Alkaline Earth Hydrides Research Articles

Laser cooling of a fermionic molecule

Only bosonic molecular species have been directly laser cooled to date, primarily due to an abundance of bosonic isotopes in nature and to their simpler hyperfine structure. Fermionic molecules provide new opportunities for ultracold chemistry, quantum simulation, and precision measurements. Here we report direct laser cooling of a fermionic molecular isotopologue, calcium monodeuteride (CaD). With a nuclear spin I=1, only 5 hyperfine states need to be addressed for rotational closure in optical cycling. These hyperfine states are unresolved for typical experimental linewidths. We present a method for efficiently producing alkaline-earth metal hydrides and deuterides. We demonstrate rotational closure and show magnetically assisted Sisyphus cooling in one dimension for a beam of CaD molecules. Our results indicate that the experimental complexity for laser cooling CaD is similar to that of calcium monohydride (CaH). Laser cooling of CaD is a promising first step for production of ultracold and trapped atomic deuterium. Published by the American Physical Society 2024

Emerging superconductivity rules in rare-earth and alkaline-earth metal hydrides

Hydrides of alkaline-earth and rare-earth metals have garnered significant interest in high-temperature superconductor research due to their excellent electron-phonon coupling and high T c upon pressurization. This study explores the electronic structures and electron-phonon coupling of metal hydrides XHn (n= 4,6), where X includes Ca, Mg, Sc, and Y. The involvement of d-orbital electrons alters the Fermi surface, leading to saddle-point nesting and a charge density wave (CDW) phase transition, which opens the superconducting gap. For instance, in YH6, the exchange coupling between Y-4d and H-1s holes in the phonon softening region results in T c values up to 230 K. The study suggests that factors, such as the origin of the CDW order, hydrogen concentration, and d-orbital contributions are crucial to superconductivity. This work proposes a new rule for high T c superconductors, emphasizing the importance of double gaps and electron-phonon interactions at exchange coupling sites, and predicts potential high-quality superconductors among rare-earth hydrides.

Superconducting ternary hydrides in Ca-U-H under high pressure

The research on hydrogen-rich ternary compounds attract tremendous attention for it paves new route to room-temperature superconductivity at lower pressures. Here, we study the crystal structures, electronic structures, and superconducting properties of the ternary Ca-U-H system, combining crystal structure predictions with ab-initio calculations under high pressure. We found four dynamically stable structures with hydrogen clathrate cages: CaUH12-Cmmm, CaUH12-Fd-3m, Ca2UH18-P-3m1, and CaU3H32-Pm-3m. Among them, the Ca2UH18-P-3m1 and CaU3H32-Pm-3m are likely to be synthesized below 1 megabar. The f electrons in U atoms make dominant contribution to the electronic density of states around the Fermi energy. The electron-phonon interaction calculations reveal that phonon softening in the mid-frequency region can enhance the electron–phonon coupling significantly. The T c value of Ca2UH18-P-3m1 is estimated to be 57.5–65.8 K at 100 GPa. Our studies demonstrate that introducing actinides into alkaline-earth metal hydrides provides possibility in designing novel superconducting ternary hydrides.

Nitrogen fixation by alkali and alkaline earth metal hydrides assisted by plasma.

The chemical behaviors of alkali and alkaline earth metal hydrides including LiH, KH, MgH2, CaH2, and BaH2 under nitrogen plasma differ significantly from one another, exhibiting an ammonia production trend that contrasts with that observed under thermal conditions. A prominent feature of KH is its ability to facilitate plasma-assisted N2 fixation without generating H2 byproduct, showing high atomic economy in utilization of hydride ions for N2 reduction.

A series of neutral alkaline earth metal hydride complexes supported by a bulky, unsymmetrical β-diketiminate ligand, [{(Dip/TCHPNacnac)M(μ-H)}2] (M = Mg, Ca, Sr or Ba).

