Hydride System Research Articles

Experimental and Theoretical Study of CO2 Insertion into Ruthenium Hydride Complexes.

The ruthenium hydride [RuH(CNN)(dppb)] (1; CNN = 2-aminomethyl-6-tolylpyridine, dppb = 1,4-bis(diphenylphosphino)butane) reacts rapidly and irreversibly with CO2 under ambient conditions to yield the corresponding Ru formate complex 2. In contrast, the Ru hydride 1 reacts with acetone reversibly to generate the Ru isopropoxide, with the reaction free energy ΔG°(298 K) = -3.1 kcal/mol measured by (1)H NMR in tetrahydrofuran-d8. Density functional theory (DFT), calibrated to the experimentally measured free energies of ketone insertion, was used to evaluate and compare the mechanism and energetics of insertion of acetone and CO2 into the Ru-hydride bond of 1. The calculated reaction coordinate for acetone insertion involves a stepwise outer-sphere dihydrogen transfer to acetone via hydride transfer from the metal and proton transfer from the N-H group on the CNN ligand. In contrast, the lowest energy pathway calculated for CO2 insertion proceeds by an initial Ru-H hydride transfer to CO2 followed by rotation of the resulting N-H-stabilized formate to a Ru-O-bound formate. DFT calculations were used to evaluate the influence of the ancillary ligands on the thermodynamics of CO2 insertion, revealing that increasing the π acidity of the ligand cis to the hydride ligand and increasing the σ basicity of the ligand trans to it decreases the free energy of CO2 insertion, providing a strategy for the design of metal hydride systems capable of reversible, ergoneutral interconversion of CO2 and formate.

Development of the Multicomponent Coupled-Cluster Theory for Investigation of Multiexcitonic Interactions.

Multicomponent systems are defined as chemical systems that require a quantum mechanical description of two or more different types of particles. Non-Born-Oppenheimer electron-nuclear interactions in molecules, electron-hole interactions in electronically excited nanoparticles, and electron-positron interactions are examples of physical systems that require a multicomponent quantum mechanical formalism. The central challenge in the theoretical treatment of multicomponent systems is capturing the many-body correlation effects that exist not only between particles of identical types (electron-electron) but also between particles of different types (electron-nuclear and electron-hole). In this work, the development and implementation of multicomponent coupled-cluster (mcCC) theory for treating particle-particle correlation in multicomponent systems are presented. This method provides a balanced treatment of many-particle correlation effects in a general multicomponent system while maintaining a size-consistent and size-extensive formalism. The coupled-cluster ansatz presented here is an extension of the electronic structure CCSD formulation for multicomponent systems and is defined as |ΨmcCC⟩ = eT1I+T2I+T1II+T2II+T11I,II+T12I,II+T21I,II+T22I,II|0I0II⟩. The cluster amplitudes in the mcCC wave function were obtained by projecting the mcCC Schrödinger equation onto a direct product space of singly and doubly excited states of type I and II particles and then solving the resulting mcCC equations iteratively. These equations were derived using an automated application of the generalized Wick’s theorem and were implemented using a computer-assisted source code generation approach. The applicability of the mcCC method was demonstrated by calculating ground state energies of multicomponent Hooke's atom and positronium hydride systems as well as by calculating exciton and biexciton binding energies in multiexcitonic systems. For each case, the mcCC results were benchmarked against full configuration interaction (FCI) calculations and were found to be in excellent agreement with the FCI results. The effect of neglecting certain classes of multicomponent connected excitation terms from the mcCC wave function was also investigated. The results from this study demonstrate that connected cluster operators that generate simultaneous excitation in type I and type II space are critical for capturing electron-hole correlation in multiexcitonic systems.

Mercury contamination in the blood, urine, hair and nails of the gold washers and its human health risk during extraction of placer gold along Gilgit, Hunza and Indus rivers in Gilgit-Baltistan, Pakistan

