Rare Earth Hydrides Research Articles

High-Temperature Superconducting Phase in Clathrate Calcium Hydride CaH_{6} up to 215K at a Pressure of 172GPa.

The recent discovery of superconductive rare earth and actinide superhydrides has ushered in a new era of superconductivity research at high pressures. This distinct type of clathrate metal hydrides was first proposed for alkaline-earth-metal hydride CaH_{6} that, however, has long eluded experimental synthesis, impeding an understanding of pertinent physics. Here, we report successful synthesis of CaH_{6} and its measured superconducting critical temperature T_{c} of 215K at 172GPa, which is evidenced by a sharp drop of resistivity to zero and a characteristic decrease of T_{c} under a magnetic field up to 9T. An estimate based on the Werthamer-Helfand-Hohenberg model gives a giant zero-temperature upper critical magnetic field of 203T. These remarkable benchmark superconducting properties place CaH_{6} among the most outstanding high-T_{c} superhydrides, marking it as the hitherto only clathrate metal hydride outside the family of rare earth and actinide hydrides. This exceptional case raises great prospects of expanding the extraordinary class of high-T_{c} superhydrides to a broader variety of compounds that possess more diverse material features and physics characteristics.

Computational materials discovery for lanthanide hydrides at high pressure for high temperature superconductivity

Hydrogen-rich superhydrides are believed to be very promising high critical temperature (high ${T}_{c}$) superconductors, with experimentally observed critical temperatures near room temperature, as shown in recently discovered lanthanide superhydrides at very high pressures, e.g., ${\mathrm{LaH}}_{10}$ at 170 GPa and ${\mathrm{CeH}}_{9}$ at 150 GPa. With the motivation of discovering new hydrogen-rich high ${T}_{c}$ superconductors at the lowest possible pressure, quantitative theoretical predictions are needed. In these promising compounds, superconductivity is mediated by the highly energetic lattice vibrations associated with hydrogen and their interplay with the electronic structure, requiring fine descriptions of the electronic properties, notoriously challenging for correlated $f$ systems. In this paper, we propose a first-principles calculation platform with the inclusion of many-body corrections to evaluate the detailed physical properties of the Ce-H system and to understand the structure, stability, and superconductivity of ${\mathrm{CeH}}_{9}$ at high pressure. We report how the calculation of ${T}_{c}$ is affected by the hierarchy of many-body corrections and obtain a compelling increase in ${T}_{c}$ at the highest level of theory, which goes in the direction of experimental observations. Our findings shed significant light on the search for superhydrides in close similarity with atomic hydrogen within a feasible pressure range. We provide a practical platform to further investigate and understand conventional superconductivity in hydrogen-rich superhydrides.

Exploring the Effect of the Number of Hydrogen Atoms on the Properties of Lanthanide Hydrides by DMFT

Lanthanide hydrogen-rich materials have long been considered as one of the candidates with high-temperature superconducting properties in condensed matter physics, and have been a popular topic of research. Attempts to investigate the effects of different compositions of lanthanide hydrogen-rich materials are ongoing, with predictions and experimental studies in recent years showing that substances such as LaH10, CeH9, and LaH16 exhibit extremely high superconducting temperatures between 150–250 GPa. In particular, researchers have noted that, in those materials, a rise in the f orbit character at the Fermi level combined with the presence of hydrogen vibration modes at the same low energy scale will lead to an increase in the superconducting transition temperature. Here, we further elaborate on the effect of the ratios of lanthanide to hydrogen in these substances with the aim of bringing more clarity to the study of superhydrides in these extreme cases by comparing a variety of lanthanide hydrogen-rich materials with different ratios using the dynamical mean-field theory (DMFT) method, and provide ideas for later structural predictions and material property studies.

