Rare Earth Hydrides Research Articles

Enhancing (de)hydrogenation kinetics properties of the Mg/MgH2 system by adding ANi5 (A = Ce, Nd, Pr, Sm, and Y) alloys via ball milling

Magnesium (Mg)-based alloys have already been widely studied as the hydrogen storage materials because of their high reversible hydrogen storage capacity, low cost, light weight, etc. However, the poor de/hydrogenation kinetic properties dramatically hinder the practical applications. In this work, the MgH2-ANi5 (A = Ce, Nd, Pr, Sm, and Y) composites were prepared by a high-energy ball milling method, which can effectively refine the particle size thus improving the kinetic properties. Experimental results reveal that the MgH2-ANi5 composites mainly consist of Mg2NiH4, MgH2 and rare earth (RE) hydride, which will be dehydrogenated to form Mg2Ni, Mg and stable RE hydride reversibly. Accordingly, the as-milled MgH2-ANi5 (A = Ce, Nd, Pr, Sm, and Y) composites with various A-elements can respectively contribute to a reversible hydrogen storage capacity of 6.16 wt%, 5.7 wt%, 6.21 wt%, 6.38 wt%, and 6.5 wt% at a temperature of 300 °C, and show much better kinetic properties in comparison to the pure MgH2 without any additive. In-situ formed Mg2Ni and stable RE hydride (such as CeH2.73 and YH2) might act as effective catalysts to significantly improve the hydrogen storage properties of MgH2. The present work provides a guideline on improving the kinetic properties of the Mg-based hydrogen storage alloys.

Stability and superconductivity of lanthanum and yttrium decahydrides

Rare-earth hydrides can exhibit high-temperature superconductivity under high pressure. Here, we apply a crystal structure prediction method to the current record-holding $T_c$ material, LaH$_{10}$, and a candidate for even higher $T_c$, YH$_{10}$. We find a pressure-induced phase transition from the experimentally observed cubic LaH$_{10}$ phase to a new hexagonal phase at around 420 GPa. This hexagonal phase could explain experimental observations of hcp impurities in fcc samples. We find that YH$_{10}$ forms similar structures to LaH$_{10}$ and discuss the sensitivity of superconductivity calculations to the computational parameters used.

Ultrafast processes in photochromic material YHxOy studied by excited-state density functional theory simulation

Oxygen-containing rare-earth metal hydride, YHxOy, is a newly found photochromic material showing fast photoresponse. While its preparation method, optical properties and structural features have been studied extensively, the photochromic mechanism in YHxOy remains unknown. Here, using excited-state molecular dynamics simulation based on the recently developed real-time time-dependent density functional theory (RT-TDDFT) method, we study the photochemical reactions in YHxOy. We find that under photoexcitation, dihydrogen defects are formed within 100 fs. The dihydrogen defect behaves as a shallow donor and renders the material strongly n-type doped, which could be responsible for the photochromic effect observed in YHxOy. We also find that oxygen concentration affects the metastability of the dihydrogen species, meaning that the energy barrier for the dihydrogen to dissociate is related to the oxygen concentration. The highest barrier of 0.28 eV is found in our model with O/Y=1:8. If the oxygen concentration is too low, the dihydrogen will quickly dissociate when the excitation is turned off. If the oxygen concentration is too high, the dihydrogen dissociates even when the excitation is still on.

Multimetallic Rare Earth and Group 4 Transition Metal Hydrides for Novel Transformations of Small Molecules

Hydrogenolysis of mono(cyclopentadienyl (Cp))-ligated rare earth metal alkyl complexes with H2 affords a variety of multimetallic rare earth hydride complexes, which can serve as a unique platform ...

