Sr‐Doped Superionic Hydrogen Glass: Synthesis and Properties of SrH ₂₂

Guan P., Hemley R.J., Viswanathan V.

Significance Superhydrides are a materials system where near–room-temperature superconductivity has been achieved but only at very high (megabar) pressures. This work proposes an approach that combines pressure and electrochemistry to stabilize superhydrides at moderate pressures. Through a computational study of the palladium–hydrogen system, we construct electrochemical phase diagrams and show that electrochemically synthesizing superhydrides may be possible when combined with moderate pressures. We generalize this to other binary metal superhydrides of interest for superconductivity, including La, Y, and Mg hydrides. Recently, superhydrides have been computationally identified and subsequently synthesized with a variety of metals at very high pressures. In this work, we evaluate the possibility of synthesizing superhydrides by uniquely combining electrochemistry and applied pressure. We perform computational searches using density functional theory and particle swarm optimization calculations over a broad range of pressures and electrode potentials. Using a thermodynamic analysis, we construct pressure–potential phase diagrams and provide an alternate synthesis concept, pressure–potential (P2), to access phases having high hydrogen content. Palladium–hydrogen is a widely studied material system with the highest hydride phase being Pd3H4. Most strikingly for this system, at potentials above hydrogen evolution and ∼ 300 MPa pressure, we find the possibility to make palladium superhydrides (e.g., PdH10). We predict the generalizability of this approach for La-H, Y-H, and Mg-H with 10- to 100-fold reduction in required pressure for stabilizing phases. In addition, the P2 strategy allows stabilizing additional phases that cannot be done purely by either pressure or potential and is a general approach that is likely to work for synthesizing other hydrides at modest pressures.

Chen W., Semenok D., Huang X., Shu H., Li X., Duan D., Cui T., Oganov A.

Wuhao Chen, Dmitrii V. Semenok, Xiaoli Huang, Haiyun Shu, Xin Li, Defang Duan, Tian Cui, and Artem R. Oganov 1 State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China 2 Skolkovo Institute of Science and Technology, Skolkovo Innovation Center, Bolshoy Boulevard 30, bld. 1 Moscow, Russia 121205 3 School of Physical Science and Technology, Ningbo University, Ningbo 315211, China 4 Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, China

Binns J., Hermann A., Peña-Alvarez M., Donnelly M., Wang M., Kawaguchi S.I., Gregoryanz E., Howie R.T., Dalladay-Simpson P.

Description

Semenok D.V., Troyan I.A., Ivanova A.G., Kvashnin A.G., Kruglov I.A., Hanfland M., Sadakov A.V., Sobolevskiy O.A., Pervakov K.S., Lyubutin I.S., Glazyrin K.V., Giordano N., Karimov D.N., Vasiliev A.L., Akashi R., et. al.

Polyhydrides offer intriguing perspectives as high-temperature superconductors. Here we report the high-pressure synthesis of a series of lanthanum-yttrium ternary hydrides: cubic hexahydride $(La,Y)H_{6}$ with a critical temperature $T_{C}$ = 237 +/- 5 K and decahydrides $(La,Y)H_{10}$ with a maximum $T_{C}$ ~${253 K}$ and an extrapolated upper critical magnetic field $B_{C2(0)}$ up to ${135 T}$ at 183 GPa. This is one of the first examples of ternary high-$T_{C}$ superconducting hydrides. Our experiments show that a part of the atoms in the structures of recently discovered ${Im3m}$-$YH_{6}$ and ${Fm3m}$-$LaH_{10}$ can be replaced with lanthanum (~70 %) and yttrium (~25 %), respectively, with a formation of unique ternary superhydrides containing incorporated $La@H_{24}$ and $Y@H_{32}$ which are specific for ${Im3m}$-$LaH_{6}$ and ${Fm3m}$-$YH_{10}$. Ternary La-Y hydrides were obtained at pressures of 170-196 GPa via the laser heating of $P6_{3}$${/mmc}$ lanthanum-yttrium alloys in the ammonia borane medium at temperatures above 2000 K. A novel tetragonal $(La,Y)H_{4}$ was discovered as an impurity phase in synthesized cubic $(La,Y)H_{6}$. The current-voltage measurements show that the critical current density $J_{C}$ in $(La,Y)H_{10}$ may exceed $2500 A/mm^{2}$ at 4.2 K, which is comparable with that for commercial superconducting wires such as ${NbTi}$, $Nb_{3}$${Sn}$. Hydrides that are unstable in a pure form may nevertheless be stabilized at relatively low pressures in solid solutions with superhydrides having the same structure.