Abstract Mercury (Hg) is considered as a global pollutant because of its presence in every environmental sector. This study was conducted to investigate the negative impacts of Hg on the human beings involved in gold mining along the three rivers (Gilgit, Hunza and Indus) in the northern areas of Pakistan. Biological samples such as blood, urine, hair and nails were obtained from the gold miners including men and women and extracted using standard procedure. The final extracts were analyzed for Hg and its species concentrations using atomic absorption spectrophotometer (Perkin Elmer-700) equipped with mercury hydride system (MHS-15). Total Hg concentrations in the blood, plasma, urine, hair and nails of male workers were 41 μg/L, 30.8 μg/L, 49.5 μg/L 0.64 μg/kg and 0.46 μg/kg, respectively, while in female workers were 44.4 μg/L, 36.8 μg/L, 41.5 μg/L, 1.1 μg/kg and 0.97 μg/kg, respectively. The results indicated that Hg in all (100%) samples exceeded the permissible limit set by US Environmental Protection Agency, (USEPA, 1997) and World Health Organization, (WHO, 2003). Being a highly toxic metal, Hg exposure has caused diseases in the workers such as inhalation problem, belly and neck pain, skin burn, stunted growth in children, teeth, heart, respiratory, kidney, joints and skin problems.

Ultrafine Nanocrystalline CeO2@C-Containing NaAlH4 with Fast Kinetics and Good Reversibility for Hydrogen Storage.

A nanocrystalline CeO2@C-containing NaAlH4 composite is successfully synthesized in situ by hydrogenating a NaH-Al mixture doped with CeO2@C. Compared with NaAlH4 , the as-prepared CeO2@C-containing NaAlH4 composite, with a minor amount of excess Al, exhibits significantly improved hydrogen storage properties. The dehydrogenation onset temperature of the hydrogenated [NaH-Al-7 wt % CeO2@C]-0.04Al sample is 77 °C lower than that of the pristine sample because of a reduced kinetic barrier. More importantly, the dehydrogenated sample absorbs ∼4.7 wt % hydrogen within 35 min at 100°C and 10 MPa of hydrogen. Compositional and structural analyses reveal that CeO2 is converted to CeH2 during ball milling and that the newly formed CeH2 works with the excess of Al to synergistically improve the hydrogen storage properties of NaAlH4. Our findings will aid in the rational design of novel catalyst-doped complex hydride systems with low operating temperatures, fast kinetics, and long-term cyclability.

Correlation between particle size and hydrogen generation properties on ammonia and lithium hydride system

Abstracts For the ammonia (NH 3 ) and lithium hydride (LiH) system, which is one of the promising hydrogen storage materials, the correlation between particle size of LiH and reactivity with NH 3 is investigated to understand the kinetic properties of the system. The pristine LiH shows low reaction yield of less than 10% due to the slow kinetics even after the reaction for 24 h. The activation by the ball-milling drastically improved both of reaction rate and yield, and then more than 80% of reaction proceeded for 24 h in the case of the LiH with the smallest particle size. Thus, the particle size strongly affects the kinetic properties. Furthermore, a thickness of possible NH 3 diffusion into the LiH surface for the 24 h reaction is estimated to be 20 nm assuming that the LiH particles are an ideal spherical shape. Therefore, the rate-determining step of the system would be the NH 3 diffusion process.

Mg-13.5wt%Ni 합금 수소화합물의 수소방출에 대한 부피법에 의한 열분석

To investigate the effect of microstructure on the formation of the desorption peak, the volumetric thermal analysis technique (VTA) was applied to the Mg-13.5 wt% Ni hydride system. The sample made by the HCS (hydriding combustion synthesis) process had two kinds of Mg microstructures. Linear heating was started with various constant heating rates. Only one peak was appeared in the case of the small initial hydrogen wt% (0.83 wt%). Yet, two peaks were appeared with increasing initial hydrogen wt% (1.85 and 3.73 wt%) when only Mg was hydrogenated. The first peak was formed through the evolution of hydrogen from , made by eutectic Mg. The second peak was formed through the evolution of hydrogen from , made by primary Mg. Therefore, this result shows that the microstructure also has a considerable effect on forming the desorption peak. We have also derived the hydrogen desorption equations by VTA to get apparent activation energy when the rate-controlling step for the desorption of the hydrided system is the diffusion of hydrogen through the phase and the chemical reaction .

Conventional superconductivity at 203 kelvin at high pressures in the sulfur hydride system.

