Auswahl der wissenschaftlichen Literatur zum Thema „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 (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.

Abstract Room-temperature superconductivity has been a long-held dream and an area of intensive research. The discovery of H3S and LaH10 under 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, H2 and H3 molecular units.

Hydrogen-rich superhydrides are promising high-Tc superconductors, with superconductivity experimentally observed near room temperature, as shown in recently discovered lanthanide superhydrides at very high pressures, e.g., LaH10 at 170 GPa and CeH9 at 150 GPa. Superconductivity is believed to be closely related to the high vibrational modes of the bound hydrogen ions. Here, we studied the limit of extreme pressures (above 200 GPa) where lanthanide hydrides with large hydrogen content have been reported. We focused on LaH16 and CeH16, two prototype candidates for achieving a large electronic contribution from hydrogen in the electron–phonon coupling. In this work, we propose a first-principles calculation platform with the inclusion of many-body corrections to evaluate the detailed physical properties of the Ce–H and La–H systems and to understand the structure, stability, and superconductivity of these systems at ultra-high pressure. We provide a practical approach to further investigate conventional superconductivity in hydrogen-rich superhydrides. We report that density functional theory provides accurate structure and phonon frequencies, but many-body corrections lead to an increase of the critical temperature, which is associated with the spectral weight transfer of the f-states.

Introduction Incited by the possibility of room-temperature superconductors in superhydrides, numerous them have been synthesized using a diamond anvil cell under high pressures up to several hundred GPa. Although palladium has widely researched for a long time as a prototype hydrogen-absorbing metal, the palladium superhydrides has not yet been obtained under such high pressures. A recent theorical calculation has predicted that the palladium superhydrides (e.g., PdH10) may be synthesized by combing electrolysis and high pressure1), but it is not certain whether it can be achieved. In this study, we attempted to synthesize the palladium superhydrides by using electrolytic hydrogen charging under high pressures, and subsequently applied protective coatings to suppress the hydrogen desorption from the palladium hydrides at ambient pressure. Experimental Figure shows a schematic illustration of the high-pressure apparatus. The high-pressure apparatus can apply the pressure of up to 400 MPa to the electrolyte in the electrolytic cell by pumping water into a sealed high-pressure vessel using a plunger pump and pressure multipliers. A cold-rolled palladium foil of 30 ~ 40 μm in thickness was used as a cathode, and a zinc plate was utilized as a soluble anode. Hydrogen was electrochemically loaded into the palladium foil at 0 V vs. Zn in an electrolyte consisting of 0.1 mol dm-3 sulfuric acid and 4.6 mmol dm-3 zinc sulfate under various pressures for 6 hours. Subsequently, the palladium foil was coated with the zinc film at constant current density 25 mA cm-2 for 120 seconds under the high pressure. The specimen was removed from the electrolytic cell in the high-pressure vessel at ambient pressure, and the hydrogen concentration of the palladium hydride PdH x was determined by thermal desorption spectroscopy. Structural analysis was conducted using X-ray diffraction and scanning electron microscopy. Results and discussion A number of bubbles were observed on the palladium foil surface during the electrolytic charging at ambient pressure. On the other hand, no bubbles were observed at 300 MPa. The zinc film with approximately 1.4 μm thick was coated on the PdH x surface after the electrolytic hydrogen charging. The hydrogen concentration of the PdH x was approximately x = 0.7 immediately after synthesis, which decreased to x = 0.2 without the zinc coating. On the other hand, the hydrogen concentration of the PdH x was maintained at least 1 week with the zinc coating. This result indicates that the zinc film acts as a barrier to hydrogen desorption from PdH x , and suggests that if the palladium superhydrides could be synthesized by electrolytic hydrogen charging under high pressure, it could be taken out to ambient pressure while maintaining its concentration. Reference 1) W. Guan, R. J. Hemley, and V. Viswanathan, PNAS, 118, e2110470118 (2021) Figure 1