Kong P., Minkov V.S., Kuzovnikov M.A., Drozdov A.P., Besedin S.P., Mozaffari S., Balicas L., Balakirev F.F., Prakapenka V.B., Chariton S., Knyazev D.A., Greenberg E., Eremets M.I.

The discovery of superconducting H3S with a critical temperature Tc∼200 K opened a door to room temperature superconductivity and stimulated further extensive studies of hydrogen-rich compounds stabilized by high pressure. Here, we report a comprehensive study of the yttrium-hydrogen system with the highest predicted Tcs among binary compounds and discuss the contradictions between different theoretical calculations and experimental data. We synthesized yttrium hydrides with the compositions of YH3, YH4, YH6 and YH9 in a diamond anvil cell and studied their crystal structures, electrical and magnetic transport properties, and isotopic effects. We found superconductivity in the Im-3m YH6 and P63/mmc YH9 phases with maximal Tcs of ∼220 K at 183 GPa and ∼243 K at 201 GPa, respectively. Fm-3m YH10 with the highest predicted Tc > 300 K was not observed in our experiments, and instead, YH9 was found to be the hydrogen-richest yttrium hydride in the studied pressure and temperature range up to record 410 GPa and 2250 K. The discovery of high temperature superconductivity in hydrogen-rich compounds stimulates further extensive studies. Here, the authors report superconductivity in pressurized yttrium-hydrogen system with highest predicted Tc among binary compounds.

Deng L., Bontke T., Dahal R., Xie Y., Gao B., Li X., Yin K., Gooch M., Rolston D., Chen T., Wu Z., Ma Y., Dai P., Chu C.

Significance As room temperature superconductivity (RTS) has been reported recently in hydrides at megabar pressures, the grand challenge in superconductivity research and development is no longer restricted to further increasing the superconducting transition temperature under extreme conditions and must now include concentrated efforts to lower, and better yet remove, the applied pressure required. This work addresses directly such a challenge by demonstrating our successful retention of pressure-enhanced and/or -induced superconducting phases and/or semiconducting phases without pressure in single crystals of superconducting FeSe and non-superconducting Cu-doped FeSe. The pressure-quenching technique developed in this work offers the possibility of future practical application and the unraveling of RTS recently detected in hydrides but only under high pressures. To raise the superconducting-transition temperature (Tc) has been the driving force for the long-sustained effort in superconductivity research. Recent progress in hydrides with Tcs up to 287 K under pressure of 267 GPa has heralded a new era of room temperature superconductivity (RTS) with immense technological promise. Indeed, RTS will lift the temperature barrier for the ubiquitous application of superconductivity. Unfortunately, formidable pressure is required to attain such high Tcs. The most effective relief to this impasse is to remove the pressure needed while retaining the pressure-induced Tc without pressure. Here, we show such a possibility in the pure and doped high-temperature superconductor (HTS) FeSe by retaining, at ambient pressure via pressure quenching (PQ), its Tc up to 37 K (quadrupling that of a pristine FeSe at ambient) and other pressure-induced phases. We have also observed that some phases remain stable without pressure at up to 300 K and for at least 7 d. The observations are in qualitative agreement with our ab initio simulations using the solid-state nudged elastic band (SSNEB) method. We strongly believe that the PQ technique developed here can be adapted to the RTS hydrides and other materials of value with minimal effort.

Peña-Alvarez M., Binns J., Martinez-Canales M., Monserrat B., Ackland G.J., Dalladay-Simpson P., Howie R.T., Pickard C.J., Gregoryanz E.

