Crystal structure, hydrogen absorption-desorption behavior and magnetic properties of the Nd3−xMgxCo9 alloys

Accepted Manuscript Crystal structure, hydrogen absorption-desorption behavior and magnetic properties of the Nd3−xMgxCo9 alloys V.V. Shtender, R.V. Denys, V. Paul-Boncour, I. Yu. Zavaliy, Yu.V. Verbovytskyy, D.D. Taylor PII:

S0925-8388(16)33404-1

DOI:

10.1016/j.jallcom.2016.10.268

Reference:

JALCOM 39437

To appear in:

Journal of Alloys and Compounds

Received Date: 16 September 2016 Revised Date:

26 October 2016

Accepted Date: 27 October 2016

Please cite this article as: V.V. Shtender, R.V. Denys, V. Paul-Boncour, I.Y. Zavaliy, Y.V. Verbovytskyy, D.D. Taylor, Crystal structure, hydrogen absorption-desorption behavior and magnetic properties of the Nd3−xMgxCo9 alloys, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.10.268. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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PC dependences for Nd2MgCo9, Nd1.5Mg1.5Co9 and NdMg2Co9 alloys at room temperature.

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Crystal structure, hydrogen absorption-desorption behavior and magnetic properties of the

ACCEPTED MANUSCRIPT Nd3-xMgxCo9 alloys

V.V. Shtender1, R.V. Denys1, V. Paul-Boncour2, I.Yu. Zavaliy1, Yu.V. Verbovytskyy1, D.D. Taylor3 1

Karpenko Physico-Mechanical Institute, NAS of Ukraine, 5 Naukova St., 79060 Lviv, Ukraine Institut de Chimie et des Matériaux Paris Est, CMTR, CNRS and U-PEC, 2-8 rue H. Dunant, 94320 Thiais, France 3 Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA

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Abstract

New Nd3-xMgxCo9 alloys have been synthesized by powder sintering method and their crystal structure, hydrogen storage and magnetic properties have been systematically studied. X-ray

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diffraction analysis showed that Nd3-xMgxCo9 (x ≤ 1.5) alloys belong to the PuNi3-type structure and NdMg2Co9 is an individual ternary compound with YIn2Ni9-type structure. Hydrogen storage capacity

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of the PuNi3-type Nd3-xMgxCo9 alloys is in the range 1.1–1.6 wt. % H at room temperature, while for NdMg2Co9 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 PuNi3-type structure with hydrogen-induced volume expansion 16.7–24 %. The positions of hydrogen atoms were determined by neutron powder diffraction study of Nd2MgCo9D10. In the Nd2MgCo9D10 structure D

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atoms partially occupy four types of interstices, including octahedral Nd2Co4 sites, tetrahedral Nd2Co2 sites and two types of trigonal bipyramids (Nd,Mg)3Co2. Electrochemical investigations of the Nd2MgCo9 electrode demonstrated rather low discharge capacity, 100 mAh/g, which is about 30 % of its theoretical capacity. Magnetic measurements have shown that Nd2MgCo9 is ferromagnetic with

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high Curie temperature (TC = 633 K) and spin reorientation at 225 K, its hydrogenation causes a

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significant decrease of the magnetization. Keywords: Rare Earth compounds, Magnesium compounds, Crystal structure, Metal hydrides, MHelectrodes, Magnetic properties.

1. Introduction

Rare earth–Mg–Ni-based superlattice alloys are amongst the most attractive materials for negative electrodes of Ni–MH secondary batteries [1–4]. Electrochemical discharge capacity of the La–Mg–Ni–Co alloy reached 410 mAh/g, which is 30 % higher than the capacity of commercial LaNi5-based electrodes [4]. These compounds have general formula A2+nB5n+4 (A = rare earth and Mg; B = Ni; n=1, 2, 3) and adopt several types of structures, which are built of Laves type A2B4 and Haucke type AB5 layers stacking along the hexagonal/trigonal c-axis. For La–Mg–Ni system, the general 1

formula for supperlatice alloys can be expressed as La2+n-xMgxNi5n+4, where x corresponds to the

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quantity of Mg substituting La in Laves type layer [5]. Liao et al. studied electrochemical performance of the PuNi3 type La3-xMgxNi9 alloy series and found that the La2MgNi9 alloy exhibited the largest discharge capacity, the most rapid activation and good high-rate dischargeability [6, 7]. Main disadvantage of La–Mg–Ni-based electrode alloys is their poor cycling stability, which is explained by high corrosion rate for both La and Mg in KOH electrolyte and large volume expansion during hydride formation [7].

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Partial substitution of La by other rare earths (Ce, Pr and Nd) and Ni by Co, Mn or Al [1, 2] has been attempted in a numerous research works in order to improve electrochemical properties of La–Mg–Ni alloys. Neodymium is more efficient in improve the performance of electrode alloys than other rare-earth elements [1, 8]. It has been shown that replacement of La with Nd improves both high

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rate dischargeability and cyclic stability for the electrodes [9, 10].

The cycling stability of the La2Mg(Ni1-xCox)9 alloys improves greatly with the increase of Co

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content as the alloy with a higher Co content has a smaller cell volume expansion on hydriding, and thus a lower rate of pulverization and corrosion during cycling [11, 12]. It has been also reported that substitution of Ni by Co in RMgNi4 (R = Y, Nd) compounds results in the increase of hydrogen storage capacity by 50–70 % [13, 14].