Ternary rare-earth hydride oxides: stability in air and potential use as precursors for the synthesis of materials

Abstract Ternary rare-earth hydride oxides (or oxyhydrides) REHO show rather high thermal stability and inertness in air. SmHO remained intact when stored in air for 12 h, while after storage for one year, it completely hydrolysed to form Sm(OH)3. In contrast, YHO and HoHO show only slight decomposition upon longer storage. The cation’s basicity and the air humidity apparently are crucial factors in the air stability of the compounds. Their reactions with various gases were investigated, in order to better understand factors governing the stability in air and to map their potential as precursors in materials synthesis. Both SmHO and YHO reduce CO2 to carbon and form the metastable C-type rare-earth sesquioxides RE 2O3 instead of the thermodynamically stable B-type. YHO reacts with gaseous ammonia to a red powder. By X-ray diffraction, this is identified as yttrium nitride, but the color of the sample suggests it to be an oxygen-poor nitride oxide (oxynitride) phase YN1−x O x . These results underline the potential of rare-earth hydride oxides as precursors for the synthesis of other rare-earth compounds. The stability in air, even at elevated temperatures of some rare-earth hydride oxides such as YHO and HoHO are advantageous for potential applications as functional materials.

High-Temperature Superconductivity in the Lanthanide Hydrides at Extreme Pressures

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.

Current research status of rare earth oxygenated hydride photochromic films

Photochromic material, as an adaptive smart material, has a wide range of applications in smart windows, photoelectric sensors, optical storage, etc. Oxygen-containing rare-earth metal hydride (REH<sub><i>x</i></sub>O<sub><i>y</i></sub>) film, a new type of photochromic material, has attracted the attention of researchers for its efficient and reversible color-changing properties, simple and reproducible preparation methods, and fast darkening-bleaching time. In this paper we review the current research status of structural composition, color change mechanism, and property modulation of oxygen-containing rare-earth metal hydride films. Exposure to visible light and ultraviolet (UV) light can lead the optical transmission of visible and infrared (IR) light to degrade. The photochromic mechanisms can be grouped into four mechanisms: lattice contraction mechanism, oxygen exchange mechanism, local metal phase change, and hydrogen migration mechanism. Currently, performance can be tuned by controlling film morphology, designing chemical components, improving substrate adaptation, multilayer film structure design, etc. Finally, the future research focus of thin film is prospected.

Current Research on Rare Earth Oxygenated Hydride Photochromic Films

Photochromic materials, as an adaptive smart material, have a wide range of applications in smart windows, photoelectric sensors, optical storage, etc. Oxygen-containing rare-earth metal hydrides (REHxOy) films, a new type of photochromic material, have attracted the attention of researchers for their efficient and reversible color-changing properties, simple and reproducible preparation methods, and fast darkening-bleaching times. This paper reviews the current status of research on the structural composition, color change mechanism, and property modulation of oxygen-containing rare-earth metal hydrides films. Exposure to visible and ultraviolet (UV) light triggers a decrease in the optical transmission of visible and infrared (IR) light. The photochromic mechanism can be categorized into four explanations: lattice contraction mechanism, oxygen exchange mechanism, local metal phase change, and hydrogen migration mechanism. Currently, performance can be tuned by controlling film morphology, designing chemical components, improving substrate adaptation, multilayer film structure design, etc. Finally, an outlook on research priorities after thin films is provided.

Superconducting ScH3 and LuH3 at Megabar Pressures

Rare-earth (RE) superhydrides have great potential as high-temperature superconductors, with recent discoveries almost achieving room-temperature superconductivity in compressed LaH10 and YH9. Here, we continue to study the rare-earth hydrides by focusing on the new hydrides that the lightest element Sc and the heaviest element Lu formed under pressure. Two new superconducting hydrides ScH3 (Tc ∼ 18.5 K at 131 GPa) and LuH3 (Tc ∼ 12.4 K at 122 GPa) have been identified both with cubic structure by combining X-ray diffraction and electrical resistance techniques. Among all of the REH3, only the superconducting properties of ScH3 and LuH3 have been experimentally confirmed. Our current results may offer guidance to other REH3, which were predicted to be superconductors but have not been experimentally confirmed.