Thermal desorption spectroscopy studies of hydrogen desorption from rare earth metal trihydrides REH3 (RE=Dy, Ho, Er)

Abstract Rare earth (RE) metals form two stoichiometric hydrides, REH2 and REH3, and for the yttrium group of RE transformation of a FCC (REH2) into an HCP (REH3) lattice takes place during the second step of hydrogenation REH2 + ½ H2 → REH3. Earlier studies of the hydrogen desorption properties of the rare earth hydrides were limited to Y and RE = La, Ce, Pr, Nd, Sm, Gd, Tb and Er. The present work is focused on the studies of the kinetics and mechanism of hydrogen desorption from trihydrides of heavy rare earths, DyH3, HoH3, and ErH3. The Thermal Desorption Spectroscopy (TDS) studies were performed at pressures below 1 × 10−5 mbar during linear heating from room temperature to 1173 K at different heating rates ranging from 1 to 5.5 K/min. Hydrogen desorption traces show the presence of two main events with the low-temperature peak appearing below 573 K, while the second peak is positioned at 1083–1159 K, with the peak temperatures gradually increasing following the rise of the heating rate. Fitting of the peak temperatures in the TDS spectra using the Kissinger method yielded activation energies of hydrogen desorption for both hydrogen desorption events. For DyH3 and ErH3, the shapes of the TDS spectra appear to be well described by a phase-stuctural transformation following a model of nucleation and growth, while for HoH3 the dehydrogenation mechanism includes a phase boundary reaction. This applied model of phase-structural transformations shows differences in dimensionality and rate-limiting steps as related to the studied compound and the desorption events, REH3 → REH2 or REH2 → RE.

Prediction and Synthesis of Dysprosium Hydride Phases at High Pressure.

Crystal structure prediction (CSP) methods recently proposed a series of new rare-earth (RE) hydrides at high pressures with novel crystal structures, unusual stoichiometries, and intriguing features such as high-Tc superconductivity. RE trihydrides (REH3) generally undergo a phase transition from ambient P63/mmc or P3̅c1 to Fm3̅m at high pressure. This cubic REH3 (Fm3̅m) was considered to be a precursor to further synthesize RE polyhydrides such as YH4, YH6, YH9, and CeH9 with higher hydrogen contents at higher pressures. However, the structural stability and equation of state (EOS) of any of the REH3 have not been fully investigated at sufficiently high pressures. This work presents high-pressure X-ray diffraction (XRD) measurements in a laser-heated diamond anvil cell up to 100 GPa and ab initio evolutionary CSP of stable phases of DyH3 up to 220 GPa. Experiments observed the Fm3̅m phase of DyH3 to be stable at pressures from 17 to 100 GPa and temperatures up to ∼2000 K. After complete decompression, the P3̅c1 and Fm3̅m phases of DyH3 recovered under ambient conditions. Our calculations predicted a series of phases for DyH3 at high pressures with the structural phase transition sequence P3̅c1 → Imm2 → Fm3̅m → Pnma → P63/mmc at 11, 35, 135, and 194 GPa, respectively. The predicted P3̅c1 and Fm3̅m phases are consistent with experimental observations. Furthermore, electronic band structure calculations were carried out for the predicted phases of DyH3, including the 4f states, within the DFT+U approach. The inclusion of 4f states shows significant changes in electronic properties, as more Dy d states cross the Fermi level and overlap with H 1s states. The structural phase transition from P3̅c1 to Fm3̅m observed in DyH3 is systematically compared with other REH3 compounds at high pressures. The phase transition pressure in REH3 shows an inverse relation with the ionic radius of RE atoms.

Quantum crystal structure in the 250-kelvin superconducting lanthanum hydride.