A bulky, unsymmetrical β-diketiminate ligand, [HC{MeCN(Dip)}{MeCN(TCHP)}]- (Dip/TCHPNacnac; Dip = 2,6-diisopropylphenyl, TCHP = 2,4,6-tricyclohexylphenyl), has been utilised in the preparation of a series of magnesium alkyl and calcium, strontium and barium amide complexes. Reaction of these with PhSiH3 afforded the first complete series of β-diketiminato heavier group 2 metal hydride complexes, [{(Dip/TCHPNacnac)M(μ-H)}2] (M = Mg, Ca, Sr or Ba). The unsymmetrical nature of the β-diketiminate ligand seemingly promotes stabilising interactions of ligand Dip groups with the metal centres in the Ca, Sr and Ba hydride complexes.

Ammonia Synthesis via an Associative Mechanism on Alkaline Earth Metal Sites of Ca3 CrN3 H.

Typically, transition metals are considered as the centers for the activation of dinitrogen. Here we demonstrate that the nitride hydride compound Ca3 CrN3 H, with robust ammonia synthesis activity, can activate dinitrogen through active sites where calcium provides the primary coordination environment. DFT calculations also reveal that an associative mechanism is favorable, distinct from the dissociative mechanism found in traditional Ru or Fe catalysts. This work shows the potential of alkaline earth metal hydride catalysts and other related 1 D hydride/electrides for ammonia synthesis.

Composition and structural characteristics of compressed alkaline earth metal hydrides.

The metallization of alkaline earth metal hydrides offers a way to achieve near-room temperature superconductivity. In order to explore the metallization mechanism of these hydrides under pressure, a detailed understanding of the property changes of alkaline earth metal hydrides is required. Based on first-principles calculations, we have systematically investigated the dihydrides (XH2, X = Be, Mg, Ca, Sr, Ba) and tetrahydrides (XH4, X = Mg, Ca, Sr, Ba) of alkaline earth metals, respectively. By applying external pressure, we show that the structures of these alkaline earth metal hydrides undergo a series of phase transitions. Moreover, we investigate how the size of the bandgap decreases and eventually closes and reveal the role of electronegativity of metal elements in the critical pressure of hydride metallization. Remarkably, the hydrogen units (H6 or H8) formed in XH4 can accelerate the metallization process. The increase of the energy level difference in hydrogen units promotes the electroacoustic coupling effect, which is conducive to realization of high superconducting transition temperature (Tc). Our theoretical findings identify MgH4-I4/mmm as having potential to be a high-temperature superconductor and provide unusual ideas for the search of unknown high-temperature superconducting materials.

Electron–phonon coupling and superconductivity in an alkaline earth hydride CaH6 at high pressures

Recently, an alkaline earth hydride CaH6 having a sodalitelike clathrate structure has been experimentally synthesized at megabar pressures with a maximum T c of 215 K, comparable to that of a rare earth hydride LaH10. Here, based on first-principles calculations, we find that CaH6 exhibits a huge peak in the Eliashberg spectral function α 2 F around the low-frequency region of H-derived phonon modes, in contrast to LaH10 having a widely spreading spectrum of α 2 F over the whole frequencies of H-derived phonon modes. It is revealed that the huge peak of α 2 F in CaH6 is associated with an effective electron–phonon coupling (EPC) between low-frequency optical phonons and hybridized H 1s and Ca 3d states near the Fermi energy. As pressure increases, the strengthened H–H covalent bonding not only induces a hardening of optical phonon modes but also reduces the electron–phonon matrix elements related to the low-frequency optical modes, thereby leading to a lowering of the EPC constant. It is thus demonstrated that H-derived low-frequency phonon modes play an important role in the pressure-induced variation of T c in CaH6. Furthermore, unlike the presence of two distinct superconducting gaps in LaH10, CaH6 is found to exhibit a single isotropic superconducting gap.

Impact of Gas-Solid Reaction Thermodynamics on the Performance of a Chemical Looping Ammonia Synthesis Process.