Ces dix dernières années, une nouvelle chimie de l'hydrogène avec les métaux a été observée sous hautes pressions. Des composés très riches en hydrogène, appelés superhydrures, se forment dans le domaine des 100 GPa. Certaines propriétés remarquables de ces composés ont été mises en évidence comme une supraconductivité BCS à très haute température critique, dans le superhydrure de lanthane (LaH₁₀) avec un sous-réseau en cages d'hydrogène et une supraconductivité à -23° C. Une question très actuelle est de savoir si de tels composés peuvent être stables à pression ambiante et la piste des hydrures ternaires est actuellement explorée. Dans un premier temps, grâce à des calculs de dynamique moléculaire ab initio, nous avons mis en évidence une nouvelle propriété de LaH₁₀ : la superionicité, qui indique une diffusion très rapide des ions hydrures. La superionicité stabilise également LaH₁₀ à très haute température. Cette propriété devrait exister pour d'autres superhydrures. Dans un second temps, nous avons recherché des hydrures ternaires dans le système Y-Fe-H. En comprimant, sous forte pression d'hydrogène dans une presse à enclumes de diamant, le composé de Laves YFe₂, bien connu pour ses capacité de stockage d'hydrogène à pression ambiante, nous avons découvert deux hydrures interstitiels, YFe₂H₆ et YFe₂H₇. Nous avons également démontré une limite à l'incorporation d'hydrogène dans ce type de composés. Ces deux composés ne sont pas stables à pression ambiante. Enfin, à l'aide d'un chauffage laser, nous avons synthétisé l'hydrure ternaire Y₃Fe₄H₂₀ qui a pu être ramené métastable à pression ambiante. La structure et les propriétés de ce superhydrure ont été caractérisées par diffraction X sur monocristal et par calculs ab initio. Une structure inédite pour un hydrure est mise en évidence avec des entités anioniques [FeH₈] reliées entre elles et formant des cages autour des cations d'yttrium. Ce composé est métallique et cette structure pourrait servir de modèle pour trouver un hydrure ternaire supraconducteur stable à pression ambiante
Over the past ten years, a new chemistry of hydrogen with metals has been observed under high pressures. Very hydrogen-rich compounds, called superhydrides, form in the 100 GPa range. Remarkable properties of these compounds have been highlighted, such as BCS superconductivity at very high critical temperatures, like LaH₁₀ with a hydrogen cage sublattice and superconductivity at -23 ° C. A current question is whether such compounds can be stable at ambient pressure, and the path of ternary hydrides is currently being explored. Firstly, using ab initio molecular dynamics calculations, we have revealed a new property of LaH₁₀ : superionicity, which indicates very rapid diffusion of hydride ions. This property should exist for other superhydrides. Secondly, we have searched for ternary hydrides in the Y-Fe-H system. By compressing, under high hydrogen pressure in a diamond anvil press, the Laves phase compound YFe₂, well known for its hydrogen storage capacity at ambient pressure, we discovered two interstitial hydrides, YFe₂H₆ and YFe₂H₇. We also demonstrated a limit to hydrogen incorporation in this type of compound. These two compounds are not stable at ambient pressure. Finally, using laser heating, we synthesized the ternary hydride Y₃Fe₄H₂₀, which was brought back metastable at ambient pressure. The structure and properties of this superhydride were characterized by single-crystal X-ray diffraction and ab initio calculations. An unprecedented structure for a hydride is highlighted with [FeH₈] anionic entities linked to each other and forming cages around yttrium cations. This compound is metallic and this structure could serve as a model to find a ternary hydride superconductor stable at ambient pressure

Abstract This thesis presents a collection of my main results I obtained studying high-pressure superhydrides, using first-principles methods for crystal structure prediction and superconductivity. Superhydrides, i.e. compounds which under high pressure (over a million atmospheres) incorporate a large amount of hydrogen in their crystal structure, are extremely exciting. Among them, superconductors with critical temperatures (Tc’s) close to, or even above room temperature were found. In less than one decade, the study of superhydrides has achieved ground-breaking results, and have given rise to a very active research community. This rapid progress was largely driven by an extremely successful synergy between first-principles calculations based on Density Functional Theory and high-pressure experiments. During the course of my thesis, the focus of the field shifted from finding materials with higher and higher superconducting Tc’s, to the search for ambient-pressure high-Tc superconductors, which may find their way into technological applications. In this regard, the attention is shifting from binary to ternary hydrides, containing two elements other than hydrogen. This thesis, based on the results contained in four papers published in 2018-2021, presents my research eorts in this direction. The main research goal was to deepen the understanding of the essential features that lead to the formation of high-Tc hydride superconductors, with the overar- ching goal of reaching high-Tc superconductivity at ambient pressure. After a general introduction to the history and open questions in the field, I discuss in some detail the theoretical foundations of methods for crystal structure prediction and superconductivity employed in the thesis. These methods are applied in the second part of the thesis to the study of three dierent systems: sodalite- like yttrium hydrides, calcium boron hydrides, and lanthanum ternary hydrides. The hope of the author is that some of the ideas outlined in this thesis will stimulate future research in ternary hydrides.

Abstract This final chapter explores what is needed in the future to realise the superconducting revolution that was promised with the discovery of high temperature superconductors. The holy grail has always been regarded as room temperature superconductivity, and this has recently been achieved in superhydride materials. The problem is that they only work at incredibly high pressures so they are by no means engineering materials. However, having a high critical temperature is not necessarily the most important property for a superconductor to be useful in real applications. How much current it can carry, how much magnetic field it can withstand and how easy it is to manufacture tend to be more important in practice. Applications where superconductors will be crucial in the future are also discussed, including compact nuclear fusion reactors, electric aircraft and quantum computers.

Superconductors are materials that conduct electricity with no resistance below its critical temperature (Tc). To date, pure metals, metal alloys, oxides, hydrides and super hydrides are among structures that have been reported to exhibit excellent superconducting properties due to their unique electronic properties and lattice structure. Most researchers have widely reported on the fabrication, structure, properties and applications of cuprate and iron-based superconducting materials. The modification of cuprate-based and iron-based superconducting materials using lanthanides have shown to massively improve their physico-chemical properties and applications. Investigations on lanthanide superhydride superconductors which contain hydrogen framework structures such as LaH10 and YbH10 are a recent adventure in the field of superconductors. Lanthanide-based structures are considered as potential high temperature superconductors (HTSC) and can be used in high performance applications. The current chapter outlines the advances and prospects observed in lanthanide-based superconductors (LBSC) as modern and fascinating functional materials. There is some literature that has been dedicated to providing a review on superconductors but very few have reported on LBSC. This review chapter provides a general insight of the development of LBSC and their potential technological applications.