The discovery of superconductivity at 200 kelvin in the hydrogen sulfide system at high pressures1 demonstrated the potential of hydrogen-rich materials as high-temperature superconductors. Recent theoretical predictions of rare-earth hydrides with hydrogen cages2,3 and the subsequent synthesis of LaH10 with a superconducting critical temperature (Tc) of 250 kelvin4,5 have placed these materials on the verge of achieving the long-standing goal of room-temperature superconductivity. Electrical and X-ray diffraction measurements have revealed a weakly pressure-dependent Tc for LaH10 between 137 and 218 gigapascals in a structure that has a face-centred cubic arrangement of lanthanum atoms5. Here we show that quantum atomic fluctuations stabilize a highly symmetrical [Formula: see text] crystal structure over this pressure range. The structure is consistent with experimental findings and has a very large electron-phonon coupling constant of 3.5. Although ab initio classical calculations predict that this [Formula: see text] structure undergoes distortion at pressures below 230 gigapascals2,3, yielding a complex energy landscape, the inclusion of quantum effects suggests that it is the true ground-state structure. The agreement between the calculated and experimental Tc values further indicates that this phase is responsible for the superconductivity observed at 250 kelvin. The relevance of quantum fluctuations calls into question many of the crystal structure predictions that have been made for hydrides within a classical approach and that currently guide the experimental quest for room-temperature superconductivity6-8. Furthermore, we find that quantum effects are crucial for the stabilization of solids with high electron-phonon coupling constants that could otherwise be destabilized by the large electron-phonon interaction9, thus reducing the pressures required for their synthesis.

Tuning LiBH4 for Hydrogen Storage: Destabilization, Additive, and Nanoconfinement Approaches

Hydrogen technology has become essential to fulfill our mobile and stationary energy needs in a global low–carbon energy system. The non-renewability of fossil fuels and the increasing environmental problems caused by our fossil fuel–running economy have led to our efforts towards the application of hydrogen as an energy vector. However, the development of volumetric and gravimetric efficient hydrogen storage media is still to be addressed. LiBH4 is one of the most interesting media to store hydrogen as a compound due to its large gravimetric (18.5 wt.%) and volumetric (121 kgH2/m3) hydrogen densities. In this review, we focus on some of the main explored approaches to tune the thermodynamics and kinetics of LiBH4: (I) LiBH4 + MgH2 destabilized system, (II) metal and metal hydride added LiBH4, (III) destabilization of LiBH4 by rare-earth metal hydrides, and (IV) the nanoconfinement of LiBH4 and destabilized LiBH4 hydride systems. Thorough discussions about the reaction pathways, destabilizing and catalytic effects of metals and metal hydrides, novel synthesis processes of rare earth destabilizing agents, and all the essential aspects of nanoconfinement are led.

YHO, an Air-Stable Ionic Hydride.

Metal hydride oxides are an emerging field in solid-state research. While some lanthanide hydride oxides (LnHO) were known, YHO has only been found in thin films so far. Yttrium hydride oxide, YHO, can be synthesized as bulk samples by a reaction of Y2O3 with hydrides (YH3, CaH2), by a reaction of YH3 with CaO, or by a metathesis of YOF with LiH or NaH. X-ray and neutron powder diffraction reveal an anti-LiMgN type structure for YHO (Pnma, a = 7.5367(3) Å, b = 3.7578(2) Å, and c = 5.3249(3) Å) and YDO (Pnma, a = 7.5309(3) Å, b = 3.75349(13) Å, and c = 5.3192(2) Å); in other words, a distorted fluorite type with ordered hydride and oxide anions was observed. Bond lengths (average 2.267 Å (Y-O), 2.352 Å (Y-H), 2.363 Å (Y-D), >2.4 Å (H-H and D-D), >2.6 Å (H-O and D-O), and >2.8 Å (O-O)) and quantum-mechanical calculations on density functional theory level (band gap 2.8 eV) suggest yttrium hydride oxide to be a semiconductor and to have considerable ionic bonding character. Nonetheless, YHO exhibits a surprising stability in air. An in situ X-ray diffraction experiment shows that decomposition of YHO to Y2O3 starts at only above 500 K and is still not complete after 14 h of heating to a final temperature of 1000 K. YHO hydrolyzes in water very slowly. The inertness of YHO in air is very beneficial for its potential use as a functional material.