Taking into account the positive effect of both Nd and Co on the hydrogenation and electrochemical performance of La–Mg–Ni-based alloys it is of interest to investigate formation and

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properties of similar alloys in the Nd–Mg–Co system. Recently, ternary phases with PuNi3-type structure were found in the R–Mg–Co (R = Ce, Tb) systems [15, 16]. They absorb up to 1.4 wt. % H at room temperature and pressures of 100 bar H2 for R = Ce [15] and 25 bar for R = Tb [16]). In this work, we systematically studied structural, hydrogenation and electrochemical

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properties of the Nd3-xMgxCo9 (x = 0–2) alloys. In addition, magnetic measurements were performed

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for Nd2MgCo9 compound and its hydride. 2. Experimental details

Starting materials for preparation of Nd3-хMgxСo9 alloys were ingots of Nd and Co (with purities ≥ 99.9 %), and Mg powder (Alfa Aesar, 325 mesh, 99.8 %). In the first step, Nd3-хСo9 alloy precursors were prepared by arc melting in purified argon atmosphere. The prepared Nd3-хСo9 buttons were ground in an agate mortar and then mixed with Mg powder in certain proportions. The powder mixtures were pressed into pellets, placed into a tantalum container, which was further loaded into a stainless steel autoclave and sealed under Ar atmosphere. Then, the samples were heated stepwise from 773 to 1073 K. Finally, the alloys were slowly cooled down to 773 K, annealed at this temperature for ~250 hours and quenched in cold water.

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Phase-structural analysis of the samples was carried out by X-ray powder diffraction (XRD)

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using Bruker D8 diffractometers (Cu-Kα radiation). The collected XRD data were processed by the Rietveld method using FullProf software [17]. Hydrogen absorption-desorption properties of the alloys were characterized using a Sieverts type apparatus. The samples were activated by heating up to 473 K in dynamic vacuum, cooled to 293 K and then hydrogenated with high purity hydrogen gas (99.999%). Pressure–composition– temperature (PCT) diagrams were measured under H2 pressures from 0.01 to 200 bars at temperatures

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between 233 and 353 K.

Hydrogen desorption from the hydrides was studied by means of thermal desorption spectroscopy (TDS) with linear heating of specimens (2 K/min) in dynamic vacuum from room temperature to 600 K. Thermal desorption by the selected hydride was also studied by in situ XRD.

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Powder XRD patterns were collected during heating (2 K/min) in argon atmosphere using Bruker C2 Discover powder diffractometer equipped Cu sealed tube, Göbbel mirror, Vantec 500 area detector and

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Anton Paar DHS 1100 Domed Heating Stage at the X-ray Crystallographic Center, University of Maryland, USA.

Neutron diffraction studies of Nd2MgCo9-based deuteride was performed at the Spallation Neutron Source SINQ at Paul Scherrer Institute, Villigen, Switzerland, using a high resolution powder diffractometer HRPT in the high intensity mode (λ = 1.494 Å, 2θ step size 0.05°). The deuteride was synthesized in a stainless steel autoclave connected to a Siverts apparatus. The sample was activated in

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vacuum at 473 K, cooled down to room temperature and then charged with deuterium gas (D2 content 99.8 %) at initial pressure ~10 bar. After fast deuterium absorption and saturation at ~4 bar, pressure in the autoclave was reduced to atmosphere. For neutron diffraction experiment the deuteride was reloaded from the autoclave into a vanadium sample holder with an inner diameter of 5 mm. The

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reloading was done in air and the vanadium container was sealed with indium wire. The collected NPD data were processed by the Rietveld method using GSAS software [18].

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Metal hydride electrodes were prepared by mixing alloy powder with carbonyl Ni powder in the weight ratio of 1:3. The powder mixture was cold-pressed under 10 ton/cm2 into a pellet with 12 mm diameter and then sandwiched between two Ni foam sheets. Electrochemical tests were performed in a three-electrode system. Platinum auxiliary and metal hydride working electrodes were placed in a glass cell filled with 6M KOH solution electrolyte, while the Ag/AgCl reference electrode was connected to the system via an agar bridge. Cycling stability of the MH electrodes was studied galvanostatically at the different current densities. The cut-off potential for the discharge was set at 0.6 V versus the Ag/AgCl electrode. Magnetization measurements were carried out using a MPMS-5S Quantum Design SQUID magnetometer and a PPMS09 from Quantum Design, using the ACMS option. The samples were placed in gelatin capsule and fixed with glass wool. Diamagnetic contribution of the gelatin capsule 3

(-2.8 10-8 emu/Oe) is really negligible compared to the sample magnetization. Isofield magnetization

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curves were recorded between 2 and 300 K with applied fields of 0.1 T. The measurements were generally performed with decreasing temperature. Isotherm magnetization curves were measured in the temperature range between 2 and 300 K with decreasing field from 9 T to -0.2 T. 3.

Results and discussions

3.1. Crystal structure of new Nd3-xMgxCo9 (x = 0–2) compounds

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Analysis of the XRD data showed formation of the Nd3-xMgxCo9 intermetallic compounds with PuNi3-type structure. These compounds can be regarded as a solid solution of Mg in NdCo3. Partial replacement of Nd atoms by smaller Mg atoms leads to a continuous shrinking of the trigonal unit cell, both in basal plane and along c-axis (Table 1). Contrary to isostructural Nd3-xMgxNi9 alloys, which exist in a wide range from NdNi3 (x = 0) up to NdMg2Ni9 (x = 2) [19], the homogeneity range of

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Nd3-xMgxCo9 solid solution is limited by the composition Nd1.5Mg1.5Co9. At the composition NdMg2Co9, a new ternary compound with distinctly different crystal structure has been identified. This

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compound crystallizes in tetragonal unit cell (sp. gr. P4/mbm; a = 8.2789(2), c = 4.8368(2) Å) and is isostructural to RIn2Ni9 compounds [20, 21]. In the Mg concentration range 1.5 < x < 2.0 we obtained two-phase alloys containing both PuNi3-type (Nd1.5Mg1.5Co9) and YIn2Ni9-type (NdMg2Co9) phases (see Fig. 1). Structural relations between these types will be further analyzed on the example of Nd2MgCo9 and NdMg2Co9.

The Nd2MgCo9 alloy crystallizes with partially ordered variant of the PuNi3 structure type (sp.