Novel ammonia catalysts seek to achieve high reaction rates under milder conditions, which translate into lower costs and energy requirements. Alkali and alkaline earth metal hydrides have been shown to possess such favorable kinetics when employed in a chemical looping process. The materials act as nitrogen carriers and form ammonia by alternating between pure nitrogen and hydrogen feeds in a two-stage chemical looping reaction. However, the thermodynamics of the novel reaction route in question are only partially available. Here, a chemical looping process was designed and simulated to evaluate the sensitivity of the energy and economic performance of the processes toward the appropriate gas–solid reaction thermodynamics. Thermodynamic parameters, such as reaction pressure and especially equilibrium ammonia yields, influenced the performance of the system. In comparison to a commercial ammonia synthesis unit with a 28% yield at 150 bar, the chemical looping process requires a yield greater than 38% to achieve similar energy consumptions and a yield greater than 26% to achieve similar costs at a given temperature and 150 bar. Entropies and enthalpies of formation of the following pairs were estimated and compared: LiH/Li2NH, MgH2/MgNH, CaH2/CaNH, SrH2/SrNH, and BaH2/BaNH. Only the LiH/Li2NH pair has satisfied the given criteria, and initial estimates suggest that a 62% yield is obtainable.

Dinitrogen fixation mediated by lanthanum hydride

Dinitrogen fixation is one of the key reactions in chemistry, which is closely associated with food, environment, and energy. It has been recently recognized that the hydride materials containing negatively charged hydrogen (H‾) show promises for N2 fixation and hydrogenation to ammonia. Herein, we report that rare earth metal hydrides such as lanthanum hydride can also fix N2 either by heating to 200 °C or ball milling under ambient N2 pressure and temperature. The N2 fixation by lanthanum hydride may proceed via an intermediate lanthanum hydride-nitride (La-H-N) structure to form the final lanthanum nitride product. The hydride ion functions as an electron donor, which provides electrons for N2 activation possibly mediated by the lanthanum atoms. It is observed that N–H bond is not formed during the N2 fixation process, which is distinctly different from the alkali or alkaline earth metal hydrides. The hydrolysis of La-H-N to ammonia is feasible using water as the hydrogen source. These results provide new insights into the nitrogen fixation by hydride materials and more efforts are needed for the development of rare earth metal-based catalysts and/or nitrogen carriers for ammonia synthesis processes.

Computational study of pressurized tetragonal magnesium hydride (MgH4) as a potential candidate for high-temperature superconducting material

The dream of realizing room temperature superconductivity is one of the most challenging problems in condensed matter physics. Currently, materials with dense hydrogen contents at high pressures hold great promise for realizing room temperature superconductivity. In particular, pressurized alkaline earth metal hydrides received particular attention following the recently predicted sodalite-like calcium hydride (CaH6) with predicted Tc about 261 K above a pressure of 150 GPa; and magnesium hydride (MgH6) with predicted Tc about 270 K above 300 GPa. In this paper, we studied magnesium hydride (MgH4) with tetragonal (I4/mmm) type symmetry, and we found that this structure shows the highest at a pressure of 280 GPa which is higher than that of MgH6. Using density-functional perturbation theory (DFPT), the superconducting transition temperature, electron-phonon coupling, Eliashberg spectral function, and the logarithmic average frequency were computed. Our results reveal that, the computed values are reasonably in agreement with previous estimates.

Pressure-induced transition from pure electronic to mixed ionic-electronic conduction in strontium hydride

The heavier alkaline-earth hydrides AeH2 (Ae = Ca, Sr, and Ba) are considered as promising materials for hydrogen energy storage. Pressure-induced structural changes in AeH2 materials could improve hydrogen transport properties and result in a better understanding of the structure-property relationship. In this work, pressure evolution of carrier transport properties of SrH2 was investigated using impedance spectroscopy measurements at room temperature and first-principles calculations. The pressure-induced structure phase transition from a Pnma phase to a P63/mmc phase was accompanied by a transition from pure electronic conduction to mixed ionic-electronic conduction, which was related to the ionic migration barrier energy. In the P63/mmc phase, the H− ionic and electronic resistances of bulk and grain boundaries were distinguished, respectively. The total resistance of SrH2 decreased by about four orders of magnitude after the phase transition. This work provides critical insight into the structure-conduction relationship and the role of grain boundaries in the transport process of alkaline-earth hydrides under high pressure.