A superconductor is a material that can conduct electricity without resistance below a superconducting transition temperature, Tc. The highest Tc that has been achieved to date is in the copper oxide system: 133 kelvin at ambient pressure and 164 kelvin at high pressures. As the nature of superconductivity in these materials is still not fully understood (they are not conventional superconductors), the prospects for achieving still higher transition temperatures by this route are not clear. In contrast, the Bardeen-Cooper-Schrieffer theory of conventional superconductivity gives a guide for achieving high Tc with no theoretical upper bound--all that is needed is a favourable combination of high-frequency phonons, strong electron-phonon coupling, and a high density of states. These conditions can in principle be fulfilled for metallic hydrogen and covalent compounds dominated by hydrogen, as hydrogen atoms provide the necessary high-frequency phonon modes as well as the strong electron-phonon coupling. Numerous calculations support this idea and have predicted transition temperatures in the range 50-235 kelvin for many hydrides, but only a moderate Tc of 17 kelvin has been observed experimentally. Here we investigate sulfur hydride, where a Tc of 80 kelvin has been predicted. We find that this system transforms to a metal at a pressure of approximately 90 gigapascals. On cooling, we see signatures of superconductivity: a sharp drop of the resistivity to zero and a decrease of the transition temperature with magnetic field, with magnetic susceptibility measurements confirming a Tc of 203 kelvin. Moreover, a pronounced isotope shift of Tc in sulfur deuteride is suggestive of an electron-phonon mechanism of superconductivity that is consistent with the Bardeen-Cooper-Schrieffer scenario. We argue that the phase responsible for high-Tc superconductivity in this system is likely to be H3S, formed from H2S by decomposition under pressure. These findings raise hope for the prospects for achieving room-temperature superconductivity in other hydrogen-based materials.

Metal Hydrides for High-Temperature Power Generation

Metal hydrides can be utilized for hydrogen storage and for thermal energy storage (TES) applications. By using TES with solar technologies, heat can be stored from sun energy to be used later, which enables continuous power generation. We are developing a TES technology based on a dual-bed metal hydride system, which has a high-temperature (HT) metal hydride operating reversibly at 600–800 °C to generate heat, as well as a low-temperature (LT) hydride near room temperature that is used for hydrogen storage during sun hours until there is the need to produce electricity, such as during night time, a cloudy day or during peak hours. We proceeded from selecting a high-energy density HT-hydride based on performance characterization on gram-sized samples scaled up to kilogram quantities with retained performance. COMSOL Multiphysics was used to make performance predictions for cylindrical hydride beds with varying diameters and thermal conductivities. Based on experimental and modeling results, a ~200-kWh/m3 bench-scale prototype was designed and fabricated, and we demonstrated the ability to meet or exceed all performance targets.

Mercury health effects among the workers extracting gold from carpets and dusted clays through amalgamation and roasting processes

Mercury (Hg) is a highly toxic metal which can cause serious health effects. The aim of this research was to determine the concentrations of total Hg (T-Hg), methyl Hg (Me-Hg), and inorganic Hg (I-Hg) in the biological samples (plasma, red blood cells (RBCs), urine, hair, and nails) of the exposed goldsmith workers. This is the first study that determines the detailed Hg concentrations in the biological samples (plasma, RBCs, urine, hair, and nails) of the exposed goldsmith workers and correlates them with the diseases noted among the workers in a single paper. Biological samples were collected from goldsmith workers (n = 40) and analyzed for T-Hg, Me-Hg, and I-Hg using atomic absorption spectrometer equipped with mercury hydride system. The mean T-Hg concentration in RBCs (33 μg L(-1)), plasma (11.8 μg L(-1)), urine (167 μg L(-1)), hair (4.21 μg g(-1)), and nails (5.91 μg g(-1)) were higher than the control RBCs (1.64 μg L(-1)), plasma (0.55 μg L(-1)), urine (2.72 μg L(-1)), hair (0.35 μg g(-1)), and nails (0.51 μg g(-1)). All workers participated in this study were suffering from physical and mental diseases. The concentration of Hg was found higher among the workers suffering from mental diseases as compared to those suffering from physical diseases. Among the physical diseases, the most serious diseases were sexual dysfunction, skin diseases, and fatigue because the workers suffering from these diseases had higher concentration of Hg than the workers with other diseases. The occurrence of physical diseases (88%) was greater than the mental diseases (53%) among the workers. The correlations of physical and mental diseases with experience (years of work) and exposure time were significant (p < 0.05), while nonsignificant (p > 0.05) correlation was observed between demographic parameters and Hg concentrations in the biological samples of the workers. The burning process of amalgamated gold is a significant source of Hg exposure to goldsmith workers; therefore, awareness and precautionary measures are needed to provide protection to them.