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gr. R3¯m). XRD pattern of this alloy is shown in Fig. S1a (Supporting Information) and the obtained structural parameters are presented in Table 2. Nd2MgCo9 has slightly larger unit cell (a = 5.0362(2), c = 24.1894(9) Å; V = 531.32(3) Å3) compared to Nd2MgNi9 (a = 4.9783(1), c = 24.1865(9) Å; V =

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519.12(4) Å3) [19]. The PuNi3-type structure can be described as a stacking of MgZn2-type Laves phase and CaCu5-type Haucke phase layers along [001] direction. Similarly with isostructural R3-xMgxNi9 alloys [19, 22–24], Mg atoms are located exclusively in the Laves-type fragments of

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Nd2MgCo9, partially substituting rare earth atoms in 6c site. Fig. S1b shows the diffraction pattern of NdMg2Co9 alloy. This ternary compound crystallizes with tetragonal structure of YIn2Ni9-type (sp. gr. P4/mbm) and it is the first representative of this type in R–Mg–T systems. In contrast, NdMg2Ni9 [25] and NdMg2Cu9 [26] compounds crystallize in trigonal PuNi3- and hexagonal CeNi3-types of crystal structure, respectively. Both structures consist of MgZn2-type and CaCu5-type slabs stacking along trigonal/hexagonal axis. YIn2Ni9-type is an ordered superstructure of the Ce(Mn0.55Ni0.45)11-type [20]. It can be viewed as intergrowth of slabs of the Zr4Al3 and CeMg2Si2 structure types. In the NdMg2Co9 structure, the positions of Nd, Mg and Co atoms correspond to the Y, In and Ni sites in YIn2Ni9, respectively. The shortest metal-metal

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separation in the structure is the following: d(Nd–Mg) = 3.317(9) Å; d(Nd–Co2) = 3.043(3) Å; d(Mg–

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Mg) = 2.74(1) Å; d(Mg–Co3) = 2.693(9) Å; d(Co3–Co3) = 2.387(6) Å. Comparison of interatomic distances shows that the Nd–Co contacts in NdMg2Co9 (3.043(3) Å) are longer than that in Nd2MgCo9 (2.908(1) Å, see Table S1). Other distances are shorter and this correspond to a decrease of the cell volume of 2.7 % for the NdMg2Co9 compound compared with the hypothetical PuNi3-type compound, as can be seen from the linear extrapolation in Fig. 1. In spite of obvious differences between the structures of Nd2MgCo9 and NdMg2Co9, they have similar

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coordination of atoms, as can be seen in Fig. 2. The cobalt atoms in the Nd2MgCo9 (PuNi3-type) structure build up a framework of edge- and face-sharing tetrahedra. The voids within this framework are filled by either Mg or Nd atoms. In the NdMg2Co9 (YIn2Ni9-type) structure Co atoms form triangular bipyramids which are composed into layers parallel to basal plane, but separated along c-

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axis. The Co–Co distances between neighboring layers are the shortest in the structure (2.387 Å). The voids between Co layers are filled by both Mg and Nd atoms. Crystal structure data for the Nd2MgCo9

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and NdMg2Co9 compounds are given in Table 2. 3.2. Hydrogenation properties of the Nd3-xMgxCo9 alloys

After a pretreatment in vacuum at 473 K, the pseudo-binary Nd3-xMgxCo9 alloys (x = 0–1) readily absorb hydrogen at room temperature and 10 bar pressure (Fig. 3). Hydrogen storage capacity reaches 11.4–14.5 hydrogen atoms per formula unit (H at./f.u.). Alloys with higher Mg content absorb hydrogen at higher pressures. The Nd1.5Mg1.5Co9 alloy absorbs only ~3 H at./f.u. under pressure of 10

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bar, 8.4. H at./f.u. at 20 bar and 9.7 H at./f.u. at 50 bar H2. In general, gravimetric storage capacity of the PuNi3-type Nd3-xMgxCo9 alloys (x = 0–1.5) is in the same range as that of the isostructural La3-xMgxNi9 alloys [22]: 1.2–1.6 wt. % H. On the contrary, the ternary NdMg2Co9 compound

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(YIn2Ni9-type) absorbs only 0.4 wt. % H (~3 H at./f.u.) at room temperature and 130 bar H2 pressure. It is worth mentioning that capacity of LaMg2Ni9 (ordered PuNi3-type) in similar conditions is three times larger, 1.2 wt. % H [22]. Such difference in hydrogenation capacity is possibly due to the

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structural differences between PuNi3- and YIn2Ni9-types, since the latter structure has a limited number of interstices suitable for occupation by H atoms. Pressure-composition isotherms of hydrogen absorption and desorption were measured for three alloys, Nd2MgCo9, Nd1.5Mg1.5Co9 and NdMg2Co9 (Fig. 4a–c). Both absorption and desorption isotherms for Nd2MgCo9 have between 0.5 and 10 at. H/f.u. (Fig. 4a). Maximum hydrogenation capacity of 1.35 wt. % (at 293 K and 10 bar H2) is close to the capacity of Nd2MgNi9 obtained in similar conditions (1.48 wt. % H at 293 K and 20 bar H2) [19]. However, equilibrium pressures of both hydrogen absorption and desorption in the Nd2MgCo9–H2 system are almost one order of magnitude lower than in the Nd2MgNi9–H2 system. The mid-plateau absorption and desorption pressures for Nd2MgCo9 at 293 K are 0.3 and 0.2 bar H2, respectively, while for Nd2MgNi9 the corresponding 5

hydrogen pressures are 2.0 and 1.3 bar, respectively. Thermodynamic parameters calculated from van’t

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Hoff dependencies (Fig. 4d) are presented in Table 3. The enthalpy change on hydrogen desorption (∆Hdes) from the Nd2MgCo9H11.4 is 39 kJ (mol H2)-1, which is slightly higher than value of 36 kJ (mol H2)-1) obtained for La2MgNi9Н13 hydride [23]. Higher equilibrium desorption pressure in the case of Nd2MgCo9H11.4, 0.2 bar compared to 0.045 bar H2 for La2MgNi9H13, can be explained by larger entropy change (∆Sdes), 120 J (K mol H2)-1 for Nd2MgCo9H11.4 versus 97 J (K mol H2)-1 for La2MgNi9H13.

