Structures Of Hydrides Research Articles

Correlating the microstructure of titanium hydride with its hydrogenation conditions via thermodynamics and kinetics

Titanium hydride is widely considered as an important hydrogen storage material due to its high capacity, stability and low cost. The microstructure of titanium hydride, which is usually induced by the hydrogenation of titanium, strongly influences its storage performance. However, the relationship between the hydrogenation conditions and the derived microstructure of titanium hydride is still unclear, limiting the development of its structure design strategy. In this work, the microstructure of titanium hydride is correlated with its hydrogenation conditions through the thermodynamics and kinetics. By evaluating the effect of the hydrogenation temperature, pressure and cooling rate, three classes of the hydrogenation processes were clarified as kinetic-limited, continuous and stepwise. Besides, according to the SEM and FIB-STEM results, these different processes are confirmed to greatly vary the bulk microstructure. It is concluded that a fast and continuous transition induces a broken bulk morphology, while a stepwise process leads to a larger bulk grain size. In summary, this study presents a framework for designing titanium hydride structures by modifying phase transition processes through careful adjustment of hydrogenation parameters, namely, a condition-process-structure relationship. This relationship offers crucial guidance in managing the grain refinement or coarsening of hydrides within hydrogen storage components and metallurgical applications.

Impact of quantum fluctuations thermally renormalize lattice vibrations on superconducting state of transition metal functionalized two-dimensional hydrides monolayer

Exploring superconducting materials stands as a pivotal pursuit in condensed matter physics, particularly, the investigation of superconductivity in two-dimensional metal hydrides is of paramount importance due to its intriguing nature. Our study focuses on elucidating the metallic state of van der Waals layered TM-H (TM = Ti, Zr, Hf) compounds, a key factor in predicting their superconducting (SC) characteristics. Leveraging an evolutionary algorithm rooted in density functional theory, we predicted the structures of hydrides, including Ti2H2, Ti2H4, Zr2H2, Zr2H4, Zr2H5, Hf2H2, Hf2H4, Hf2H5, and determined their energetically stable structures. Along with exploring the potential for SC, we conducted a comprehensive examination of relevant electronic properties. A significant aspect addressed was the influence of anharmonic phonon properties in determining the stochastic self-consistent harmonic approximation. These findings underscore the pivotal role of anharmonicity in determining the SC of two-dimensional metal hydrides.

Data-driven design of high pressure hydride superconductors using DFT and deep learning

The observation of superconductivity in hydride-based materials under ultrahigh pressures (for example, H3S and LaH10) has fueled the interest in a more data-driven approach to discovering new high-pressure hydride superconductors. In this work, we performed density functional theory (DFT) calculations to predict the critical temperature ( Tc ) of over 900 hydride materials under a pressure range of (0–500) GPa, where we found 122 dynamically stable structures with a Tc above MgB2 (39 K). To accelerate screening, we trained a graph neural network (GNN) model to predict Tc and demonstrated that a universal machine learned force-field can be used to relax hydride structures under arbitrary pressures, with significantly reduced cost. By combining DFT and GNNs, we can establish a more complete map of hydrides under pressure.

Raman spectroscopy of zirconium hydride

The characterization of zirconium hydride is important in the nuclear industry because of the hydrogen-induced embrittlement of Zircaloy cladding and its use as a neutron moderator. This paper introduces the use of Raman spectroscopy for the characterization of zirconium hydride. First-principles density functional theory (DFT) calculations were used to predict the Raman spectra of ζ-ZrH0.5, γ-ZrH, δ-ZrH1.5, δ-ZrH1.66, and ε-ZrH2 with all their predicted symmetries; ζ-ZrH0.5 (P3m1, R3¯m, C2/m, Cm, Cmmm, and Pn3¯m); γ-ZrH (P222, Ccce, and P42/mmc); δ-ZrH1.5 (P4¯m2, P42/mcm, Fmmm, Pn3¯m, Ibam, P2/c, PI, and P42/nnm); δ-ZrH1.66 (Fm3¯m); and ε-ZrH2 (Fm3¯m, R3¯m, and I4/mmm). Two samples of Zircaloy-4 containing 133 wt ppm and 360 wt ppm hydrogen were characterized by Raman spectroscopy, showing two signal lines at 215 cm−1 and 1,187 cm−1, which were assigned to the presence of δ-ZrH1.66. These signals had a good spatial correlation with visible hydride precipitates in Raman spectroscopy images. This work provides the basis for the characterization of all possible zirconium hydride compositions and structures using Raman spectroscopy.