Accessing the main-group metal formyl scaffold through CO-activation in beryllium hydride complexes

Carbon monoxide (CO) is an indispensable C1 building block. For decades this abundant gas has been employed in hydroformylation and Pausen-Khand catalysis, amongst many related chemistries, where a single, non-coupled CO fragment is delivered to an organic molecule. Despite this, organometallic species which react with CO to yield C1 products remain rare, and are elusive for main group metal complexes. Here, we describe a range of amido-beryllium hydride complexes, and demonstrate their reactivity towards CO, in its mono-insertion into the Be-H bonds of these species. The small radius of the Be2+ ion in conjunction with the non-innocent pendant phosphine moiety of the developed ligands leads to a unique beryllium formyl complex with an ylidic P-COC fragment, whereby the carbon centre, remarkably, datively binds Be. This, alongside reactivity toward carbon dioxide, sheds light on the insertion chemistry of the Be-H bond, complimenting the long-known chemistry of the heavier Alkaline Earth hydrides.

From SmOF to SmH0.78OF0.22: H/F Substitution in Oxide Fluorides as a Synthesis Route to Heteroanionic Compounds.

Mixed anionic hydrides of the rare earths are a fascinating class of compounds as potential functional materials, especially in luminescence, as photochromic thin films and for ion conduction. For exploratory studies, the effectiveness of various synthesis methods must be investigated, which is done here for metathesis reactions. The reaction of Sm2O3 with PTFE yields SmOF (P21/c, a = 5.60133(19) Å, b = 5.65567(19) Å, c = 5.6282(2) Å, β = 90.169(5)°, V = 178.295(11) Å3, and Z = 4) in a new, probably metastable, polymorph of the baddeleyite-type structure. Metathesis reactions of SmOF with LiH, NaH, or CaH2 led to a samarium hydride oxide fluoride, SmHxOF1-x; i.e., incomplete H/F exchange occurs. X-ray diffraction and neutron diffraction on a compound with x = 0.78 obtained via NaH reveal hydride, oxide, and fluoride ions to be partially ordered. SmH0.78OF0.22 (Ia3̅, a = 10.947(2) Å, V = 1311.7(4) Å3, Z = 32) crystallizes in an anti-Li3AlN2-type structure with distorted cubic anion coordination for samarium atoms (site symmetry 3̅ and 2) and distorted tetrahedral arrangement of samarium atoms around the anions (site symmetry 1 and 3). It is a fully structurally characterized hydride oxide fluoride and shows a rare crystal chemical feature─the occupation of a crystallographic site by three different anions (0.188 H + 0.667 O + 0.145 F). Interatomic distances between samarium and hydrogen and samarium and the mixed hydrogen/oxygen/fluorine site range from 2.45 to 2.48 Å and 2.29 to 2.42 Å, respectively, and are similar to those in samarium hydride, samarium oxide, and samarium fluoride. Fluoride extraction by reaction with alkali and alkaline earth hydrides has thus proven to be a useful synthesis route to hydride oxides and also hydride oxide halogenides, which might be further exploited in exploratory research on heteroanionic metal hydrides.