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As can be seen from Table 3, increasing Mg content in the PuNi3-type Nd3-xMgxCo9 alloys leads to decrease of the thermodynamic stability of hydrides. Indeed, when Mg content increases from x = 1 to 1.5, equilibrium pressure of hydrogen desorption at room temperature increases by more than

wt. % H for Nd2MgCo9 to 1.23 wt. % H for Nd1.5Mg1.5Co9.

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40 times, from 0.2 to 8.3 bar. At the same time, hydrogen storage capacity slightly decreases from 1.35

The capacity dramatically drops when going from Nd1.5Mg1.5Co9 (PuNi3-type) to NdMg2Co9

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(YIn2Ni9-type). For the latter, hydrogenation capacity of only 0.48 wt. % H (3.5 H at./f.u.) was reached during room temperature hydrogenation in maximum H2 pressure of 185 bar available in our PCT setup. At lower temperatures, NdMg2Co9 demonstrated hydrogen capacity up to 5 H at./f.u. with a hardly distinguishable pressure plateau between 2.5 and 4.5 at H at./f.u. (see inset in Fig. 4c). Note very small hysteresis for absorption and desorption pressures. The enthalpy of hydrogen desorption for NdMg2Co9 is only 20.8 kJ (mol H2)-1, being almost two times smaller than for Nd2MgCo9. Crystal structure of the Nd3-xMgxCo9-based hydrides

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3.3.

The saturated hydrides of the Nd3-xMgxCo9 compounds with x = 0, 0.45, 0.85 and 1.0 were exposed to air and characterized by XRD. These hydrides retain initial trigonal symmetry with

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isotropic expansion of the unit cell by 16.7–24 % (Table 4). As an example, XRD pattern of the hydrogenated Nd2MgCo9 alloy is shown in Fig. S2. In addition to the main Nd2MgCo9H11.4 phase, the sample also contains a small amount of unhydrogenated NdCo5 impurity (a = 5.037(2) Å, c = 3.993(3)

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Å). The hydrides of Nd1.5Mg1.5Co9 and NdMg2Co9 alloys are characterized by high desorption pressures at room temperature (~10 and ~200 bar H2, respectively) and immediately decompose upon exposure to air, thus, their structural characterization is possible only at in situ conditions under hydrogen pressure. Lattice parameters of the NdCo3H4.8 hydride (a = 5.368(5) Å, c = 27.354(3) Å) are in good agreement with literature data [27]. Crystal structure of Nd2MgCo9-based hydride has been studied in more detail by neutron diffraction. A freshly prepared Nd2MgCo9 alloy (a = 5.0344(6) Å, c = 24.188(3) Å) was loaded into stainless steel autoclave, activated in vacuum at 573 K and exposed to D2 gas at room temperature and initial pressure 9.5 bar. In such conditions, we observed a very fast reaction (when 90 % of maximum capacity was reached within 5 min) leading to a pressure drop to 4.3 bar and formation 6

Nd2MgCo9D11.1. Further pressure in the autoclave was gradually reduced to 1 bar (that caused to a

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partial release of deuterium from the sample) and the deuterated alloy was exposed to air and reloaded into vanadium container. According to volumetric measurement, the final composition of the deuteride at 1 bar D2 was Nd2MgCo9D10.2. Fig. S3 shows neutron diffraction pattern of this deuteride collected at room temperature. The obtained structural data are summarized in Table 5. As a starting structural model in Rietveld refinements of the deuteride, we used structural parameters for Nd2MgCo9H11.4 obtained from XRD

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data. The positions of the deuterium atoms have been determined by the use of differential Fourier synthesis. In the structure of Nd2MgCo9D10, deuterium atoms partially occupy four types of interstitial sites: D1 inside Nd2Co4 octahedra; D4 in Nd2Co2 tetrahedra; D5 and D6 in the centers of trigonal bypiramids (Nd2/Mg)3Co2. Note that for the sake of comparison we use the same labeling of D-sites as

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for the La2MgNi9D13 deuteride [19, 23]. Two occupied D-sites (D1 and D4) are located within the CaCu5-type AB5 slab and other two sites (D5 and D6) are within the Laves-type AB2 slab. The

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composition of these slabs can be presented as NdCo5D4.9(1) and NdMgCo4D4.7(1), respectively. Total deuterium content obtained from Rietveld refinements is 9.6(2) D at./f.u. which is in good agreement with volumetrically calculated value of 10.2 D at./f.u.

Fig. 5 compares the crystal structure of Nd2MgCo9D10 with related γ-ErCo3D3.71 [28] and Nd2MgNi9D11.9 [19] deuterides. All three deuterides form via isotropic expansion of a trigonal unit cell. However, in the case of Nd2MgCo9D10 the deformation along c-axis (∆c/c = 4.9 %) is less

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pronounced than for both γ-ErCo3D3.71 (∆c/c = 7.4 %) and Nd2MgNi9D11.9 (∆c/c = 9.6 %), see Table 5. Nevertheless, in all three structures D atoms fill the same four types of interstitial sites (D1, D4, D5 and D6). In Nd2MgNi9D11.9 a small fraction of D atoms is also located in two types of T4 tetrahedral sites (D2 and D8) which are vacant in the case of Co-containing deuterides. The main difference

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between the structures of Nd2MgCo9D10 and ErCo3D3.71 is splitting of some deuterium positions in the latter. For example, D5 atoms in Nd2MgCo9D10 are located in the centers of R3Co2 triangular

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bipyramids (18h site), while in the structure of ErCo3D3.71 this position is split in two 36i sites separated by 0.48 Å. Similarly, D4-site in R2Co2 interstice between Laves and Haucke type slabs is described as two different crystallographic positions in ErCo3D3.71 at the distance 0.37 Å between them. In spite of apparent similarity of the D atom coordination of cobalt to that in ErCo3D3.71 (Fig.5), in general, the structure of Nd2MgCo9D10 is closer to Nd2MgNi9D11.9. A common feature of the Nd2MgT9-based deuterides is triangular MgT2 coordination of D atoms in D5- and D6-sites located in the Laves-type slab. Such conclusion is derived from the analysis of the distances between the metal and deuterium atomic positions. The mixed Nd2/Mg site inside the Laves-type slab has an equal probability to be occupied by Mg or Nd atoms. However, both Nd2/Mg–D5 and Nd2/Mg–D6 distances are below 2 Å (see Table S1) which is too short for the large Nd atom (rNd = 1.82 Å, rMg = 1.60 Å); however, these distances are in the same range as Mg–D bond distances in MgD2 [29]. Thus, it means 7

that D atoms fill D5- and D6-sites only if they have Mg atoms in their nearest surrounding. It is worth

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mentioning that triangular MgT2 coordination of D atoms was earlier observed in the deuterides of cubic RMgT4 compounds, e.g. in CeMgCo4D4.2 [15]. 3.4.