Towards the Operational Window for Nitridic and Carbidic Palladium Nanoparticles for Directed Catalysis

AbstractThe reactions under which interstitial structures of Pd form are profoundly important and prevalent in catalysis; the formation and stability of Pd hydride structures are well understood, however, interstitial structures of the carbide and nitride are relatively under explored. This work reports a systematic study of the formation and stability of PdCx and PdNx at elevated temperatures and different atmospheres using in situ Pd L3 edge XANES spectroscopy. These studies were further complemented by the application of 14N MAS‐NMR experiments and computational DFT investigations. The experiments confirmed that PdCx was significantly more stable than PdNx; 14N MAS‐NMR provided direct confirmation on the formation of the nitride, however, the XANES studies evidenced very limited stability under the conditions employed. Moreover, the results suggest that the formation of the nitride imparts some structural changes that are not entirely reversible under the conditions used in these experiments. This work provides important insights into the stability of interstitial structures of Pd and the conditions in which they could be employed for directed catalytic processes.

Structural Features and Phase Transitions during Dehydrogenation of a Composite Based on Magnesium Hydride and Metal-Organic Framework Structures MIL-101 (Cr)

Structural Features and Phase Transitions during Dehydrogenation of a Composite Based on Magnesium Hydride and Metal-Organic Framework Structures MIL-101 (Cr)

Metal Flux Growth of Lanthanide Carbide Hydrides Using Anthracene.

Reactions of anthracene in Ln/T eutectic mixtures (Ln = La, Yb; T = Ni, Cu) have produced crystals of new complex lanthanide carbide hydride phases. The thermal decomposition of anthracene provides a source of both carbon and hydrogen. LaCHx forms in space group Pnma with unit cell parameters a = 7.2736(4) Å, b = 3.7218(2) Å, and c = 13.0727(7) Å. Yb2CHx forms in space group P-3m1 with unit cell parameters a = 3.5659(4) Å and c = 5.8000(8) Å, with a structure related to that of Ho2CF2. The presence of hydride in interstitial sites is supported by the formation of different compounds in the absence of hydrogen and by comparison to known hydride structures. 1H nuclear magnetic resonance (NMR) spectra collected on LaCHx show two unique hydride resonances, in agreement with the two available interstitial sites. Density of states calculations show that filling the tetrahedral sites in LaCHx with hydrogen increases metallic behavior, while adding hydrogen into the tetrahedral sites in Yb2CHx induces the formation of a band gap.

Dehydrogenation of Alkali Metal Aluminum Hydrides MAlH4 (M = Li, Na, K, and Cs): Insight from First-Principles Calculations

Complex aluminum hydrides with high hydrogen capacity are among the most promising solid-state hydrogen storage materials. The present study determines the thermal stability, hydrogen dissociation energy, and electronic structures of alkali metal aluminum hydrides, MAlH4 (M = Li, Na, K, and Cs), using first-principles density functional theory calculations in an attempt to gain insight into the dehydrogenation mechanism of these hydrides. The results show that the hydrogen dissociation energy (Ed-H2) of MAlH4 (M = Li, Na, K, and Cs) correlates with the Pauling electronegativity of cation M (χP); that is, the Ed-H2 (average value) decreases, i.e., 1.211 eV (LiAlH4) < 1.281 eV (NaAlH4) < 1.291 eV (KAlH4) < 1.361 eV (CsAlH4), with the increasing χP value, i.e., 0.98 (Li) > 0.93 (Na) > 0.82 (K) > 0.79 (Cs). The main reason for this finding is that alkali alanate MAlH4 at higher cation electronegativity is thermally less stable and held by weaker Al-H covalent and H-H ionic interactions. Our work contributes to the design of alkali metal aluminum hydrides with a favorable dehydrogenation, which is useful for on-board hydrogen storage.

Investigations of Microstructure and Mechanical Properties of Lap Jointed Dissimilar Metals by Friction Stir Spot Welding Process

The joining of different materials is required for industrial application to utilize the hydride structures with attractive advantages such as superior strength to weight ratio, low cost, high tensile strength and less weight of the component. Specifically, Al-Cu composite material is widely used in foil conductors of a transformer, electrical connectors, foil windings in capacitors and tubes in heat exchangers. However, joining or spot welding the Al parts to Cu parts are significant challenge owing to variation in mechanical properties and chemical compositions of joining materials. In this present research work, the friction stir spot welded process (FSSWP) is carried out to join the Al and Cu materials. Further, microstructure and mechanical properties of friction stir spot welded specimens (FSSW) are studied at different tool rotational speeds likely from 1000rpm to 1500rpm. The microstructural study is carried out using scanning electron microscope images at the interface and overall welded region. The tensile strength of both single and double spot-welded specimens is analyzed using a universal tensile test machine. The output of this study states that the optimal tool rotational speed is 1500rpm for both single and double spot-welded specimens. Moreover, the double spot-welded specimen exhibits more tensile with a crack-free spot-welded surface than that of the single spot-welded specimen. The tensile strength double spot-welded specimen has a 6.8% higher strength than that of a single spot-welded specimen. Based on the present study, it is concluded that the double spot-welded specimen can be used for different industrial applications to replace Cu material with this Al-Cu material that gives added advantages to those components.