By combining pressures up to 50 GPa and temperatures of 1200 K, we synthesize the novel barium hydride, Ba8H46, stable down to 27 GPa. We use Raman spectroscopy, X-ray diffraction, and first-principles calculations to determine that this compound adopts a highly symmetric Pm3¯n structure with an unusual 534:1 hydrogen-to-barium ratio. This singular stoichiometry corresponds to the well-defined type-I clathrate geometry. This clathrate consists of a Weaire-Phelan hydrogen structure with the barium atoms forming a topologically close-packed phase. In particular, the structure is formed by H20 and H24 clathrate cages showing substantially weakened H-H interactions. Density functional theory (DFT) demonstrates that cubic Pm3¯n Ba8H46 requires dynamical effects to stabilize the H20 and H24 clathrate cages.

Snider E., Dasenbrock-Gammon N., McBride R., Wang X., Meyers N., Lawler K.V., Zurek E., Salamat A., Dias R.P.

The recent observation of room-temperature superconductivity will undoubtedly lead to a surge in the discovery of new, dense, hydrogen-rich materials. The rare earth metal superhydrides are predicted to have very high-T_{c} superconductivity that is tunable with changes in stoichiometry or doping. Here we report the synthesis of an yttrium superhydride that exhibits superconductivity at a critical temperature of 262 K at 182±8  GPa. A palladium thin film assists the synthesis by protecting the sputtered yttrium from oxidation and promoting subsequent hydrogenation. Phonon-mediated superconductivity is established by the observation of zero resistance, an isotope effect and the reduction of T_{c} under an external magnetic field. The upper critical magnetic field is 103 T at zero temperature.

Troyan I.A., Semenok D.V., Kvashnin A.G., Sadakov A.V., Sobolevskiy O.A., Pudalov V.M., Ivanova A.G., Prakapenka V.B., Greenberg E., Gavriliuk A.G., Lyubutin I.S., Struzhkin V.V., Bergara A., Errea I., Bianco R., et. al.

AbstractPressure‐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‐TC superconductors—yttrium hexahydride ‐YH6 is reported, which displays a superconducting transition at ≈224 K at 166 GPa. The extrapolated upper critical magnetic field Bc2(0) of YH6 is surprisingly high: 116–158 T, which is 2–2.5 times larger than the calculated value. A pronounced shift of TC in yttrium deuteride YD6 with the isotope coefficient 0.4 supports the phonon‐assisted superconductivity. Current–voltage measurements show that the critical current IC and its density JC 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.

Chen W., Semenok D.V., Kvashnin A.G., Huang X., Kruglov I.A., Galasso M., Song H., Duan D., Goncharov A.F., Prakapenka V.B., Oganov A.R., Cui T.

AbstractFollowing the discovery of high-temperature superconductivity in the La–H system, we studied the formation of new chemical compounds in the barium-hydrogen system at pressures from 75 to 173 GPa. Using in situ generation of hydrogen from NH3BH3, we synthesized previously unknown superhydride BaH12 with a pseudocubic (fcc) Ba sublattice in four independent experiments. Density functional theory calculations indicate close agreement between the theoretical and experimental equations of state. In addition, we identified previously known P6/mmm-BaH2 and possibly BaH10 and BaH6 as impurities in the samples. Ab initio calculations show that newly discovered semimetallic BaH12 contains H2 and H3– molecular units and detached H12 chains which are formed as a result of a Peierls-type distortion of the cubic cage structure. Barium dodecahydride is a unique molecular hydride with metallic conductivity that demonstrates the superconducting transition around 20 K at 140 GPa.

Semenok D.V., Zhou D., Kvashnin A.G., Huang X., Galasso M., Kruglov I.A., Ivanova A.G., Gavriliuk A.G., Chen W., Tkachenko N.V., Boldyrev A.I., Troyan I., Oganov A.R., Cui T.

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 TN of up to 24 K, whereas hexagonal EuH9 and Pm3n-Eu8H46 possess ferromagnetic ordering with TC = 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.

Monacelli L., Errea I., Calandra M., Mauri F.