Hydrogen thermal desorption by the Nd3-xMgxCo9Hy hydrides We have found that hydrides of the Nd3-xMgxCo9 alloys with x ≤ 1 become very stable when

exposed to air. Thermal stability of such stabilized hydrides NdCo3H4.8, Nd2.15Mg0.85Co9H13.5 and Nd2MgCo9H11.4 was studied by thermal desorption spectroscopy (TDS) with linear heating of the

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specimen in dynamic vacuum at rate 2 K/min (Fig. 6). At first, we have performed a blank experiment keeping the sample under dynamic vacuum (<10-3 mbar) at room temperature for 5 h. XRD analysis showed that the hydrides remained unchanged after such processing. Further TDS experiment has showed that hydrogen desorption from the hydrides into vacuum occurs in a single event at

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temperatures above 330 K (Fig. 6). The thermal desorption plot of Nd2MgCo9H11.4 has a single peak at 353 K. In the case of Nd2.15Mg0.85Co9H13.5 the hydrogen desorption occurs at higher temperatures

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between 330 and 400 K. The thermal desorption of hydrogen from NdCo3H4.8 is observed in the temperature range 360-450 K with maximum at 405 K. TDS curve of NdCo3H4.8 is similar to that of CeCo3H3.7 [30]. Thus, partial replacement of Nd by Mg in NdCo3 results in the decrease of hydrogen desorption temperature (Fig. 6). This is in agreement with PCT measurements which show the decrease of thermodynamic stability of the hydrides with increasing Mg content. The XRD analysis of

intermetallic compounds.

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the samples after TDS experiment showed complete transformation of the hydrides back to parent

Thermal desorption from the Nd2.15Mg0.85Co9H13.5 hydride was studied in detail by means of in situ powder X-ray diffraction under argon atmosphere. A series of powder XRD patterns was collected in the temperature range from 293 to 573 K, as shown in Fig. S4a. Starting from room temperature up

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to 400 K only diffraction peaks of the hydride are present. In the temperature range 400–460 K, the XRD patterns contain peaks from both hydride and intermetallic phase. This region corresponds to the

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direct hydride → IMC transformation accompanied by intense hydrogen release from the sample. Above 460 K only peaks from the Nd2.15Mg0.85Co9 intermetallic phase can be observed, constituting completion of hydrogen desorption process. From the changes of the unit cell parameters (Fig. S4b) we can conclude that hydrogen desorption starts at ~350 K, when the parameters of the hydride phase begin to decrease. Such shrinking of the unit cell is due to gradual decrease of hydrogen concentration in the hydride phase. The end of desorption process is reached at 513 K, when minimum volume of IMC is observed corresponding to the release of hydrogen dissolved in the IMC phase. 3.5.

Electrochemical studies of the Nd3-xMgxCo9 alloys Fig. 7a shows electrochemical discharge capacity of the Nd3-xMgxCo9 (x = 0–1) electrodes

during cycling at a current density of 50 mA/g. It can be seen that increasing Mg content facilitates 8

activation of the electrodes. The Nd2MgCo9 electrode reached its maximum capacity after 6 charge-

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discharge cycles, Nd2.55Mg0.45Co9 required 20 activation cycles and Mg-free alloy was not fully activated even after 30 cycles. Surprisingly, despite the favorable hydrogen storage properties, Nd2MgCo9 alloy demonstrated rather poor electrochemical discharge capacity. The maximum experimental value obtained in this study was 103 mAh/g (Fig. 7b), while, according to hydrogen storage capacity of 1.3 wt. %, the theoretical electrochemical capacity of Nd2MgCo9 is 360 mAh/g. La2MgNi9 and Nd2MgNi9 alloys with similar H storage capacity showed discharge capacity of ~400

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mAh/g and ~370 mAh/g, respectively [9]. It is also worth mentioning that equilibrium hydrogen desorption pressure for the Nd2MgCo9−H2 system, 0.2 bar at 293 K (Table 3), is intermediate between desorption pressures for the La2MgNi9 and Nd2MgNi9−H2 systems, 0.045 and 1.3 bar, respectively [19]. The observed discrepancy between gaseous and electrochemical hydrogenation capacities of requires

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investigation.

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possible

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Nd2MgCo9

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oxidation/passivation which prevents hydrogen desorption from the hydride during electrochemical

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discharge. Indeed, XRD analysis of charged and discharged electrodes showed that Nd2MgCo9 alloy completely transformed into hydride on electrochemical charging and it remained in hydrogenated state after discharging. Thus, just a small amount of hydrogen evolves from the hydride on discharging process giving a low discharge capacity, which is only 20–30 % of the theoretical value. 3.6.

Magnetic properties of the Nd2MgCo9 and its hydride

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The magnetic properties of Nd2MgCo9 alloy and its hydride will be compared with those of NdCo3 and corresponding hydride NdCo3H4.1.

