and 157-175 K at 300 and 400 GPa, respectively. The high contributions of H-derived states at the Fermi level play an important role in the superconductivity of calcium hydrides.

High-Pressure Bonding Mechanism of Selenium Nitrides.

The high-pressure phase diagrams of binary Se-N system have been constructed using the CALYPSO method and first-principles calculations. Four stable compounds ( Cmc21-SeN2, P21 /m-SeN3, P1̅-SeN4, and P1̅-SeN5) were identified at high pressures. Various peculiar nitrogen polymerization forms composed of single/double nitrogen-nitrogen bonds were found at the nitrogen-rich condition, such as N∞-chains in P21/ m-SeN3, oligomeric N8-chains in P1̅-SeN4, and distorted N63- anion rings in P1̅-SeN5. Peculiar nitrogen polymerization forms make these compounds potential high-energy-density materials (HEDMs). Especially, P1̅-SeN5 has the highest energy density of 4.08 kJ g-1 among the selenium nitrides. The polymerization mechanism of nitrogen in the Se-N system has been explored using the "Lewis-like" two-center-two-electron and three-center-two-electron bonding analysis. Using the nitrogen-rich P1̅-SeN5 as a prototype, it is found that the famous N6 distortion in the polymerized nitrogen HEDM can be explained by the interatomic mechanical unbalance which is induced by the three-center two-electron bonding between the metal atom and the two neighboring nitrogen atoms.