Improved hydrogen storage kinetics and thermodynamics of RE-Mg-based alloy by co-doping Ce–Y

Aiming at improving hydrogen storage performance of Mg-base alloy, the Mg90Ce5Y5 alloy, which has high capacity and high stability, was prepared by vacuum induction melting. The XRD, SEM, TEM, PCI, and DSC were used to characterize the microstructure and phase transformation of alloy as well as hydrogen storage property. The results indicate that the Mg90Ce5Y5 alloy consists of multiphase structure, including the CeMg12, Y5Mg24, Ce2Mg17 as well as residual Mg phase, besides, part Y dissolved in both Mg and CeMg12/Ce2Mg17 phase to form solid solutions. After hydrogen absorption, these phases transform into the MgH2, CeH2.73 and YH2 phase, while after hydrogen desorption, the MgH2 transforms into the Mg phase, but the rare earth hydride phase was not changed. There is another reversible transformation between the CeH2.73 and CeO2 phase, which is beneficial for the cyclic stability of the alloy. The alloy has the reversible hydrogen capacity of about 6.0 wt% H2 as well as the activation energy of 114.3 kJ/mol, and also exhibits enhanced kinetics compared with the pure MgH2 sample, as a result of the synergistic effect of rare earth hydride phase. Meanwhile, it is also noted that the Mg90Ce5Y5 alloy begins to release hydrogen below 250 °C and the rate of hydrogen desorption is mainly dominated by surface controlled.

Experimental investigation of nitrogenation process for heavy rare earth nitrides from their hydrides

We experimentally investigated the nitrogenation process of heavy rare earth (Gd, Tb, Dy, Ho and Er) hydrides dynamically by thermal analysis. The onset temperature of nitrogenation and the absolute value of nitrogenation enthalpy increased as the atomic number of the heavy rare earth elements increased and the onset temperature of nitrogenation with a sharp exothermic peak exhibited a large gap between Tb (406 °C) and Dy (806 °C). The reaction route R→RHx→RN has been done at a relatively low temperature and atmospheric pressure, indicating that the diffusion coefficient of nitrogen into the rare earth hydride is much higher than that of nitrogen into the rare earth metal and/or the activation energy of RHx→RN is lower than that of R→RN. For complete nitriding, ErN requires the highest nitrogenation temperature of at most 1000 °C in N2 flow from ErHx. We obtained the magnetic entropy change ΔS of -14 J/kg K (144 kJ/m3 K, calculated by using the theoretical density of 10.3 g/cm3) at ΔH = 2 T for HoN prepared at the nitrogenation temperature of 1000 °C. This value reached the reported value, indicates that this simple synthesis route without any toxic compounds as a precursor and with normal equipment of an atmospheric furnace without high temperature and pressure can adequately exploit the ability of the rare earth nitride as a magnetic refrigerant for magnetic refrigeration.

A rare earth hydride supported ruthenium catalyst for the hydrogenation of N-heterocycles: boosting the activity via a new hydrogen transfer path and controlling the stereoselectivity.

Hydrogenation of N-heterocycles is of great significance for their wide range of applications such as building blocks in drug and agrochemical syntheses and liquid organic hydrogen carriers (LOHCs). Pursuing a better hydrogenation performance and stereoselectivity, we successfully developed a rare earth hydride supported ruthenium catalyst Ru/YH3 for the hydrogenation of N-heterocycles, especially N-ethylcarbazole (NEC), the most promising LOHC. Full hydrogenation of NEC on Ru/YH3 can be achieved at 363 K and 1 MPa hydrogen pressure, which is currently the lowest compared to previous reported catalysts. Furthermore, Ru/YH3 shows the highest turnover number, namely the highest catalytic activity among the existing catalysts for hydrogenation of NEC. Most importantly, Ru/YH3 shows remarkable stereoselectivity for all-cis products, which is very favorable for the subsequent dehydrogenation. The excellent performance of Ru/YH3 originates from the new hydrogen transfer path from H2 to NEC via YH3. Ru/LaH3 and Ru/GdH3 also reveal good activity for hydrogenation of NEC and Ru/YH3 also possesses good activity for hydrogenation of 2-methylindole, indicating that the use of rare earth hydride supported catalysts is a highly effective strategy for developing better hydrogenation catalysts for N-heterocycles.