Upon heating NdCo3 shows a spin reorientation from the easy axis direction in the basal plane to the c-axis in the temperature range 220–255 K [31]. In addition a field-induced transition is observed in

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fields below 0.7 T and temperatures below 115 K from collinear to noncollinear magnetic structures occurring in magnetic fields along the c axis direction. This results from of reorientation of Nd2 moment in

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cubic slabs to one of the other local easy directions. NdCo3H4.1 becomes antiferromagnetic with TN = 214 K as results of Co–Co negative exchange interaction under hydrogenation. Metamagnetic transition of the Co sublattice occurred in hydride from anti-ferromagnetic to ferromagnetic state for magnetic field below 5.5 T [31].

As shown in Fig. 8a the magnetization of Nd2MgCo9 display two peaks at 225 K an 250 K which corresponds to the spin reorientation observed for NdCo3 [31]. Surprisingly, the Curie temperature of Nd2MgCo9 (633 K) is much higher than for NdCo3 (381 K) [31]. The magnetization at 2 K (Fig. 8b) shows a change of slope at 5.5 T indicating the beginning of a field induced transition and a small hysteresis loop with a coercive field of 0.035 T. Assuming the same magnetic structure than NdCo3, i.e. an easy axis in the basal plane, the field induced transition from a collinear to non collinear occurs at critical field much larger than in NdCo3. Since Mg atom substitutes on the Nd2 site, the increase of critical field is in agreement with 9

the assumption that Nd2 is responsible of this transition as proposed to explain the difference between

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NdCo3 and Nd2Co7. The saturation magnetization Ms is 14.5 µ B at 2K and 15.2 µ B at 300 K. Assuming that the Nd moment is 2.4 µ B like in NdCo3 and a collinear orientation of Nd and Co, the average Co moment is 1.08 µ B/Co and 1.15 µ B/Co at 2 and 300 K respectively, close to the value obtained for NdCo3. Corresponding ternary hydride shows three transitions with maxima at 45 K, 115 and 265 K. Assuming that TN = 265 K, this value is larger than TN = 214 K observed for NdCo3 hydride [31] like TC for the alloy. At 2 K the saturation magnetization of the hydride (MS = 8.6 µ B) is reduced of 40 %

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compared to the parent compound (MS = 14.2 µ B) (Fig. 8b), this can be explained by either a canting of the magnetic moments or a reduction of the Co moment (µ Co = 0.42 µ B) as also observed in NdCo3 hydride. In both alloy and hydride a hysteresis loop is observed at 2 K with coercive fields of 0.035 T and 0.13 T respectively and showing ferromagnetic interactions. No change of slope is observed up to the maximum

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field of 9 T, and a field induced transition probably occurs at a much higher field. Above 45 K, there is no more coercive field, which could be explained by a change of magnetic order or a spin reorientation. MS

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decreases linearly versus field up to 200 K. The small spontaneous magnetization remaining at 300 K (MS = 1.5 µ B/f.u.), i.e. above TN may partly result from the presence of segregated Co at the surface. Neutron diffraction experiments at low temperature will be necessary to solve the magnetic structure of Nd2MgCo9 and its hydride. 4.

Conclusions The effect of Mg on the structure and hydrogenation properties of the Nd3-xMgxCo9 alloys has

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been studied. Alloys with Mg content x ≤ 1.5 have a trigonal structure of PuNi3 type and can be considered as pseudo-binary compounds related to NdCo3, where Nd is partially replaced with Mg. Similarly to isostructural Nd3-xMgxNi9 [19], the unit cell dimensions of Nd3-xMgxCo9 decrease with the increase in Mg/Nd ratio and Mg atoms are located exclusively in the Laves type layers of the PuNi3-

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type structure. At the maximum Mg content (x = 2), a new ternary compound NdMg2Co9 with tetragonal structure of YIn2Ni9 type was found. Hydrogenation capacity of NdMg2Co9 is small,

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reaching only 0.5 wt. % at room temperature and 185 bar. On the contrary, the PuNi3-type Nd3-xMgxCo9 alloys (x ≤ 1.5) absorb up to 1.6 wt. % H at much lower pressures (1-20 bar). The Nd3-xMgxCo9-based hydrides retain initial trigonal structure with volume expansion up to 24 %. Nd3-xMgxCo9-based hydrides with x ≤ 1 are stable in air and desorb hydrogen only when heated above 350 K. TDS measurements showed that the temperature of the desorption peak decreases with increasing Mg content. In situ XRD study of thermal desorption process in Nd2.15Mg0.85Co9H13.5 showed that hydrogen release is associated with direct transformation of the hydride back to parent intermetallic compound. The thermodynamic stability of the hydrides has strong dependence on Mg content, where increasing Mg/Nd ratio leads to increase of the equilibrium desorption pressure. Among the studied 10

alloys, Nd2MgCo9 shows the most attractive hydrogen storage properties: high reversible capacity (1.3

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wt. % H), flat pressure plateau and low absorption/desorption hysteresis. Neutron diffraction study of the Nd2MgCo9D10 deuteride revealed its significant similarity to Nd2MgNi9D12 [19]. Deuterium atoms are evenly distributed in the layered structure: partially occupying two types of sites in the CaCu5-type layer (octahedral Nd2Co4 and tetrahedral Nd2Co2) and two types of sites the Laves-type layer (trigonal bipyramids (Nd,Mg)3Co2). The electrochemical discharge capacity of the Nd3-xMgxCo9-based electrodes does not exceed of a surface oxide layer with reduced electrocatalytic activity.

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100 mAh/g, which is less than 30 % of the hydrogenation capacity. This is likely caused by formation

The Nd2MgCo9 compound is ferromagnetic with higher Curie temperature (TC = 633 K) than NdCo3 (381 K). Like NdCo3 it displays a spin reorientation between 220 and 255 K. Hydrogenation of

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Nd2MgNi9 causes the decrease of the transition temperatures due to a weakening of the magnetic interactions and probably a change of magnetic order (to antiferromagnetic with TN = 265 K) and

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various spin reorientations at lower temperatures. The saturation magnetization at 2 K is reduced of 40 % compared to the parent alloy, due either to a canting or a reduction of the Co moments. Acknowledgments

We thank to Dr. Denis Sheptyakov for his assistance with the neutron diffraction experiment at Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland. Dr. Peter Y. Zavalij is

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highly appreciated for in situ high temperature XRD measurements at X-ray Crystallographic Center, University of Maryland, College Park, MD 20742, USA. This work was supported partially by the French government scholarship program for short-term research visits in 2014 (№ 815302G).