Effect of hole doping on superconductivity in compressed CeH9 at high pressures

The experimental realization of high-temperature superconductivity in compressed hydrides H$_3$S and LaH$_{10}$ at high pressures over 150 GPa has aroused great interest in reducing the stabilization pressure of superconducting hydrides. For cerium hydride CeH$_9$ recently synthesized at 80$-$100 GPa, our first-principles calculations reveal that the strongly hybridized electronic states of Ce 4$f$ and H 1$s$ orbitals produce the topologically nontrivial Dirac nodal lines around the Fermi energy $E_F$, which are protected by crystalline symmetries. By hole doping, $E_F$ shifts down toward the topology-driven van Hove singularity to significantly increase the density of states, which in turn raises a superconducting transition temperature $T_c$ from 74 K up to 136 K at 100 GPa. The hole-doping concentration can be controlled by the incorporation of Ce$^{3+}$ ions with varying their percentages, which can be well electronically miscible with Ce atoms in the CeH$_9$ matrix because both Ce$^{3+}$ and Ce behave similarly as cations. Therefore, the interplay of symmetry, band topology, and hole doping contributes to enhance $T_c$ in compressed CeH$_9$. This mechanism to enhance $T_c$ can also be applicable to another superconducting rare earth hydride LaH$_{10}$.

Computational Chemistry-Guided Syntheses and Crystal Structures of the Heavier Lanthanide Hydride Oxides DyHO, ErHO, and LuHO

Heteroanionic hydrides offer great possibilities in the design of functional materials. For ternary rare earth hydride oxide REHO, several modifications were reported with indications for a significant phase width with respect to H and O of the cubic representatives. We obtained DyHO and ErHO as well as the thus far elusive LuHO from solid-state reactions of RE2O3 and REH3 or LuH3 with CaO and investigated their crystal structures by neutron and X-ray powder diffraction. While DyHO, ErHO, and LuHO adopted the cubic anion-ordered half-Heusler LiAlSi structure type (F4¯3m, a(DyHO) = 5.30945(10) Å, a(ErHO) = 5.24615(7) Å, a(LuHO) = 5.171591(13) Å), LuHO additionally formed the orthorhombic anti-LiMgN structure type (Pnma; LuHO: a = 7.3493(7) Å, b = 3.6747(4) Å, c = 5.1985(3) Å; LuDO: a = 7.3116(16) Å, b = 3.6492(8) Å, c = 5.2021(7) Å). A comparison of the cubic compounds’ lattice parameters enabled a significant distinction between REHO and REH1+2xO1−x (x < 0 or x > 0). Furthermore, a computational chemistry study revealed the formation of REHO compounds of the smallest rare earth elements to be disfavored in comparison to the sesquioxides, which is why they may only be obtained by mild synthesis conditions.

Environmental dependence of the photochromic effect of oxygen-containing rare-earth metal hydrides

We study the dependence of the photochromic effect on environment and triggering light. We demonstrate that the first darkening/bleaching cycle of freshly grown films is accompanied by a release of weakly bound hydrogen, most likely present at the grain boundaries. For consecutive photochromic cycles, we do not find further exchange of material with the environment. Moreover, we report bleaching kinetics dependent on the gas environment after darkening with light of energies below the optical bandgap of the film. For darkening with photon energies above the bandgap of the film, we report a linear relation between the degree of darkening and bleaching relaxation time irrespective of gas environment.

Activation of D2 by Neodymium Cation (Nd+): Bond Energy of NdH+ and Mechanistic Insights through Experimental and Theoretical Studies.

The kinetic-energy-dependent cross section for the reaction of Nd+ with D2 was studied by using a guided ion beam tandem mass spectrometer. The formation of NdD+ is endothermic, and analysis of the reaction cross section gave an NdH+ 0 K bond dissociation energy (BDE) of 1.99 ± 0.06 eV. Theoretical calculations for the NdH+ BDE were performed for comparison with the experimental thermochemistry and generally gave accurate results. Additionally, relaxed potential energy surfaces for NdH2+ were performed, and no strongly bound dihydride intermediate was located. The Nd+ + D2 reactivity and BDE of NdH+ are compared with analogous results for the lanthanide cations La+, Ce+, Pr+, Sm+, Gd+, and Lu+ to establish periodic trends and insight into the role of the electronic configurations on this reactivity and the lanthanide hydride cation bond strengths.