Hydrogen metallization under stable conditions is a substantial step towards the realization of the first room-temperature superconductor. Recent low-temperature experiments1–3 report different metallization pressures, ranging from 360 GPa to 490 GPa. In this work, we simulate the structural properties and vibrational Raman, infrared and optical spectra of hydrogen phase III, accounting for proton quantum effects. We demonstrate that nuclear quantum fluctuations downshift the vibron frequencies by 25%, introduce a broad lineshape into the Raman spectra and reduce the optical gap by 3 eV. We show that hydrogen metallization occurs at 380 GPa in phase III due to band overlap, in good agreement with transport data2. Our simulations predict that this state is a black metal—transparent in the infrared—so the shiny metal observed at 490 GPa (ref. 1) is not phase III. We predict that the conductivity onset and optical gap will substantially increase if hydrogen is replaced by deuterium, underlining that metallization is driven by quantum fluctuations and is thus isotope-dependent. We show how hydrogen acquires conductivity and brightness at different pressures, explaining the apparent contradictions in existing experimental scenarios1–3. Numerical calculations that include the quantum fluctuations of protons explain the optical properties of hydrogen at high pressure.

Gonze X., Amadon B., Antonius G., Arnardi F., Baguet L., Beuken J., Bieder J., Bottin F., Bouchet J., Bousquet E., Brouwer N., Bruneval F., Brunin G., Cavignac T., Charraud J., et. al.

Abstract Abinit  is a material- and nanostructure-oriented package that implements density-functional theory (DFT) and many-body perturbation theory (MBPT) to find, from first principles, numerous properties including total energy, electronic structure, vibrational and thermodynamic properties, different dielectric and non-linear optical properties, and related spectra. In the special issue to celebrate the 40th anniversary of CPC, published in 2009, a detailed account of Abinit  was included [Gonze et al. (2009)], and has been amply cited. The present article comes as a follow-up to this 2009 publication. It includes an analysis of the impact that Abinit  has had, through for example the bibliometric indicators of the 2009 publication. Links with several other computational materials science projects are described. This article also covers the new capabilities of Abinit  that have been implemented during the last three years, complementing a recent update of the 2009 article published in 2016. Physical and technical developments inside the abinit  application are covered, as well as developments provided with the Abinit  package, such as the multibinit  and a-tdep  projects, and related Abinit  organization developments such as AbiPy  . The new developments are described with relevant references, input variables, tests, and tutorials. Program summary Program Title: Abinit  Program Files doi: http://dx.doi.org/10.17632/csvdrr4d68.1 Licensing provisions: GPLv3 Programming language: Fortran2003, Python Journal reference of previous version: X .Gonze et al, Comput. Phys. Commun. 205 (2016) 106–131 Does the new version supersede the previous version?: Yes. The present 8.10.3 version is now the up-to-date stable version of abinit  , and supercedes the 7.10.5 version. Reasons for the new version: New developments Summary of revisions: • Many new capabilities of the main abinit  application, related to density-functional theory, density-functional perturbation theory, GW, the Bethe-Salpeter equation, dynamical mean-field theory, etc. • New applications in the package: multibinit  (second-principles calculations)and tdep  (temperature-dependent properties) Nature of problem: Computing accurately material and nanostructure properties: electronic structure, bond lengths, bond angles, primitive cell, cohesive energy, dielectric properties, vibrational properties, elastic properties, optical properties, magnetic properties, non-linear couplings, electronic and vibrational lifetimes, etc. For large-scale systems, second-principles calculations, building upon the first-principles results, are also possible. Solution method: Software application based on density-functional theory and many-body perturbation theory, pseudopotentials, with plane waves or wavelets as basis functions. Different real-time algorithms are implemented for second-principles calculations.

Semenok D.V., Kvashnin A.G., Ivanova A.G., Svitlyk V., Fominski V.Y., Sadakov A.V., Sobolevskiy O.A., Pudalov V.M., Troyan I.A., Oganov A.R.