Internal Resistance Reduction of a Membrane Electrolyzer for Electrohydrogenation of Toluene As Hydrogen Carrier Synthesis

Introduction Potential of renewable energy is uneven distribution with fluctuation; therefore, to increase renewable energies, energy carrier technology is needed for storage and transportation. Hydrogen can be produced with various energies, but its volumetric energy density is too low to storage and transport. Toluene-methylcyclohexane organic chemical hydride system is one of promising technologies as hydrogen storage and transportation by invention of the dehydrogenation catalyst (1). Electrohydrogenation of toluene with water splitting has higher theoretical energy conversion efficiency compare to a series process of water electrolysis and hydrogenation. Direct and indirect electrohydrogenation processes had been proposed (2, 3). However, further challenge is needed to improve efficiency, durability, and increasing of the scale. We focused on a membrane electrolysis as a direct process with water splitting. Cathode side is a cathode membrane assembly, which is applied PEFC technology with precious metal loaded carbon electrocatalyst. Anode is a dimensionally stable electrode for oxygen evolution reaction in acidic electrolyte using industrial electrolysis technology. In our previous study, cell voltage of electrohydrogenation of toluene was almost as same as water electrolysis, but internal resistance of the electrolyzer was not small enough and conversion was very low (4). In this study, the membrane of anode side was hydrophilized to release oxygen from anode to reduce internal resistance, and characteristics of low concentration of toluene has been evaluated to reach high conversion to methylcyclohexane. Experimental A cathode was a carbon paper (35BC, SGL) coated 0.5 mgcm-2 of PtRu (TEC61E54, TKK) with Nafion dispersion. The cathode was pressed on a perfluoroethylene sulfuric acid (PFSA) membrane (Nafion® 212CS, DuPont) for a cathode membrane assembly. The membrane of the cathode side was hydrophilized by ZrO2 power coating or mechanically roughing. A non-treated membrane was also used for comparison. A DSE® anode with IrO2 based electrocatalyst is used for oxygen evolution. Toluene/methylcyclohexane mixture and 1M (=moldm-3) of H2SO4 were supplied to the cathode and anode for hydrogenation of toluene, respectively. The toluene concentration was in the range from 1 to 100 %. Cell voltage was determined with 4 mVs-1 of voltage sweep from 0.6 for toluene hydrogenation. Current efficiency was determined with constant cell voltage electrolysis with the volume measurement of gas evolution from cathode during the electrolysis. Internal resistance (iR) was determined with higher frequency intercept of AC impedance method in the cell voltage range from 1.0 to 2.5 V. Results and discussion Figure 1 shows the internal resistance and cell voltage as a function of current density for 100 % TL fed with various membrane treatments of anode side. The output of H2SO4 pulsed especially in high current density region, and current fluctuated above 2.2 V of the cell voltage with hydrogen evolution from cathode. This behavior would be affected by oxygen release between the membrane and the anode. The cell voltage and resistance of the hydrophilized membranes was smaller than those of non-treated membrane. At this moment, the output of H2SO4 and oxygen mixture was smooth, and there was no gas evolution reaction from cathode. Figure 2 shows the cell voltage (a) and current efficiency of hydrogenation as a function of current density for 1 to 100 % of TL-MCH fed with the rough surface membrane cell. The side reaction is only hydrogen evolution for this system. Error bars were current variation during constant cell voltage electrolysis for current and current efficiency measurements. The lines show the polarization curves with 4 mVs-1 of cell voltage sweep. For 100 and 50 % of TL fed, the current efficiency of electrohydrogenation was above 97 % up to the limit of current source of 0.44 Acm-2. At this moment, electrolysis was stable, and the cell voltage was around 2.0 V at 0.4 Acm-2. Below 10 % of TL fed, simultaneously hydrogen evolution proceeded at higher current density region with current variation. Therefore, this process should be feasible, and improvement of mass transfer in the cathode catalyst layer is important issue for real application. Acknowledgment This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “energy carrier” (Funding agency: JST). We applicate the person concerned them. References 1) Y. Okada, E. Sasaki, E. Watanabe, S. Hyodo, H. Nishijima, J. Hydrogen Energy, 2006, 31, 1348. 2) T. Hirashige, T. Ishikawa, M. Sugimasa, WO 2011/122155 Al (PCT/JP2011/053478) 2011. 3) C. Iwakura, Y. Yoshida, S. Ogata, H. Inoue, J. Mater. Res., 2011, 13, 821. 4) S. Mitsushima, Y. Takakuwa, Y. Kohno, K. Matsuzawa, A. Kato, Y. Nishiki, of 226th meeting of the ECS, #619 (2014). Figure 1

Reversible hydrogen storage properties of LiBH4 combined with hydrogenated Mg11CeNi alloy