Computational description of surface hydride phases on Pt(111) electrodes.

Surface platinum hydride structures may exist and play a potentially important role during electrocatalysis and cathodic corrosion of Pt(111). Earlier work on platinum hydrides suggests that Pt may form clusters with multiple equivalents of hydrogen. Here, using thermodynamic methods and density functional theory, we compared several surface hydride structures on Pt(111). The structures contain multiple monolayers of hydrogen in or near the surface Pt layer. The hydrogen in these structures may bind the subsurface or reconstruct the surface both in the set of initial configurations and in the resulting (meta)stable structures. Multilayer stable configurations share one monolayer of subsurface H stacking between the top two Pt layers. The structure containing two monolayers (MLs) of H is formed at -0.29V vs normal hydrogen electrode, is locally stable with respect to configurations with similar H densities, and binds H neutrally. Structures with 3 and 4 ML H form at -0.36 and -0.44V, respectively, which correspond reasonably well to the experimental onset potential of cathodic corrosion on Pt(111). For the 3 ML configuration, the top Pt layer is reconstructed by interstitial H atoms to form a well-ordered structure with Pt atoms surrounded by four, five, or six H atoms in roughly square-planar and octahedral coordination patterns. Our work provides insight into the operando surface state during low-potential reduction reactions on Pt(111) and shows a plausible precursor for cathodic corrosion.

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.

Theory-directed discovery of high-temperature superconductivity in clathrate hydrides at high pressure

Theory-directed discovery of high-temperature superconductivity in clathrate hydrides at high pressure

Missing theoretical evidence for conventional room-temperature superconductivity in low-enthalpy structures of carbonaceous sulfur hydrides

To elucidate the geometric structure of the putative room-temperature superconductor, carbonaceous sulfur hydride (C-S-H), at high pressure, we present the results of an extensive computational structure search of bulk C-S-H at 250 GPa. Using the minima hopping structure prediction method coupled to the GPU-accelerated sirius library, more than 17 000 local minima with different stoichiometries in large simulation cells were investigated. Only 24 stoichiometries are favorable against elemental decomposition, and all of them are carbon-doped ${\mathrm{H}}_{3}\mathrm{S}$ crystals. The absence of van Hove singularities or similar peaks in the electronic density of states of more than 3000 candidate phases rules out conventional superconductivity in C-S-H at room temperature.

Hydrogen Clathrate Structures in Uranium Hydrides at High Pressures.

Room-temperature superconductivity has always been an area of intensive research. Recent findings of clathrate metal hydrides structures have opened up the doors for achieving room-temperature superconductivity in these materials. Here, we report first-principles calculations for stable H-rich clathrate structures of uranium hydrides at high pressures. The clathrate uranium hydrides contain H cages with stoichiometries of H24, H29, and H32, in which H atoms are bonded covalently to other H atoms, and U atoms occupy the centers of the cages. Especially, a UH10 clathrate structure containing H32 cages is predicted to have an estimated Tc higher than 77 K at high pressures.

Distinguishing the Structures of High-Pressure Hydrides with Nuclear Magnetic Resonance Spectroscopy.

The structural characterization of high-pressure hydrides has encountered many difficulties mainly due to the weak X-ray scattering of hydrogen. Herein, we investigate the prospect of detecting the H3S and LaH10 structures with nuclear magnetic resonance (NMR) spectroscopy. Our calculations demonstrate that the different candidate structures of H3S (or LaH10) exhibit significant differences in the electric field gradient (EFG) tensor of the 33S (or 139La) sites, indicating that the NMR spectroscopy can well capture the structural differences, even the small changes in the atomic position, and hence can be used to effectively probe the structures and the phase transitions of H3S and LaH10. Our results clarify the relationship between the structures and the EFG tensor parameters and provide a potential means to detect the structures of high-pressure hydrides.

Carbide Dihydrides: Carbonaceous Species Identified in Ta4 + -Mediated Methane Dehydrogenation.

