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Abstract This work was focused on studies of structural and electrochemical properties of the La-doped AB 2 -type Zr/Ti-based metal hydride anode alloys. The Ti 0.2 Zr 0.8 La 0-0.05 Ni 1.2 Mn 0.7 V 0.12 Fe 0.12 alloys were characterized using SEM, EDS and XRD, which concluded that the major C15 Laves type AB 2 compound co-exists with a minor La-Ni intermetallic. NPD study indicated that vanadium together with Ti and Zr partially fills the A site, while the rest of V together with Ni, Mn and Fe statistically fills the B site. NPD showed that in a trihydride (Ti,Zr,V)(Ni,Mn,Fe,V) 2 D 2.9 D atoms occupy A 2 B 2 tetrahedra. The alloys were characterized during high-rate discharge and on cycling. La addition resulted in a significant improvement of the activation performance caused by a catalytic influence of LaNi hydride. The highest content of La greatly accelerated the activation, but it also caused an obvious decrease in discharge capacity and cycling stability. The alloy with an optimized La addition (x = 0.03) demonstrated a maximum discharge capacity of 420 mAh g−1 and the discharge capacity maintained at 79% at 0.71 C, while the capacity retention after 500 cycles was also high, 63%. Based on the EIS results, we conclude that the decay of the alloy electrode is related to the irreversible capacity loss and its pulverization. Graphical abstract Image 1 Highlights • Annealed multi-component Zr/Ti-based alloy has a FCC C15 Laves type structure. • The maximum electrochemical discharge capacity of the alloy reaches 420 mAhg−1. • La addition significantly improves the electrochemical performance of AB 2 alloy. • D atoms occupy the 96 g sites only while V atoms occupy both 8 a and 16 d sites. [ABSTRACT FROM AUTHOR]

New Nd 3− x Mg x Co 9 alloys have been synthesized by powder sintering method and their crystal structure, hydrogen storage and magnetic properties have been systematically studied. X-ray diffraction analysis showed that Nd 3− x Mg x Co 9 ( x ≤ 1.5) alloys belong to the PuNi 3 -type structure and NdMg 2 Co 9 is an individual ternary compound with YIn 2 Ni 9 -type structure. Hydrogen storage capacity of the PuNi 3 -type Nd 3− x Mg x Co 9 alloys is in the range 1.1–1.6 wt % H at room temperature, while for NdMg 2 Co 9 it is below 0.5 wt % H. PСT measurements demonstrated that thermodynamic stability of hydrides decrease with increasing Mg content. Hydrides of the alloys with lower Mg content ( x ≤ 1.0) are stable in air at room temperature. XRD study of these hydrides showed preserved PuNi 3 -type structure with hydrogen-induced volume expansion 16.7–24%. The positions of hydrogen atoms were determined by neutron powder diffraction study of Nd 2 MgCo 9 D 10 . In the Nd 2 MgCo 9 D 10 structure D atoms partially occupy four types of interstices, including octahedral Nd 2 Co 4 sites, tetrahedral Nd 2 Co 2 sites and two types of trigonal bipyramids (Nd,Mg) 3 Co 2 . Electrochemical investigations of the Nd 2 MgCo 9 electrode demonstrated rather low discharge capacity, 100 mAh/g, which is about 30% of its theoretical capacity. Magnetic measurements have shown that Nd 2 MgCo 9 is ferromagnetic with high Curie temperature (T C = 633 K) and spin reorientation at 225 K, its hydrogenation causes a significant decrease of the magnetization. [ABSTRACT FROM AUTHOR]

The isothermal section of the Tb–Mg–Co phase diagram at 500 °C has been built on the basis of XRD analysis of forty samples prepared by powder metallurgy. The existence of two ternary compounds Tb 4 Mg 3 Co 2 and Tb 4 MgCo was confirmed. The formation of two solid solutions, Tb 1− x Mg x Co 3 (0≤ x ≤0.4) and Tb 1- −x Mg x Co 2 (0≤ x ≤0.6), was found for the first time. It is shown that Tb 5 Mg 24 also dissolves a small amount of Co. Other binary compounds do not dissolve the third component. The Tb 4 MgCo and TbMgCo 4 compounds form hydrides (12.7 and 5.3 at.H/f.u. capacity, respectively) that retain the original structure of metallic matrices. Upon thermal desorption the Tb 4 MgCoH 12.7 hydride was stable up to 300 °C and disproportionated at higher temperature. Two other hydrides, Tb 4 Mg 3 Co 2 H ∼4 and Tb 2 MgCo 9 H 12 , are unstable in air and decompose into the initial compounds. [ABSTRACT FROM AUTHOR]