The lanthanide hydride oxides SmHO and HoHO

Abstract Metal hydride oxides are an interesting class of compounds with potential for hydride ion conduction and as host materials for luminescence. SmHO and HoHO were prepared from mixtures of the sesquioxides Ln 2O3 and the hydrides LnH2+ x at 1173 K as gray powders (Ln=Sm, Ho). They crystallize in a fluorite type crystal structure with disordered anion distribution (Fm3̅m; SmHO: a=5.46953(6) Å, V=163.625(5) Å3; HoHO: a=5.27782(3) Å, V=147.016(2) Å3, based on powder X-ray diffraction) and show stability towards air. Lanthanide-oxygen and -hydrogen distances are 2.36838(3) Å in SmHO and 2.28536(1) Å in HoHO and comparable to those in binary lanthanide oxides and hydrides. Elemental analyses confirm the composition LnHO. Quantum-mechanical calculations show a negative enthalpy for the reaction RE 2O3+REH3→3 REHO for all lanthanides and Y, with increasing values for decreasing ionic radii.

Enabling the development of ductile powder metallurgy titanium alloys by a unique scavenger of oxygen and chlorine

NdB6 can effectively react with the surface titanium oxide films to form an initial NdBO3 layer at about 670 °C, just prior to their dissolution. Two thirds of all the oxygen scavenged during sintering occurred in the α-Ti phase region through this reaction. NdBO3 layer continues to grow by diffusion of oxygen until 1130 °C, beyond which NdBO3 decomposes into Nd2O3 dispersoids and Nd2O3 further reacts with chlorine to form NdClxOy. The unique oxygen-scavenging process prior to the active dissolution of oxide film makes NdB6 more effective in scavenging oxygen than other rare earth hydrides, resulting in better tensile elongation. The addition of NdB6 also leads to the improved sintered density, substantial microstructural refinement and dispersion strengthening by Nd-containing compounds and TiB. The optimal addition level of NdB6 is 0.3 wt.%, beyond which large Nd2O3 aggregates and an increased number of TiB whiskers were formed. This is detrimental to the ductility.

Microstructure evolution, mechanical properties and diffusion behaviour of Mg-6Zn-2Gd-0.5Zr alloy during homogenization

The microstructure evolution and mechanical properties of Mg-6Zn-2Gd-0.5Zr alloy during homogenization treatment were investigated. The as-cast alloy was found to be composed of dendritic primary α-Mg matrix, α-Mg+W (Mg3Zn3Gd2) eutectic along grain boundaries, and icosahedral quasicrystalline I (Mg3Zn6Gd) phase within α-Mg matrix. During homogenization process, α-Mg+W (Mg3Zn3Gd2) eutectic and I phase gradually dissolved into α-Mg matrix, while some rod-like rare earth hydrides (GdH2) formed within α-Mg matrix. Both the tensile yield strength and the elongation showed a similar tendency as a function of homogenization temperature and holding time. The optimized homogenization parameter was determined to be 505°C for 16h according to the microstructure evolution. Furthermore, the diffusion kinetics equation of the solute elements derived from the Gauss model was established to predict the segregation ratio of Gd element as a function of holding time, which was proved to be effective to evaluate the homogenization effect of the experimental alloy.

The Smaller the Better: Hosting Trivalent Rare-Earth Guests in Cu–P Clathrate Cages

Summary The most striking difference between the trivalent rare-earth cations and their alkali and alkaline-earth peers is the presence of localized 4 f electrons. Placing trivalent rare-earth cations inside fullerene molecules or in between blocks of intermetallics has given rise to a plethora of fascinating properties and materials. A long-sought-after yet undiscovered material is a semiconducting compound with rare-earth cations situated inside the oversized polyhedral cages of a three-dimensional framework. In this work, we present a synthesis of unconventional clathrates, Ba 8−x R x Cu 16 P 30 , with the smallest pentagonal dodecahedral cages encapsulating La 3+ and Ce 3+ . Their composition and crystal structure were unambiguously determined by a combination of synchrotron powder diffraction, time-of-flight neutron powder diffraction, scanning transmission electron microscopy, and electron energy-loss spectroscopy. Our quantum-mechanical calculations and experimental characterizations show that the incorporation of the rare-earth cations significantly enhances hole mobility and reduces hole concentration, resulting in a drastic (700%) increase in the thermoelectric performance.