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Appendix A. Supplementary material

Diffraction patterns and interatomic distances for intermetallics and hydrides are given in the

version.

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Supplementary Material. Supplementary data associated with this article can be found in the online

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Fig. 1. V/Z ratio for the Nd3-xMgxCo9 (0 ≤ x ≤ 2) intermetallics as a function of magnesium content.

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Fig. 2. Crystal structures of the Nd2MgCo9 (a) and NdMg2Co9 (b) compounds and coordination polyhedrons in these structures (c).

Fig. 3. Hydrogenation curves of the Nd3-xMgxCo9 alloys: NdMg2Co9 at 130 bar H2, Nd1.5Mg1.5Co9 at

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20 bar H2, Nd2MgCo9 at 3 bar H2 and others − at 10 bar H2.

Fig. 4. Hydrogen absorption/desorption PCT diagrams for Nd2MgCo9 (a), Nd1.5Mg1.5Co9 (b) and NdMg2Co9 (c) alloys and corresponding van’t Hoff plots (d).

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Fig. 5. Comparison of the γ-ErCo3D3.71, Nd2MgNi9D11.9 and new Nd2MgCo9D10 deuterides with filled

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PuNi3-type structure; D-atom environments around Me atoms are highlighted. Fig. 6. Thermal hydrogen desorption spectra for the Nd3-xMgxCo9Hy hydrides. Fig. 7. Discharge capacity as a function of cycle number (a) and Discharge curves for the cycle with maximum capacity (b) for the Nd3-xMgxCo9 electrodes.

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Fig. 8. M(T) at 0.1 T (a) and M(H) curves at 2 K and 300 K (b) for the Nd2MgCo9 compound and its hydride. Inset: (a) zoom on the M(T) curves near the Curie temperature, (b) zoom of the M(H) curves

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measured at 2 K.

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Table 1. Crystallographic data for the Nd3-xMgxCo9 intermetallics.

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Alloy compositions NdCo3 Nd2.55Mg0.45Co9 Nd2.15Mg0.85Co9 Nd2MgCo9 Nd1.5Mg1.5Co9 NdMg2Co9

Str. type–sp. gr.

a (Å)

c (Å)

c/a

V (Å3)

V/Z (Å3)*

¯m PuNi3–R3 ¯m PuNi3–R3 ¯m PuNi3–R3 ¯m PuNi3–R3 ¯m PuNi3–R3 YIn2Ni9–P4/mbm

5.0692(8) 5.0572(2) 5.0411(1) 5.0362(2) 5.0017(4) 8.2789(2)

24.745(6) 24.4225(11) 24.2420(8) 24.1894(9) 23.992(2) 4.8368(2)

4.88 4.83 4.81 4.80 4.80 –

550.7(2) 540.92(4) 533.51(4) 531.32(3) 519.78(7) 331.51(2)

183.57 180.31 177.84 177.11 173.26 165.76

*

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V/Z ratio is presented in order to compare volume changes for compounds with different structures. Z = 3 and Z = 2 for PuNi3 and YIn2Ni9 types respectively.

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Table 2. Crystal structure data for the Nd2MgCo9 and NdMg2Co9 compounds from Rietveld refinements of X-ray diffraction data. Refined composition Nd1.96(2)Mg1.04(2)Co9 PuNi3-type, sp. gr. R3¯m; a = 5.0362(2) Å; c = 24.1894(9) Å; V = 531.32(3) Å3; Z = 3 RBragg = 8.72 %; χ2 = 1.44 Uiso × 100 (Å2) Atom Site x y z Nd1 3a 0 0 0 1.2(2) 0.52(1)Mg/0.48(1)Nd2 6c 0 0 0.1439(2) 1.1(2) Co1 3b 0 0 1/2 0.9(3) Co2 6c 0 0 0.3340(3) 0.3(2) Co3 18h 0.5016(6) –x 0.0833(2) 1.0(1) Refined composition NdMg2Co9 YIn2Ni9-type; sp. gr. P4/mbm; a = 8.2789(2) Å; c = 4.8368(2) Å; V = 331.51(2) Å3; Z = 2 RBragg = 9.91 %; χ2 = 1.19 Uiso × 100 (Å2) Atom Site x y z Nd 2a 0 0 0 0.7(1) Mg 4g 0.617(1) x+1/2 0 0.3(-) Co1 8j 0.0681(6) 0.2125(5) 1/2 0.2(1) Co2 2c 0 1/2 1/2 0.5(3) Co3 8k 0.1768(5) x+1/2 0.2468(9) 0.3(1)

Table 3. Equilibrium pressures of hydrogen absorption/desorption and

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NdMg2Co9–H2 –/20.1 45.2/44.0 –/91.4 –/181.0 – – –/20.8±0.6 –/113.8±2.1

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thermodynamic parameters for Nd3-xMgxCo9–H2 systems. Nd1.5Mg1.5Co9–H2 Nd2MgCo9–H2 T (K) Pabs/Pdes (bar) 233 – – 253 – – 273 – 3.99/3.51 293 0.31/0.20 9.63/8.26 323 1.18/0.88 28.5/25.1 353 4.01/3.05 – -1 ∆Habs/∆Hdes (kJ (mol H2) ) -36.5±1.1/39.1±0.2 -28.81±0.24/28.84±0.19 -1 ∆Sabs/∆Sdes (J (K mol H2) ) -114.9±3.4/120.0±0.5 -117.1±0.8/116.1±0.6

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Table 4. Crystal structure data for the Nd3-xMgxCo9Hy hydrides (synthesized at room temperature and H2 pressure 10 bar) from Rietveld refinements of X-ray diffraction data. a c V ∆a/a ∆c/c ∆V/V ∆V/at. H Hydride (Å) (Å) (Å3) (%) (%) (%) (Å3) NdCo3H4.8 5.368(5) 27.354(3) 682.7(6) 5.9 10.5 24.0 3.1 Nd2.55Mg0.45Co9H12.7 5.3778(2) 26.329(2) 659.44(7) 6.3 7.8 21.9 3.1 Nd2.15Mg0.85Co9H13.5 5.3546(1) 25.5644(9) 634.79(3) 6.2 5.5 19.0 2.7 Nd2MgCo9H11.4 5.3267(2) 25.219(2) 619.68(5) 5.8 4.3 16.7 2.6

Table 5. Crystal structure data for the γ-ErCo3D3.71, Nd2MgNi9D11.9 and new Nd2MgCo9D10 ACCEPTED MANUSCRIPT deuterides (PuNi3-type, sp.gr. R3¯m) from Rietveld refinements of neutron data.