Magnetic and structural characteristics of ambient pressure fcc phase Ho and Tb thin films

We report the results of an investigation into the structural and magnetic properties of thin films of Ho and Tb sputtered on Ta-buffered Si substrates. As is often reported in thin films and nanoparticles of the rare earth (RE) metals, we observe both hcp and fcc phases where the relative fraction of each depends on the deposition conditions. The presence of a fcc RE phase at ambient conditions is generally claimed to be strain stabilised as in the bulk the fcc phase is only thermodynamically stable at elevated pressures. We find the lattice constants of the fcc phases in our films to coincide with values for the Ho- and Tb- hydrides, and analysis of the magnetic measurements shows that the Ho fcc phase is paramagnetic at ambient temperatures and antiferromagnetic at low temperatures, also a feature of the RE-hydrides. By considering both the structural and magnetic measurements on Ho and Tb films together, we demonstrate that the observed fcc phase is the antiferromagnetic rare earth hydride, which readily forms at the RE/Ta interface in the presence of the residual and outgassed hydrogen in vacuum chambers.

Stability and bonding nature of clathrate H cages in a near-room-temperature superconductor LaH10

Lanthanum hydride (${\mathrm{LaH}}_{10}$) with a sodalitelike clathrate structure was experimentally synthesized to exhibit a near-room-temperature superconductivity under megabar pressures. Based on first-principles density-functional theory calculations, we reveal that the metal framework of La atoms has excess electrons at interstitial regions. Such anionic electrons are easily captured to form a stable clathrate structure of H cages. We thus propose that the charge transfer from La to H atoms is mostly driven by the electride property of the La framework. Furthermore, the interaction between La atom and H cage induces a delocalization of La $5p$ semicore states to hybridize with the H $1s$ state. Consequently, the bonding nature of ${\mathrm{LaH}}_{10}$ is characterized as a mixture of ionic and covalent bonding between La atom and H cage. Our findings demonstrate that anionic and semicore electrons play important roles in stabilizing clathrate H cages in ${\mathrm{LaH}}_{10}$, which can be broadly applicable to other compressed rare-earth hydrides with clathrate structures.

Cage Structure and Near Room-Temperature Superconductivity in TbHn (n = 1–12)

Hydrogen-rich compounds are considered most likely to achieve room-temperature superconductivity since the critical temperature (Tc) above 250 K was observed in lanthanum hydride. Exploring the high-temperature superconductivity in rare-earth metal hydrides becomes very interesting. Based on the particle swarm optimization for crystal structures and first-principles calculations, we investigate the crystal structures, phase stability, metallization, and possible superconducting properties of terbium hydride (TbHn, n = 1 – 12) under pressure. Our results show that terbium hydride is a potential high-temperature superconductor under high pressures. It stably exists at different pressure conditions by adjusting the H content. Specifically, the H atomic cage structure can be observed in most terbium hydrides, and the number of H atoms in the cage sublattice increases with the stoichiometry of H in TbHn. We demonstrate that the high Tc value is closely related to this cage sublattice and it increases with increasing H content in terbium hydride. The highest Tc above 270 K is predicted in TbH10 at 250 GPa for Fm3̅m and 310 GPa for R3̅m space group. This result indicates that the superconductivity with Tc close to or beyond lanthanum hydride can be achieved in other rare-earth metal hydrides.

Nanoparticulate Pt-catalyzed hydrogenation of Yb films: Towards hydrides with higher hydrogen compositions

• Moderate condition synthesis of β-YbH 3− δ was achieved by nanoparticulate Pt-catalyzed hydrogenation. • Hydrogenation using the Pt nanoparticle catalyst capping method demonstrated room-temperature synthesis of α-YbH 2± δ . • Thermodynamic kinetics in syntheses of β-YbH 3− δ and α-YbH 2± δ were analyzed. It is more difficult to synthesize β-phase ytterbium trihydride (β-YbH 3− δ ) under moderate conditions than other rare-earth metal trihydrides because it requires a valence change from a fully occupied 4 f shell ( 4 f 14 ) in Yb 2+ to an unclosed shell ( 4 f 13 ) in Yb 3+ . We synthesized β-YbH 3− δ at ambient pressure through hydrogenation of a Yb film capped with platinum (Pt) nanoparticle (NP) catalysts. In addition, Pt NP capping enabled us to synthesize α-phase Yb dihydride (α-YbH 2± δ ) at room temperature. The difficulty of trihydrogenating Yb to β-YbH 3− δ is explained by kinetic analyses based on temperature and time dependences of molar fractions of α-YbH 2± δ and β-YbH 3− δ . Pt NP capping can be used to synthesize other hydrogen-rich rare-earth metal hydrides under ambient pressure and lower temperatures.