The products of methane dehydrogenation by gas‐phase Ta4 + clusters are structurally characterized using infrared multiple photon dissociation (IRMPD) spectroscopy in conjunction with quantum chemical calculations. The obtained spectra of [4Ta,C,2H]+ reveal a dominance of vibrational bands of a H2Ta4C+ carbide dihydride structure over those indicative for a HTa4CH+ carbyne hydride one, as is unambiguously verified by studies employing various methane isotopologues. Because methane dehydrogenation by metal cations M+ typically leads to the formation of either MCH2 + carbene or HMCH+ carbyne hydride structures, the observation of a H2MC+ carbide dihydride structure implies that it is imperative to consider this often‐neglected class of carbonaceous intermediates in the reaction of metals with hydrocarbons.

Lithium metal hydrides (Li2CaH4 and Li2SrH4) for hydrogen storage; mechanical, electronic and optical properties

Hydrogen storage is a critical step for commercialisation of hydrogen consumed energy production. Among other storage methods, solid state storage of hydrogen attracts much attention and requires extensive research. This study rationally and systematically designs novel solid state hydrides; Li2CaH4 (GHD is obtained as −6.95 wt %) and Li2SrH4 (GHD is obtained as −3.83 wt %) using computational method. As a first step, we suggest and predict crystal structures of solid state Li2CaH4 and Li2SrH4 hydrides and look for synthesizability. Then, the mechanical stabilities of hydrides are identified using elastic constants. Both hydrides fulfil the well-known Born stability criteria, indicating that both Li2CaH4 and Li2SrH4 are mechanically stable materials. Several critical parameters, bulk modulus, shear modulus, Cauchy pressures, anisotropy factors of hydrides and bonding characteristics are obtained and evaluated. Furthermore, electronic and optical band structures of hydrides are computed. Both Li2CaH4 and Li2SrH4 have indirect bands gaps as 0.96 eV (Г-U) and 1.10 eV (Г-R). Thus, both materials are electronically semiconducting. Also, Bader charge analysis of hydrides have been carried out. Charge density distribution suggests an ionic-like (or polarized covalent) bonding interaction between the atoms.

Fracture toughness evaluation for Zr-4 clad tubing structure with pellet inserts

Applying fracture mechanics approach to spent nuclear fuel (SNF) system reliability investigation is warranted due to the inherent flaws and hydride structures existed in a SNF system after nuclear reactor operation. However, none of the existing fracture toughness data deal with fuel cladding specific geometry or spent fuel material conditions, such as cladding structure with the pellet-inserts and the associated pellet clad mechanical interactions induced mixed-mode damage mechanisms. Thus, the development of an intrinsic fracture mechanics approach that is suitable for SNF materials is needed. Due to thin wall and small dimension of clad tubing structure, the spiral notch torsion test (SNTT) method of small specimen approach was used to estimate the clad tubing structure fracture toughness. The estimated fracture toughness for Zr-4 cladding with alumina-pellet inserts are presented in this report. For SNTT samples with a short or medium crack length, between 5.4-mm and 8-mm, the estimate JIQ upon fracture initiation for the baseline Zr-4 cladding is at 50 kJ/m2 with 2-sigma uncertainty of 3.26 kJ/m2, and the associated KIQ is at 67.46 MPa√m. For SNTT samples with a long crack length, around 13-mm, the crack initialization is deviated from that of the Mode-I tensile fracture and appears to be a mixed-mode fracture of Mode I – tensile stress and Mode III – out of plane shear stress; the estimated JMQ is at 18.9 kJ/m2, the associated KMQ is at 41.4 MPa√m.

Hydride Mapping in Uranium Using MLLS Fitting of Electron Energy-Loss Spectra

Previous studies of the reaction of hydrogen gas with uranium metal have been mainly limited to microscale or higher observations of uranium surfaces. We have characterized subsurface features of uranium hydride structures formed in a depleted uranium pellet aged for over 9 years under a controlled atmosphere with an initial H2 partial pressure of 0.667 kPa (5 torr) using electron energy-loss spectroscopy (EELS), giving information at lower length scales than previously reported. We demonstrate that multiple linear least-squares fitting of the O4,5 edge of the uranium EEL spectrum, with input spectra generated from the dataset being analyzed, can be used to map hydride and metal phases in aged uranium metal at the nanometer scale. While the interface between the bulk metal and the subsurface hydride structures is sharp and well defined, the interior of the reacted region is heterogeneous, containing a mix of polycrystalline UH3 and stranded, apparently unreacted, U metal.

Hydrogenation Character and Crystal Structures of Hydrides in RMgNi<sub>4</sub> (R = Nd, Gd, Er)

Hydrogenation Character and Crystal Structures of Hydrides in RMgNi<sub>4</sub> (R = Nd, Gd, Er)