The hydrogenated Mg11CeNi alloy was utilized as a destabilizer to combine with LiBH4 in this paper. It was found that thermal dehydrogenation of the LiBH4/Mg11CeNi hydride system with a molar ratio of 17:2.5 proceeds in two-step reactions: one initiates at around 280 °C with about 4.0 wt.% of hydrogen released from MgH2 and Mg2NiH4, and the other occurs in the temperature range 350–425 °C that can be assigned to the decomposition of LiBH4 co-destabilized by Mg2Ni, CeH2 and Mg. These two-step dehydrogenation reactions for the LiBH4/Mg11CeNi hydride system are partially reversible, and about 4.9 wt.% of hydrogen can be reloaded at 400 °C under an initial hydrogen pressure of 9 MPa.

Generalized computational model for high-pressure metal hydrides with variable thermal properties

This study considers a detailed 1D fueling model applied to a metal hydride system, with Ti1.1CrMn as the absorbing alloy, to predict the weight fraction of the absorbed hydrogen and the solid bed temperature. Dependencies of thermal conductivity and specific heat capacity upon pressure and hydrogen content, respectively, are accounted for by interpolating experimental data. The effect of variable parameters on the critical metal hydride thickness is investigated and compared to results obtained from a constant-parameter analysis. Finally, the discrepancy in the metal hydride thickness value is estimated to be approximately 10%.

Effect of Lithium Ion Conduction on Hydrogen Desorption of LiNH2–LiH Solid Composite

This paper presents a relationship between ionic mobility and reaction kinetics for the Li–N–H system after doping LiTi2O4. The structural characteristic of this fast ionic conductor was introduced to the complex–hydride system. On one hand, the properties of the dehydrogenation process were improved significantly. On the other hand, the relationship between lithium ionic conductivity and the catalytic effect on the dehydrogenation was investigated according to the alternating current (AC) impedance results. The lithium ionic conductivity of samples with catalyst was higher than the samples without catalyst. Especially, the conductivity of LiNH2 and LiH mixtures with LiTi2O4 was almost 1.5 times higher than that of LiNH2 and LiH. The mobility of the Li+ ions between LiH and LiNH2 solid phases was enhanced by adding LiTi2O4.

Metal hydride systems for hydrogen storage and supply for stationary and automotive low temperature PEM fuel cell power modules

Metal Hydrides (MH) provide efficient hydrogen storage for various applications, including Low Temperature PEM Fuel Cells (LT PEMFCs), when system weight is not a major and critical issue. Endothermic dehydrogenation of MH leading to decreased rates of H2 evolution eliminates the risk of accidents even in the case of rupture of the hydrogen storage containment. At the same time, it poses a number of challenges related to the constant, stable and sufficient H2 supply for stable FC operation.This paper reviews recent efforts in MH hydrogen storage and supply systems for LT PEMFC applications, including the ones developed at HySA Systems/SAIAMC/University of the Western Cape. The systems are characterised by a series of hydrogen storage capacities ranging from 10 NL to ∼10 Nm3 H2 in turns providing stable operation for stationary and mobile FC power modules (from a few W to several kW). The MH systems use unstable hydride materials (equilibrium H2 pressure at ambient temperature around 10 bar) that, in combination with special engineering solutions of MH containers (both liquid- and air-heated-cooled), and optimised system layout, facilitates H2 supply to LT PEMFC stacks.

Niche applications of metal hydrides and related thermal management issues

This short review highlights and discusses the recent developments and thermal management issues related to metal hydride (MH) systems for hydrogen storage, hydrogen compression and heat management (refrigeration, pump and upgrade, etc.). Special attention is paid to aligning the system features with the requirements of the specific application. The considered system features include the MH material, the MH bed on the basis of its corresponding MH container, as well as the layout of the integrated system.

Investigation of the dehydrogenation behavior of the 2LiBH4:CaNi5 multicomponent hydride system

LiBH4 has gained much attention as a potential hydrogen storage material due to its high hydrogen storage capacity of 18.5 wt%. However, LiBH4 only releases its full hydrogen capacity at temperatures greater than 600 °C and requires hydrogen pressures of at least 350 bar to rehydrogenate the end products. The dehydrogenation temperature can be altered by thermodynamic tuning through the addition of a reactive agent resulting in a lower enthalpy of dehydrogenation. Most multicomponent hydride systems display dehydrogenation temperatures above 300 °C, making them less desirable for automotive applications. In this work we report the solid-state decomposition of LiBH4 in the 2LiBH4:CaNi5 system below the LiBH4 melting temperature of 270 °C. In situ neutron diffraction measurements confirmed the decomposition took place in the solid state at 200 °C, forming LiD, CaD2, Ni3B and Ni2B phases as end products. The solid-state decomposition was further supported by SEM and TEM measurements showing the presence of nano-crystalline particles.