Here we report targeted high-pressure synthesis of two novel high-$T_C$ hydride superconductors, $P6_3/mmc$-$ThH_9$ and $Fm\bar{3}m$-$ThH_{10}$, with the experimental critical temperatures ($T_C$) of 146 K and 159-161 K and upper critical magnetic fields ($\mu$$H_C$) 38 and 45 Tesla at pressures 170-175 Gigapascals, respectively. Superconductivity was evidenced by the observation of zero resistance and a decrease of $T_C$ under external magnetic field up to 16 Tesla. This is one of the highest critical temperatures that has been achieved experimentally in any compounds, along with such materials as $LaH_{10}$, $H_3S$ and $HgBa_2Ca_xCu_2O_{6+z}$. Our experiments show that $fcc$-$ThH_{10}$ has stabilization pressure of 85 GPa, making this material unique among all known high-$T_C$ metal polyhydrides. Two recently predicted Th-H compounds, $I4/mmm$-$ThH_4$ (> 86 GPa) and $Cmc2_1$-$ThH_6$ (86-104 GPa), were also synthesized. Equations of state of obtained thorium polyhydrides were measured and found to perfectly agree with the theoretical calculations. New phases were examined theoretically and their electronic, phonon, and superconducting properties were calculated.

Zhou D., Semenok D.V., Duan D., Xie H., Chen W., Huang X., Li X., Liu B., Oganov A.R., Cui T.

We have successfully realized the synthesis and superconductivity of praseodymium superhydrides above megabar pressures. 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 (Tc) below 9 K and Tc 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.

Yang F., Zhang C., Wang Y., Zhang J., Kong P., Li S., Chen F., Jin Y., Ju M., Gao K.

Shan P., Ma L., Yang X., Li M., Liu Z., Hou J., Jiang S., Zhang L., Shi L., Yang P., Lin C., Wang B., Sun J., Guo H., Ding Y., et. al.

Jiang Q., Duan D., Song H., Zhang Z., Huo Z., Jiang S., Cui T., Yao Y.

AbstractAchieving 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.

He X., Zhao W., Xie Y., Hermann A., Hemley R.J., Liu H., Ma Y.

The recent theory-driven discovery of a class of clathrate hydrides (e.g., CaH 6 , YH 6 , YH 9 , and LaH 10 ) with superconducting critical temperatures ( T c ) 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 H 24 and H 30 hydrogen clathrate cages at megabar pressures. Among these phases, LaSc 2 H 24 shows potential hot superconductivity at the thermodynamically stable pressure range of 167 to 300 GPa, with calculated T c s 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 H 30 as well as H 24 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.

Deng C., Wang M., Huang H., Xu M., Zhao W., Du M., Song H., Cui T.

Zhou D., Semenok D., Galasso M., Alabarse F.G., Sannikov D., Troyan I.A., Nakamoto Y., Shimizu K., Oganov A.R.

AbstractA new method for synthesis of metal polyhydrides via high‐pressure thermal decomposition of corresponding amidoboranes in diamond anvil cells is proposed. Within this approach, molecular semiconducting cesium (P4/nmm‐CsH7, P1‐CsH15+x) and rubidium (RbH9‐x) polyhydrides with a very high hydrogen content reaching 93 at.% are synthesized. Preservation of CsH7 at near ambient conditions, confirmed both experimentally and theoretically, represents a significant advance in the stabilization of hydrogen‐rich compounds. In addition, two crystalline modifications of RbH9‐x with pseudohexagonal and pseudotetragonal structures identified by synchrotron X‐ray diffraction, and Raman measurements are synthesized. Both phases are stable at 8–10 GPa. This is an unprecedentedly low stabilization pressure for polyhydrides. These discoveries open up possibilities for modifying existing hydrogen storage materials to increase their efficiency.

Zhilyaev P., Brekhov K., Mishina E., Tantardini C.

Shutov G.M., Semenok D., Kruglov I.A., Oganov A.R.