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γ-ErCo3D3.71 [28] Nd2MgNi9D11.9 [19] Nd2MgCo9D10 a (Å) 5.2218(3) 5.3236(2) 5.3272(4) c (Å) 26.046(2) 26.506(2) 25.368(4) V (Å3) 615.05(7) 650.55(7) 623.5(1) ∆a/a (%) 4.9 6.9 5.8 ∆c/c (%) 7.4 9.6 4.9 ∆V/V (%) 18.1 25.3 17.4 11.8 26.1 15.2 ∆V(AB5) (%) ∆V(AB2) (%) 24.1 24.6 19.5 R1 in 3a (0, 0, 0) Uiso × 100 (Å2) 2.3(3) 1.3(2) 1.3(2) R2/Mg in 6c (0, 0, z) z 0.1342(1) 0.1405(2) 0.1430(2) nMg, (nR = 1 - nMg) 0.5(-) 0.5(-) 2 1.6(2) 2.8(2) 2.3(2) Uiso × 100 (Å ) T1 in 3b (0, 0, 1/2) Uiso × 100 (Å2) 0.1(2) 2.5(2) 6.3(9) T2 in 6c (0, 0, z) z 0.3342(5) 0.3278(2) 0.3312(7) 2 Uiso × 100 (Å ) 5.3(6) 1.5(1) 3.5(5) T3 in 18h (x, -x, z) x 0.4933(7) 0.4966(4) 0.5000(10) z 0.0767(2) 0.0830(1) 0.0818(2) Uiso × 100 (Å2) 1.4(1) 1.21(6) 0.9(1) D1 *D3 in 9e (1/2, 0, 0) 36i (x, y, z) 18h (x, -x, z) x 0.537(3) 0.501(2) y 0.553(2) –x z 0.0189(4) 0.0132(3) n 0.348(7) 0.239(4) 0.273(5) D2 in 6c (0, 0, z) z vacant 0.3907(9) vacant n 0.31(1) D3 in 18h (x, -x, z) vacant vacant vacant D4 in 18h (x, -x, z) *D4/*D5 x 0.8427(9)/0.8532(10) 0.851(2) 0.8518(6) z 0.0621(4)/0.0760(6) 0.0710(5) 0.0693(2) n 0.50(2)/0.33(2) 0.47(1) 0.547(9) D5 in 18h (x, -x, z) *D1 in 36i (x, y, z) x 0.4536(9) 0.502(1) 0.4968(7) y 0.4544(9) -x -x z 0.1389(1) 0.149(3) 0.1434(2) n 0.366(3) 0.476(7) 0.485(7) D6 in 18h (x, -x, z) x 0.771(2) 0.820(2) 0.8186(12) z 0.1177(7) 0.1001(6) 0.1002(3) n 0.124(5) 0.34(1) 0.294(8) D7 in 6c (0, 0, z) vacant vacant vacant D8 in 6c (0, 0, z) z vacant 0.4431(8) vacant n 0.33(2) Uiso × 100 (Å2) for all D 1.77(6) 2.4(1) 1.99(9) Calculated D content 3.71(6) 11.9(1) 9.6(1) Volumetric capacity, – 12.1 10.2 D at./f.u. * Original positions of D-atoms in all presented deuterides are labelled according to [19, 23] nomenclature.

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Fig. 1. V/Z ratio for the Nd3-xMgxCo9 (0 ≤ x ≤ 2) intermetallics as a function of magnesium content.

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b с Fig. 2. Crystal structures of the Nd2MgCo9 (a) and NdMg2Co9 (b) compounds and coordination polyhedrons in these structures (c).

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Fig. 3. Hydrogenation curves of the Nd3-xMgxCo9 alloys: NdMg2Co9 at 130 bar H2, Nd1.5Mg1.5Co9 at 20 bar H2, Nd2MgCo9 at 3 bar H2 and others − at 10 bar H2.

Fig. 4. Hydrogen absorption/desorption PCT diagrams for Nd2MgCo9 (a), Nd1.5Mg1.5Co9 (b) and NdMg2Co9 (c) alloys and corresponding van’t Hoff plots (d).

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Fig. 5. Comparison of the γ-ErCo3D3.71, Nd2MgNi9D11.9 and new Nd2MgCo9D10 deuterides with filled PuNi3-type structure; D-atom environments around Me atoms are highlighted.

Fig. 6. Thermal hydrogen desorption spectra for the Nd3-xMgxCo9Hy hydrides.

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Fig. 7. Discharge capacity as a function of cycle number (a) and discharge curves for the cycle with maximum capacity (b) for the Nd3-xMgxCo9 electrodes.

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Fig. 8. M(T) at 0.1 T (a) and M(H) curves at 2 K and 300 K (b) for the Nd2MgCo9 compound and its hydride. Inset: (a) zoom on the M(T) curves near the Curie temperature, (b) zoom of the M(H) curves measured at 2 K.

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▶ The crystal structure of the Nd3-xMgxCo9 alloys have been determined. ▶ The hydrogenation properties of the Nd3-xMgxCo9 alloys have been studied. ▶ The crystal structure of the Nd2MgCo9D10 deuteride was studied by NPD.

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▶ Electrochemical and magnetic properties of selected samples have been studied.

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