Theoretical investigations of alkaline earth hydrides XH 2 (XCa, Sr, and Ba) for hydrogen storage applications

The structural, electronic, optical, and mechanical properties of XH2 (XCa, Sr, and Ba) earth hydrides) are obtained. Modified Becke-Johnson (mBJ) exchange potential with a proper choice of basic parameters is used to calculate the electronic band-structure, density of states, and optical properties. Strikingly, an excellent agreement between calculations and experiment is obtained. Under ambient conditions, XH2 (XCa, Sr, and Ba) are found to be structurally stable in the orthorhombic (PbCl2-type) structure. The calculated structural parameters, such as the lattice constant, bulk modulus are investigated. Total energy minimization indicates that examined alkaline hydrides undertake a structural phase transition from the orthorhombic (PbCl2-type) to hexagonal (Ni2In-type) phase and the transition pressures were calculated. The chemical bonding of these alkaline earth hydrides is delineated using ab initio calculation. Investigation of the electronic density of states reveals that these hydrides are insulators. Furthermore, optical parameters such as, dielectric function, reflectivity, and absorption coefficients as functions of the wavelength of incident light are computed and analyzed. Remarkably, XH2 (XCa, Sr, and Ba) are found to exhibit strong optical anisotropy. In addition, elastic constants of the single-crystal and polycrystalline forms of the investigated hydrides are numerically estimated and interpreted. The Voigt-Reuss-Hill (VRH) approximation is used to estimate the elastic constants of a surfaced polycrystalline hydrides in terms of its crystallographic texture and the elastic constants of the constituting single crystals under high pressure. The calculated optical and mechanical properties are in good agreement with previous theoretical and experimental studies.

Interplay of Alkali, Transition Metals, Nitrogen, and Hydrogen in Ammonia Synthesis and Decomposition Reactions.

ConspectusThe fixation of dinitrogen to ammonia is critically important for the biogeochemical cycle on earth. Ammonia also holds promise as a sustainable energy carrier. Tremendous effort has been devoted to the development of green processes and advanced materials for ammonia synthesis and decomposition under milder conditions, and encouraging progress has been made.The reduction of dinitrogen to ammonia needs electrons and protons, which hydridic hydrogen H- could supply. Polarized, electron-rich NxHy intermediates, on the other hand, can be stabilized by alkali or alkaline earth metal cations to lower kinetic barriers in the transformation. The inherent properties of alkali/alkaline earth metal hydrides (denoted as AH) endow them with a unique function in ammonia synthesis.In this Account, recent efforts in the exploration of alkali or alkaline earth metal hydrides (denoted as AH), amides, and imides (denoted as ANH hereafter) for ammonia synthesis and decomposition reactions will be summarized and discussed. We begin with an introduction to the chemistry of A with N2, NH3, and H2, highlighting the interconversion between AH and ANH that has profound implications on the formation and decomposition of NH3. We then present our finding on the strong synergistic effect between ANH and transition metals (TM) in ammonia decomposition catalysis, which stimulated our subsequent research on AH for ammonia synthesis. We discuss the effect and function mechanism of AH in the thermocatalytic and chemical looping ammonia synthesis processes. In the thermocatalytic process, AH cooperates with both early and late TM forming either composite catalysts with two active centers or complex metal hydride catalysts with electron- and hydrogen-rich ionic centers facilitating ammonia synthesis with high activities at lower temperatures. Very interestingly, AH levels the catalytic performances of TMs and intervenes in the energy-scaling relations of TM-only catalysts. Moreover, ANH serves as a new type nitrogen carrier effectively mediating ammonia synthesis via a low-temperature chemical looping process, in which N2 is fixed by AH forming ANH. Subsequently, ANH is hydrogenated to ammonia and AH. Late TMs have a strong catalytic effect on the chemical looping process. The unique interplay of A, N, TM, and H- offers plenty of opportunities for achieving dinitrogen conversion under mild conditions, while further efforts are needed to address the challenges in the fundamental understanding and practical application.