Ternary or more complex hydrogen-rich hydrides are the main hope of reaching room-temperature superconductivity at high pressures. Their chemical space is vast and its exploration is challenging. Here we report the investigation of the La–Mg–H ternary system using the evolutionary algorithm USPEX at pressures on the range 150–300 GPa. Several ternary superconducting hydrides were found, including thermodynamically stable P6/mmm-LaMg3H28 with TC = 164 K at 200 GPa, P/2m-LaMgH8, C2/m-La2MgH12 and P2/m-La3MgH16. In addition, novel binary hydrides were predicted to be stable at various pressures, such as Cm–Mg6H11, P1–MgH26, Fmm2-MgH30, P1–MgH38 and R3̄m-LaH13. We also report several novel low-enthalpy metastable phases, both ternary and binary ones. Finally, we demonstrate important methods of exploring very large chemical spaces and show how they can improve crystal structure prediction.

Shuttleworth H.A., Osmond I., Strain C., Binns J., Buhot J., Friedemann S., Howie R.T., Gregoryanz E., Peña-Alvarez M.

Troyan I.A., Semenok D.V., Ivanova A.G., Sadakov A.V., Zhou D., Kvashnin A.G., Kruglov I.A., Sobolevskiy O.A., Lyubutina M.V., Perekalin D.S., Helm T., Tozer S.W., Bykov M., Goncharov A.F., Pudalov V.M., et. al.

AbstractThe chemical interaction of Sn with H2 by X‐ray diffraction methods at pressures of 180–210 GPa is studied. A previously unknown tetrahydride SnH4 with a cubic structure (fcc) exhibiting superconducting properties below TC = 72 K is obtained; the formation of a high molecular C2/m‐SnH14 superhydride and several lower hydrides, fcc SnH2, and C2‐Sn12H18, is also detected. The temperature dependence of critical current density JC(T) in SnH4 yields the superconducting gap 2Δ(0) = 21.6 meV at 180 GPa. SnH4 has unusual behavior in strong magnetic fields: B,T‐linear dependences of magnetoresistance and the upper critical magnetic field BC2(T) ∝ (TC – T). The latter contradicts the Wertheimer–Helfand–Hohenberg model developed for conventional superconductors. Along with this, the temperature dependence of electrical resistance of fcc SnH4 in non‐superconducting state exhibits a deviation from what is expected for phonon‐mediated scattering described by the Bloch‐Grüneisen model and is beyond the framework of the Fermi liquid theory. Such anomalies occur for many superhydrides, making them much closer to cuprates than previously believed.

Li X., Huang X., Cui T.

Abstract This study explores the behavior of ruthenium hydrides under high-pressure conditions through three thermodynamical paths using laser-heated diamond anvil cells. The synthesis of RuH0.9 occurs gradually exceeding the pressure of 23.5 GPa in the ambient temperature path, while RuH is successfully synthesized at pressures above 20 GPa and a temperature of 1500 K. High-temperature conditions are found to reduce the pressure required for synthesis. The results demonstrate that the hydrogen occupancy of octahedral interstitial sites in the ruthenium hydrides is found to reach saturation with complete hydrogen absorption in the high-temperature path. Moreover, the crystallinity of the ruthenium hydride samples improves at higher temperatures, with the grain size increasing from 10 um in the ambient temperature path to submicron in the high-temperature path. However, the predicted RuH6 and RuH3 were not observed in the present work.

He X., Zhang P., Ma Y., Li H., Zhong X., Wang Y., Liu H., Ma Y.