The influence of Co substitution on the structural and magnetic properties of NdMgNi 4− x Co x ( x ⩽ 3) alloys and their hydrides has been investigated. The formation of NdMgCo 4 compound has not been observed. The gas hydrogenation of these alloys resulted in the formation of NdMgNi 4 H 4 , NdMgNi 3 CoH 4.8 , NdMgNi 2 Co 2 H 5.6 and NdMgNiCo 3 H 6.2 hydrides. Transformation from cubic into orthorhombic structure upon hydrogenation has been confirmed for NdMgNi 4 H 4 hydride, whereas it has been found that Co-containing hydrides preserved the parent cubic structure. PC diagrams at 50 °C for NdMgNi 4− x Co x –H 2 ( x = 0, 2, 3) showed only one absorption/desorption plateau, which equilibrium pressure decrease as the cell volume increase. The NdMgCu 4 ternary compound has been synthesized, but it does not interact with hydrogen under normal conditions. Electrochemical investigations demonstrated that substitution of the nickel atoms by the cobalt ones in NdMgNi 4 does not significantly influence on the maximum discharge capacity (244–261 mA h/g) and all alloys are characterized by high cyclic stability. NdMgNi 4 and its hydride display a Pauli paramagnet behavior. NdMgNi 2 Co 2 displays a magnetic order below 50 K whereas its hydride shows a Pauli paramagnet behavior similar to that of NdMgNi 4 H 4 . [ABSTRACT FROM AUTHOR]

Abstract: The present work is focused on studies of the influence of magnesium on the hydrogenation behaviour of the (La,Mg)2Ni7 alloys. Substitution of La in La2Ni7 by Mg to form La1.5Mg0.5Ni7 preserves the initial Ce2Ni7 type of the hexagonal P63/mmc structure and leads to contraction of the unit cell. The system La1.5Mg0.5Ni7–H2 (D2) was studied using in situ synchrotron X-ray and neutron powder diffraction in H2/D2 gas and pressure–composition–temperature measurements. La replacement by Mg was found to proceed in an ordered way, only within the Laves-type parts of the hybrid crystal structure, yielding formation of LaMgNi4 slabs with statistic and equal occupation of one site by La and Mg atoms. Mg alters structural features of the hydrogenation process. Instead of a strong unilateral anisotropic expansion which takes place on hydrogenation of La2Ni7, the unit cell of La1.5Mg0.5Ni7D9.1 is formed by nearly equal hydrogen-induced expansions proceeding in the basal plane (Δa/a=7.37%) and along [001] (Δc/c=9.67%). In contrast with La2Ni7D6.5 where only LaNi2 layers absorb hydrogen atoms, in La1.5Mg0.5Ni7D9.1 both LaNi5 and LaMgNi4 layers become occupied. Nine types of sites were found to be filled by D in total, including tetrahedral (La,Mg)2Ni2, (La,Mg)Ni3, Ni4, tetragonal pyramidal La2Ni3 and trigonal bipyramidal (La,Mg)3Ni2 interstices. The hydrogen sublattice around the La/Mg site shows formation of two co-ordination spheres of D atoms: an octahedron MgD6 and a 16-vertex polyhedron LaD16 around La. The interatomic distances are in the following ranges: La–D (2.28–2.71), Mg–D (2.02–2.08), Ni–D (1.48–1.86Å). All D–D distances exceed 1.9Å. Thermodynamic PCT studies yielded the following values for the ΔH and ΔS of hydrogenation/decomposition; ΔH H=−15.7±0.9kJ(molH)−1 and ΔS H=−46.0±3.7J(KmolH)−1 for H2 absorption, and ΔH H=16.8±0.4kJ(molH)−1 and ΔS H=48.1±1.5J(KmolH)−1 for H2 desorption. [Copyright &y& Elsevier]