Potassium, rubidium and cesium hydrides as dehydrogenation catalysts for the lithium amide/magnesium hydride system

In this study, the effectiveness of several alkali metal hydrides (KH, RbH and CsH) for improving the hydrogen desorption properties of a 2LiNH2/MgH2 mixture was studied. Results showed that the relative effectiveness of these additives in decreasing the hydrogen desorption temperature, lowering the activation energy and increasing desorption rates from the mixtures is in the order: RbH > KH > CsH > Un-catalyzed. Modeling studies showed that diffusion through a Li2Mg(NH)2 product layer is the rate-controlling process. It is believed that the alkali elements: K, Rb and Cs partially replace the Li in the product layer. This may have an inductive effect in which the N–H bond is weakened thus leading to lower desorption enthalpies. The lattice expansion caused by substitution of the larger alkali elements for Li may also allow for faster diffusion and increased desorption rates.

Nanostructured, complex hydride systems for hydrogen generation

Complex hydride systems for hydrogen (H2) generation for supplying fuel cells are being reviewed. In the first group, the hydride systems that are capable of generating H2 through a mechanical dehydrogenation phenomenon at the ambient temperature are discussed. There are few quite diverse systems in this group such as lithium alanate (LiAlH4) with the following additives: nanoiron (n-Fe), lithium amide (LiNH2) (a hydride/hydride system) and manganese chloride MnCl2 (a hydride/halide system). Another hydride/hydride system consists of lithium amide (LiNH2) and magnesium hydride (MgH2), and finally, there is a LiBH4-FeCl2 (hydride/halide) system. These hydride systems are capable of releasing from ~4 to 7 wt.% H2 at the ambient temperature during a reasonably short duration of ball milling. The second group encompasses systems that generate H2 at slightly elevated temperature (up to 100 °C). In this group lithium alanate (LiAlH4) ball milled with the nano-Fe and nano-TiN/TiC/ZrC additives is a prominent system that can relatively quickly generate up to 7 wt.% H2 at 100 °C. The other hydride is manganese borohydride (Mn(BH4)2) obtained by mechano-chemical activation synthesis (MCAS). In a ball milled (2LiBH4 + MnCl2) nanocomposite, Mn(BH4)2 co-existing with LiCl can desorb ~4.5 wt.% H2 at 100 °C within a reasonable duration of dehydrogenation. Practical application aspects of hydride systems for H2 generation/storage are also briefly discussed.

Non-precious bimetallic catalysts for selective dehydrogenation of an organic chemical hydride system.

Methylcyclohexane (MCH)-toluene (TOL) chemical hydride cycles as hydrogen carrier systems are successful with the selective dehydrogenation of MCH to TOL, which has been achieved only using precious Pt-based catalysts. Herein, we report improved selectivity using non-precious metal nickel-based bimetallic catalysts, where the second metal occupies the unselective step sites.

First principles study of structural, electronic, elastic and magnetic properties of gadolinium hydride system GdHx (x=1, 2, 3)

The structural, electronic, elastic and magnetic properties of gadolinium and its hydrides GdHx (x=1, 2, 3) are investigated by using Vienna ab-initio simulation package with the generalized gradient approximation parameterized by Perdew, Burke and Ernzerhof (GGA-PBE) plus a Hubbard parameter (GGA-PBE+U) in order to include the strong Coulomb correlation between localized Gd 4f electrons. At ambient pressure all the hydrides are stable in the ferromagnetic state. The calculated lattice parameters are in good agreement with the experimental results. The bulk modulus is found to decrease with the increase in the hydrogen content for the gadolinium hydrides. A pressure-induced structural phase transition is predicted to occur from cubic to hexagonal phase in GdH and GdH2 and from hexagonal to cubic phase in GdH3. The electronic structure reveals that mono and di-hydrides are metallic, whereas trihydride is half-metallic at normal pressure. On further increasing the pressure, a half-metallic to metallic transition is also observed in GdH3. The calculated magnetic moment values of GdHx (x=1, 2, 3) are in accord with the experimental values.