The recently synthesized ${\mathrm{SrH}}_{22}$, with a rich amount of ${\mathrm{H}}_{2}$ units, is predicted with low superconductivity, since two hydrogen (H) atoms in ${\mathrm{H}}_{2}$ units are inclined to stay together by forming a well-known sigma bond, where H electrons tend to occupy the low-lying energy level far below the Fermi energy, resulting in a less H populated Fermi surface. Of particular interest, for ${\mathrm{SrH}}_{22}$ or other similar ${\mathrm{H}}_{2}$-rich hydrides, is to optimize the H electron density of states in the search for high superconductivity. Here, via the strategy of bringing an additional metal element into the binary hydride, in combination with our developed global structure-searching method, we predict a ternary hydride of ${\mathrm{YSrH}}_{22}$. Compared with the parent hydride of ${\mathrm{SrH}}_{22}$, the H electron density of states at the Fermi level of ${\mathrm{YSrH}}_{22}$ is significantly enhanced, due to the favorable charge transfer from metal elements, such as Y, to the antibonding state of the sigma bond of ${\mathrm{H}}_{2}$, where such a bond is broken and H electrons come back to the Fermi surface. Our in-depth analysis indicates that this hydride could be viewed as a substitutional alloy superhydride of $(\mathrm{Y},\mathrm{Sr}){\mathrm{H}}_{11}$ with an estimated superconducting critical temperature ${T}_{c}$ of 240 K at 175 GPa, which is much higher than that of ${\mathrm{SrH}}_{22}$ (${T}_{c}=21\phantom{\rule{0.28em}{0ex}}\mathrm{K})$ and ${\mathrm{LaH}}_{11}$ (${T}_{c}=13\phantom{\rule{0.28em}{0ex}}\mathrm{K}$) both at 200 GPa. Our current findings not only offer a platform to tune the superconductivity of binary superhydrides ${\mathrm{SrH}}_{22}$ and ${\mathrm{LaH}}_{11}$, via the strategy of metal element doping, but also provide a roadmap in the search for high superconductivity, even toward room-temperature superconductivity, in the family of ternary alloy superhydrides.

Zhou D., Semenok D.V., Volkov M.A., Troyan I.A., Seregin A.Y., Chepkasov I.V., Sannikov D.A., Lagoudakis P.G., Oganov A.R., German K.E.

In this work, we synthesize and investigate lower technetium hydrides at pressures up to 45 GPa using synchrotron x-ray diffraction, reflectance spectroscopy, and ab initio calculations. In the Tc-H system, the hydrogen content in $\mathrm{Tc}{\mathrm{H}}_{x}$ phases increases when the pressure rises, and at 27 GPa we found a hexagonal (hcp) nonstoichiometric hydride $\mathrm{Tc}{\mathrm{H}}_{1.3}$. The formation of technetium hydrides is also confirmed by the emergence of a reflective band at 450--600 nm in the reflectance spectra of $\mathrm{Tc}{\mathrm{H}}_{1+x}$ samples synthesized at 45 GPa. On the basis of theoretical analysis, we propose crystal structures for the phases $\mathrm{Tc}{\mathrm{H}}_{0.45\ifmmode\pm\else\textpm\fi{}0.05} ({\mathrm{Tc}}_{16}{\mathrm{H}}_{7})$ and $\mathrm{Tc}{\mathrm{H}}_{0.75\ifmmode\pm\else\textpm\fi{}0.05} ({\mathrm{Tc}}_{4}{\mathrm{H}}_{3})$ previously obtained at 1--2 GPa. Calculations of the electron-phonon interaction show that technetium hydrides $\mathrm{Tc}{\mathrm{H}}_{1+x} (x=0--0.3)$ do not possess superconducting properties due to low electron-phonon interaction parameter $(\ensuremath{\lambda}\ensuremath{\sim}0.23)$.

Zhao W., Song H., Liu Z., Du M., Zhang Z., Liu Z., Jiang Q., Chen L., Duan D., Cui T.

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.

Peña-Alvarez M., Binns J., Marqués M., Kuzovnikov M.A., Dalladay-Simpson P., Pickard C.J., Ackland G.J., Gregoryanz E., Howie R.T.

Through a series of high pressure diamond anvil experiments, we report the synthesis of alkaline earth (Ca, Sr, Ba) tetrahydrides, and investigate their properties through Raman spectroscopy, X-ray diffraction, and density functional theory calculations. The tetrahydrides incorporate both atomic and quasi-molecular hydrogen, and we find that the frequency of the intramolecular stretching mode of the H2δ- units downshifts from Ca to Sr and to Ba upon compression. The experimental results indicate that the larger the host cation, the longer the H2δ- bond. Analysis of the electron localization function (ELF) demonstrates that the lengthening of the H-H bond is caused by the charge transfer from the metal to H2δ- and by the steric effect of the metal host on the H-H bond. This effect is most prominent for BaH4, where the precompression of H2δ- units at 50 GPa results in bond lengths comparable to that of pure H2 above 275 GPa.