Abstract: ‘Hybrid’ RNi3 (R=rare earth metal) crystal structures are built of the slabs of simpler types, CaCu5 and MgZn2. Different affinities of these slabs to hydrogen result in unusual “anisotropic” expansion of the RNi3 and R2Ni7 (R=La, Ce) structures upon hydrogenation. This work focuses on studies of the hydrogenation behaviour of LaNi3 and on the properties of the hydrides of the modified by Mg and Mn La–Ni alloys. The crystal structure of LaNi3D2.8 and the crystal structure and hydrogen storage behaviours of the La1.5Mg0.5Ni7 and La(Ni1−x Mn x )3 (x =0; 0.067; 0.133; 0.2; 0.267; 0.3; 0.333; 0.4) alloys were in focus. The deuteration of LaNi3 with PuNi3 type of structure leads to the formation of LaNi3D2.8 and is accompanied by a deformation of the metal matrix causing a change of the initial rhombohedral symmetry (space group ) to a monoclinic one (space group C2/m; a =8.6408(7)Å, b =4.9281(4)Å, c =32.774(3)Å; β =90.850(8)°; V =1395.5(2)Å3). Similar to the earlier studied CeNi3D2.8, preferential occupation by deuterium atoms of the AB2 layers takes place, leading to the “anisotropic” expansion of the unit cell along [001] (Δc/c =30.6%). 14 occupied D crystallographic sites have 4 chemically different types of metal-atom surroundings, including Ni4 (2), La2Ni2 (2), La3Ni (6), and La3Ni3 (4). Modification of the La–Ni alloys by magnesium and manganese leads to the formation of intermetallic compounds crystallising with the PuNi3, CeNi3, and Ce2Ni7-type structures. An ordered substitution of La by Mg in the MgZn2-type slabs was observed, causing a complete alteration of the hydrogenation behaviour of the original LaNi3 alloy. La1.5Mg0.5Ni7D 9 isotropically expands upon its formation and leads to a substantial increase of the stability against hydrogenation-induced amorphisation. On the other hand, replacement of Ni by Mn leads to the change in crystal-structure type from PuNi3 to CeNi3 in the LaNi3−x Mn x alloys (x >0.1). An ordered substitution of Ni by Mn proceeds inside the RNi5 slabs only. This decreases the stability of the initial alloy against amorphisation on hydrogenation. [Copyright &y& Elsevier]

Abstract: A new intermetallic deuteride Ce2Ni7D4.7 with an anomalous volume expansion has been studied. Its structure was solved on the basis of in situ neutron diffraction data. Expansion proceeds along the c-axis and within the CeNi2 slabs only. All D atoms are located inside these slabs and on the border between CeNi2 and CeNi5. Ordering of D atoms in the bulk of CeNi2 is accompanied by substantial deformation of these slabs thus lowering the hexagonal symmetry to orthorhombic [space group Pmcn (No. 62); a=4.9251(3)Å, b=8.4933(4)Å, c=29.773(1)Å]. Inside the CeNi2 layer the hydrogen sublattice is completely ordered; all D–D distances exceed 2.0Å. Local coordination of Ni by D inside the CeNi2 blocks is of “open”, saddle-like type. Hydrogen ordering is mainly determined by Ce–H and H–H interactions. The pressure–composition–temperature measurements yielded the following thermodynamic parameters of the formation of the hydride: ΔH=−22.4kJ/molH, ΔS=−59.9J/(KmolH). [Copyright &y& Elsevier]

Abstract: The crystal and magnetic structure of TbNiSnD has been studied by neutron powder diffraction. The deuteride crystallizes with the filled TiNiSi type structure (sp. gr. Pnma; a=7.0354(1); b=4.20662(9); c=8.1808(2)Å at 293K; a=7.008(1); b=4.1954(6); c=8.153(1)Å at 2K) and is canted ferromagnetic below 10K. The deuterium atoms occupy the tetrahedral Tb3Ni sites. The Tb atoms carry a magnetic moment of 6.6(1) μ B /Tb. They order antiferromagnetically along the a-axis and ferromagnetically along the c-axis. No significant magnetic moment on the Ni atoms has been observed by neutron diffraction. Insertion of deuterium in TbNiSn induces both a change of the magnetic structure and a reduction of the Tb magnitude of the magnetic moment. [Copyright &y& Elsevier]

The recent powder neutron diffraction study of the crystal structure of LaNiInD1.22 [J. Alloys Comp. 330–322 (2002) 132] concluded on the formation of a D…D pair with an unusually short interatomic distance of 1.63 A˚. Hydrogen atoms in LaNiInD1.22 occupy a single crystallographic site and are coordinated by face-sharing La3Ni tetrahedra (92% occupancy). PCT measurements show that hydrogen storage capacity of LaNiIn, 1.63 at.H/formula unit, exceeds the limit of 1.33 at.H/LaNiIn when the La3Ni sites are completely occupied. In the present work, in situ powder neutron diffraction data were collected under D2 pressure of 4.6 bar in order to study the deuteride with a maximum D content in the metal matrix. In the hexagonal structure of LaNiInD1.63 (space group P6¯2m; a=7.3874(4); c=4.6816(2) A˚) Rietveld refinements showed that deuterium atoms occupy 36% of the available distorted La3NiIn2 octahedra, in addition to the 96% filled La3Ni sites. The structure of LaNiInD1.63 represents the first example of a deuteride containing direct In–D bonds (2.346(2) A˚). H bonding to the La3NiIn2 sites is rather weak and a desorption from these sites takes place at room temperature and hydrogen pressures below 1 bar. [Copyright &y& Elsevier]