Electronic structure of the SrH+ and BaH+ molecules with dipole moment and rovibrational calculations

The alkaline-earth metal hydrides and their corresponding ions are heteronuclear molecules that are essential in many fields especially in astrophysics and spectroscopy such as sunspots, stars, nebulae, the interstellar medium and chemical engineering. They are important systems in spectroscopy due to their visible bands which emerge in the absorption spectrum of sun. Due to the importance of hydride species and the lack of theoretical studies on some astronomical diatomic molecules is a main reason to investigate their electronic structure. The adiabatic potential energy curves and the static dipole moment curves of the low-lying electronic states of strontium hydride cation SrH+ and barium hydride cation BaH+ have been investigated using ab initio CASSCF/(MRCI + Q) calculations. The spectroscopic parameters Te, Re, ωe, Be, the dipole moment μe, and the dissociation energy De were calculated for the bound states. Using the canonical function approach, the rovibrational constants Ev, Bv, Dv and the turning points Rmin and Rmax for the ground and several excited states have been determined.

Metal carbonates-induced solution-free dehydrogenation of alkaline earth metal hydrides at room temperature

We report an efficient, convenient, solution-free method to achieve the room-temperature dehydrogenation of alkaline earth metal hydrides (MgH2 and CaH2). It is found, for the first time, that metal carbonates, which are readily available and cheap, can efficiently induce the hydrogen desorption of alkaline earth metal hydrides at room temperature under mechanical ball milling conditions. We demonstrate that the hydrogen yield and purity mainly depend on the nature of metal carbonates, rate and duration of ball milling, and reaction mixture composition. The best performances are obtained with the systems of MgCO3-MgH2 and MgCO3-CaH2 milled at 350 ​rpm for 12 ​h, with the COx-free hydrogen yield of 94% and 90%. This work opens a new way to achieve the efficient dehydrogenation of alkaline earth metal hydrides at room temperature.

Ab-initio calculations of the electronic structure of the alkaline earth hydride anions XH- (X = Mg, Ca, Sr and Ba) toward laser cooling experiment.

By the use of the ab initio CASSCF/(MRCI+Q) calculations in the representation 2s+1Λ(+/-), the adiabatic potential energy curves and the dipole moment curves of the low lying states of the alkaline earth hydride anions (MgH-, CaH-, SrH- and BaH-) have been investigated in their singlet and triplet multiplicities. The spectroscopic parameters Te, Re, ωe, Be, αe, the dipole moment μe, and the dissociation energy De have been also calculated for the bound states of the considered molecules. In addition, a systematic investigation of the transition dipole moment curves for the lowest 1Σ+-1Π transitions has been done along with the Franck-Condon factor (FCF) corresponding to the X1Σ+-(1)1Π transition. Using the canonical function approach, a rovibrational study has been performed for finding the rovibrational constants Ev, Bv, Dv and the turning points Rmin and Rmax for the ground and different excited bound state. Efficient routes may be achieved via the diagonal FCF for the formation of cold and ultracold alkaline earth hydride anions. PACS N: 31.10. + z, 31.15.A, 31.15.vn, 31.50.Df.

Crystal structure evolution of complex metal aluminum hydrides upon hydrogen release

Abstract Complex aluminum hydrides have been widely studied as potential hydrogen storage materials but also, for some time now, for electrochemical applications. This review summarizes the crystal structures of alkali and alkaline earth aluminum hydrides and correlates structure properties with physical and chemical properties of the hydride compounds. The crystal structures of the alkali metal aluminum hydrides change significantly during the stepwise dehydrogenation. The general pathway follows a transformation of structures built of isolated [AlH4]− tetrahedra to structures built of isolated [AlH6]3− octahedra. The crystal structure relations in the group of alkaline earth metal aluminum hydrides are much more complicated than those of the alkali metal aluminum hydrides. The structures of the alkaline earth metal aluminum hydrides consist of isolated tetrahedra but the intermediate structures exhibit chains of corner-shared octahedra. The coordination numbers within the alkali metal group increase with cation sizes which goes along with an increase of the decomposition temperatures of the primary hydrides. Alkaline earth metal hydrides have higher coordination numbers but decompose at slightly lower temperatures than their alkali metal counterparts. The decomposition pathways of alkaline metal aluminum hydrides have not been studied in all cases and require future research.