Coercivity Enhancement of Nd-Fe-B HDDR Powder by Grain Boundary Diffusion Process with Rare-Earth Hydride

The grain boundary diffusion (GBD) process with rare-earth hydride was performed to increase the coercivity of hydrogenation–disproportionation–desorption–recombination (HDDR) powder. Before the GBD process, we investigated the effect of post-annealing of the initial HDDR powder on its magnetic properties. Low-temperature annealing reduced the coercivity of the HDDR powder. However, the coercivity decline decreased with increasing annealing temperature, becoming similar to that of the initial powder at 900°C. After the GBD process at 850°C for 1 h, the coercivity increased by about 4 kOe with 4 wt.% NdH x -Cu, forming a thick and continuous grain boundary phase. In addition, the coercivity and remanence of the HDDR powder produced by the GBD process with NdH x -Cu were higher when using NdH x in spite of the same amount of diffusion as at 2 wt.%.

Hydrogenation effect on microstructure and mechanical properties of Mg-Gd-Y-Zn-Zr alloys

This work explores the ways of manipulating the microstructure and mechanical properties of Mg-Gd-Y-Zn-Zr alloys of various compositions using hydrogen treatment. Changes to phase composition, microstructure, and mechanical properties of the alloys upon hydrogenation were studied. Prior to hydrogenation, the alloys were extruded at different temperatures with or without subsequent aging. Hydrogen treatment was performed on bulk rods after thermo-mechanical processing. X-ray diffraction and scanning electron microscopy studies showed that a single rare-earth (RE) hydride phase, Gd0.5Y0.5H2, was formed in all samples. As a result, the 14H long period stacking ordered (LPSO) structure, detected before hydrogenation, is completely destroyed due to the clustering of RE atoms into large hydride crystals and annihilation of the specific spatial order within the superlattice. Prolonged hydrogen treatment at high temperature (≈ 0.74·Tm) causes recrystallization and grain growth in the magnesium matrix, defect annealing and reduction of preferred orientation, which together with the complete destruction of the LPSO phase lead to the substantial decrease of alloys’ strength and concomitant increase of their ductility to the record-high value of above 20%.

Interaction of R1 – xZr x Si (R = La, Ce, Nd, Dy; x = 0–0.05) Silicides with Hydrogen

The interaction of R1 – xZr x (R = La, Ce, Nd, Dy; x = 0–0.05) silicides with hydrogen has been studied at hydrogen pressures of up to 250 MPa. At a high pressure and a temperature of 20°C, hydrogen solubility in the solid phase is very low. Raising the reaction temperature leads to disproportionation of the silicides. According to X-ray diffraction data, the disproportionation reaction yields a rare-earth hydride and RSi2 disilicide.

Hydrogen Clathrate Structures in Rare Earth Hydrides at High Pressures: Possible Route to Room-Temperature Superconductivity.

Room-temperature superconductivity has been a long-held dream and an area of intensive research. Recent experimental findings of superconductivity at 200K in highly compressed hydrogen (H) sulfides have demonstrated the potential for achieving room-temperature superconductivity in compressed H-rich materials. We report first-principles structure searches for stable H-rich clathrate structures in rare earth hydrides at high pressures. The peculiarity of these structures lies in the emergence of unusual H cages with stoichiometries H_{24}, H_{29}, and H_{32}, in which H atoms are weakly covalently bonded to one another, with rare earth atoms occupying the centers of the cages. We have found that high-temperature superconductivity is closely associated with H clathrate structures, with large H-derived electronic densities of states at the Fermi level and strong electron-phonon coupling related to the stretching and rocking motions of H atoms within the cages. Strikingly, a yttrium (Y) H_{32} clathrate structure of stoichiometry YH_{10} is predicted to be a potential room-temperature superconductor with an estimated T_{c} of up to 303K at 400GPa, as derived by direct solution of the Eliashberg equation.