The isothermal sections of the Nd–Mg–Co phase diagram at 300 and 500 °C have been constructed on the basis of X-ray diffraction (XRD), Wavelength Dispersive Spectrometry (WDS) and Electron Probe Micro Analyzer (EPMA) analysis. The existence of two ternary compounds Nd 4 Mg 3 Co 2 (own structure type) and Nd 4 MgCo (Gd 4 RhIn type) was confirmed. The latter has homogeneity range defined as Nd 4- x Mg 1+ x Co with 0 ≤ x ≤ 0.2 at 300 °С and 0 ≤ x ≤ 0.3 at 500 °С. Four new ternary compounds were identified in this work: Nd 23- x Mg 4+ x Co 7 (Pr 23 Mg 4 Ir 7 type; −1.5 ≤ x ≤ 0.1 at 300 °С and −0.6 ≤ x ≤ 0.3 at 500 °С); NdMg 2 Co 9 (YIn 2 Ni 9 type), ∼Nd 77 Mg 14 Co 9 (cubic) and ∼Nd 56 Mg 6 Co 38 (unknown crystal structure). Two binary compounds, NdCo 2 and NdCo 3 , were found to dissolve significant amount of Mg with the formation of substitutional solid solutions Nd 1- x Mg x Co 2 (MgCu 2 type; x ≤ 0.1 at 300 °С and x ≤ 0.15 at 500 °C) and Nd 1- x Mg x Co 3 (PuNi 3 type; x ≤ 0.33 at 300 °С and x ≤ 0.5 at 500 °C), respectively. The Nd 4 Mg 3 Co 2 , Nd 4- x Mg 1+ x Co, Nd 23- x Mg 4+ x Co 7 and Nd 1- x Mg x Co 2 compounds irreversibly absorb hydrogen at room temperature and pressure 1–10 bar to form crystalline or amorphous hydrides with hydrogen content 1.6–2.1 wt. %. Hydrogen thermal desorption from these hydrides in vacuum results in their disproportionation with formation of binary NdH 2 hydride. The Nd 1- x Mg x Co 3 (0 ≤ x ≤ 0.5) alloys reversibly absorb up to 1.3 wt. % of hydrogen. NdMg 2 Co 9 compound has small hydrogenation capacity, absorbing only 0.5 wt. % at room temperature and hydrogen pressure 185 bar. [ABSTRACT FROM AUTHOR]

Our earlier study Yartys et al. (2015) showed that at high hydrogen pressures, hexagonal MgNi 2 undergoes a hydrogen assisted phase transition into the orthorhombic MoSi 2 -type structure. Here we report on a combined high resolution synchrotron and neutron diffraction investigation of the crystal structure of MgNi 2 D 3 , and ab initio calculation of its electronic structure that revealed the nature of the metal–hydrogen bonding. The diffraction data (293 and 1.8 K) are well described with a Cmca unit cell with H atoms filling the deformed octahedra Mg 4 Ni 2 and the positions within the buckled nets –Ni–H–Ni–H– penetrating through the structure. DFT and phonon calculations showed that the Cmca structure of MgNi 2 D 3 is the most stable, both from the electronic structure and the lattice dynamical arguments. The Bader charge analysis indicates an electronic transfer from Mg (−1.59e − ) to Ni (+0.21e − ), H1 (+0.55e − ) and H2 (+0.31e − ). The phonon dispersion curves of MgNi 2 H 3 show positive frequencies, indicating that the structure is mechanically stable. The calculated gross heat of formation for the Cmca phase of MgNi 2 H 3 is −37.3 kJ/mol-H 2 , which makes it more stable by 3 kJ/mol-H 2 than the prototype structures tested in Yartys et al. (2015). The stability of the Cmca crystal structure of MgNi 2 H 3 is enhanced by the formation of the directional Ni–H covalent bonds supplemented by the electron transfer from Mg to both Ni and H. The heat capacity as a function of temperature is obtained by phonon calculation in the quasi-harmonic approximation. [ABSTRACT FROM AUTHOR]