Rare-Earth-Catalyzed Hydrosilylation and Dehydrogenative Coupling of Hydrosilanes

AbstractActivation of Si–H bonds with rare-earth complexes could generate the highly reactive rare-earth hydrides or silyl complexes, which are key intermediates for hydrosilylation and cross-coupling reactions. This Account summarizes the recent advances in the rare-earth-catalyzed hydrosilylation of unsaturated substrates and dehydrogenative coupling of hydrosilanes with amines in our laboratory. The results demonstrated that rare-earth catalysts are unique in their reactivity and selectivity, enabling some unprecedented reactions.1 Introduction2 Dehydrogenative Coupling of Hydrosilanes with Amines3 Catalytic Dehydrogenative Coupling of Hydrosilanes with Amines4 Catalytic Hydrosilylation of Terminal Alkenes and Polymerization of Styrene5 Catalytic Hydrosilylation of Internal Alkenes6 Catalytic Dihydrosilylation of Internal Alkynes7 Conclusions and Outlook

Origin of enhanced chemical precompression in cerium hydride hbox {CeH}_{{9}}

The rare-earth metal hydrides with clathrate structures have been highly attractive because of their promising high-T_{rm{c}} superconductivity at high pressure. Recently, cerium hydride hbox {CeH}_9 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 hbox {LaH}_{{10}} and hbox {YH}_6. 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 hbox {CeH}_9.

Dehydrogenative Coupling of Terminal Alkynes with O/N-Based Monohydrosilanes Catalyzed by Rare-Earth Metal Complexes.

Newly synthesized rare-earth metal alkyl complexes bearing a tripyrrolyl ligand act as excellent precatalysts for the cross-dehydrogenative coupling between various terminal alkynes and O/N-based monohydrosilanes of HSi(OEt)3/HSi(NMe2)3, leading to the formation of a variety of alkoxysilylalkyne and aminosilylalkyne derivatives in good to high yields. The precatalysts LRE(CH2SiMe3)(thf)2 (RE = Y(1a), Er(1b), Yb(1c), L = 2,5-[(2-C4H3N)CPh2]2(C4H2NMe), thf = tetrahydrofuran) were easily prepared in high yields via the reactions of RE(CH2SiMe3)3(thf)2 with the proligand H2L in a single step. Mechanistic studies reveal that treatment of 1 with phenylacetylene could generate the active catalytic species: dinuclear rare-earth metal alkynides (L(thf)n[RE(μ-C≡CPh)]2L) (RE = Y(5a), n = 1; Yb(5c), n = 0), which could react with HSi(OEt)3 to produce the coupling product 4aa and the dinuclear rare-earth metal hydrides (L (thf)[RE(μ-H)]2L) (RE = Y(6a); Yb(6c)). By contrast, prior treatment of 1c with HSi(OEt)3 proceeds via cleavage of the Si-O bond to produce the dinuclear ytterbium alkoxide (LYb(μ-OEt))2 7c, which is inert in the dehydrogenative coupling reaction. The results of the mechanistic studies are consistent with the observation that the reaction is greatly influenced by the addition sequence of precatalyst/alkynes/silanes.

Selection of metal hydrides for a thermal energy storage device to support low-temperature concentrating solar power plants

Metal hydrides have become more and more significant both as hydrogen storage devices and as basic elements in energy conversion systems. Besides the well-known rare earth hydrides, magnesium alloys are very promising in the field of thermal energy storage for concentrating solar power plants. There is interest in analysing the performances of such materials in this context; for this purpose, a numerical model to describe hydrogen absorption and desorption processes of a metal hydride has been connected to a model elaborated with the help of Cycle-Tempo software to simulate a CSP plant operation. The integration of this plant with four metal hydride systems, based on the combination of two low-temperature hydrides (LaNi5, LaNi4.8Al0.2) and two high-temperature hydrides (Mg, Mg2Ni) has been studied. The investigation has taken into account CSP overall performances, transfer surfaces and storage efficiencies, to determine the feasibility of designed plants. Results show that the selection of the optimal hydrides must take into account hydride operation temperatures, reaction enthalpies, storage capacities and kinetic compatibility. In the light of the calculated parameters, a solar ORC plant using R134a as the working fluid is a valuable choice if matched to a storage system composed of LaNi5 and Mg2Ni hydrides.