MgNi 2 intermetallic was synthesized by powder metallurgy and crystallizes with a Laves-type C 36 structure (space group P 6 3 / mmc (No. 194); a = 4.826; c = 15.832 Å). At 300 °C during interaction with hydrogen (deuterium) gas compressed to 2.8–7.4 GPa, a trihydride MgNi 2 H(D) 3.2 was synthesized. The trihydride remained metastable at ambient conditions allowing its structure, stability and magnetic properties to be studied. The formation of MgNi 2 H 3.2 is associated with a complete rebuilding of the initial hexagonal structure into the orthorhombic distorted MoSi 2 -type sublattice (space group Fmmm (No. 69); a = 4.55; b = 4.69; c = 8.80 Å). Neutron diffraction of the MgNi 2 D 3.2 demonstrated that D atoms fill sites having octahedral Mg 4 Ni 2 (D1/4 b ) and planar Ni2 (D2/8 f ) coordination. Within the framework of the density functional theory, density of states (DOS) calculations showed the formation of a structure around −10 to −6 eV caused by the chemical bonds of hydrogen and its 1 s states mainly via interaction with the 3 d states of Ni. Analysis of the electronic structure revealed a charge transfer from Mg to Ni, and to the H atoms. The calculated enthalpy of formation of MgNi 2 H 3 is about −30 kJ/mol-H 2 , which is consistent with the stability of the hydride at normal conditions. The initial sample contained a small amount of a secondary MgNi 3 intermetallic, which has been formed during the equilibrium interaction of magnesium and nickel at 800 °C. Thus this compound should be included in the phase diagram of the Mg–Ni system. MgNi 3 decomposes under high-temperature/high-pressure hydrogenation conditions and forms nickel monohydride. [ABSTRACT FROM AUTHOR]

Hydrogenation of RECu2 (RE=Dy, Ho, Y) at room temperature and pressures of 100–1500-bar H2 has not resulted in the formation of ternary hydrides. The interaction of hydrogen with Pr(Cu1−xNix)2 (x=0, 0.1, 0.17, 0.25, 0.32) at room temperature and pressure of 25 bar resulted in the formation of Pr(Cu1−xNix)2H∼3 hydrides. It was found that the PrCu2H3 and Pr(Cu0.9Ni0.1)2H2.9 hydrides are poorly crystallized, but that an increase of the Ni-content leads to improved crystallinity of the hydrides. The hydrides Pr(Cu0.75Ni0.25)2H∼3 and Pr(Cu0.68Ni0.32)2H∼3 preserve, shortly after the hydrogen absorption, the CeCu2 type structure of their metallic matrix with a hydrogen induced volume expansion up to 28% compared to the parent compound. During long-term exposure in the air they undergo a structural transformation from the orthorhombic CeCu2 to the hexagonal Fe2P type with a hydrogen induced volume expansion up to 16.6% compared to the parent compound. [Copyright &y& Elsevier]

This work focused on studies of the phase composition and crystal structure of LaxMm1−xNi4.7Sn0.3 (x=0, 0.5 and 1.0) alloys, where Mm denotes La-rich mischmetal produced by Norsk Hydro ASA. The alloys have been studied using conventional powder and synchrotron X-ray diffraction (XRD), powder neutron diffraction and scanning electron microscopy (SEM). After annealing at 950 °C the alloys were nearly single-phase materials with small precipitations of RENiSn equiatomic intermetallics formed at grain boundaries. Hydrogen absorption–desorption studies at 298, 323 and 348 K showed (a) a ‘flat’ structure of the pressure–composition isotherms; (b) a rather broad range of working H storage capacities; (c) an increase of the plateau pressures and decrease in both enthalpy and entropy changes following an increase of the Mm content in the alloy; and (d) fast formation–decomposition rate. Desorption isotherms were approximated by a recently developed model of phase equilibria in the metal–hydrogen systems. In situ powder neutron diffraction studies of the system MmNi4.7Sn0.3–D2 show that in the region of α-solid solution deuterium insertion leads to a gradual increase of the unit cell volume reaching 1.9%. In the hexagonal β-MmNi4.7Sn0.3 D4.51 deuteride D atoms occupy two of four available types of tetrahedral interstices including 6m RE2(Ni,Sn)2 and 12n RE(Ni,Sn)3. [Copyright &y& Elsevier]