Experimental and Theoretical Investigations on the Influence of A on

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C: Energy Conversion and Storage; Energy and Charge Transport

Experimental and Theoretical Investigations on the Influence of A on the Hydrogen Sorption Properties of ANi Compounds, A = {Y, Sm, Gd} y

Jean-Claude Crivello, Nicolas Madern, Junxian Zhang, Judith Monnier, and Michel Latroche J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04600 • Publication Date (Web): 20 Aug 2019 Downloaded from pubs.acs.org on August 28, 2019

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The Journal of Physical Chemistry

Experimental and Theoretical Investigations on the Inuence of

A

on the Hydrogen

Sorption Properties of ANiy Compounds,

A=

{Y, Sm, Gd} ∗

Jean-Claude Crivello,

Nicolas Madern, Junxian Zhang, Judith Monnier, and

Michel Latroche

Université Paris Est, ICMPE (UMR 7182), CNRS, UPEC, F-94320 Thiais, France

E-mail: [email protected]

1

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Abstract The phase stability, electronic structure, magnetism and mechanical properties of ANiy compounds, A = {Y, Sm, Gd}, y = {2, 3,

7 19 2, 5 ,

4, 5} and of the A2 Ni7 H8

hydrides have been investigated by systematic rst principle calculations. Supported by a discussion to understand the role of A and y parameters, the results are compared with published and new experimental data regarding the heat of formation of some compounds and their hydrides. The main results can be summarized as follows: (i) ANi3 and A2 Ni7 are clearly stable by DFT calculation whereas the stability of ANiy for y >

7 2

is not highlighted

distinctively, in agreement with the diculty to identify a single phase by experimental synthesis for some compounds such as Y5 Ni19 ; (ii) ANiy compounds are found more rigid for A=Y and may explained the diculty to observe Y5 Ni19 in comparison with Sm5 Ni19 and Gd5 Ni19 ; (iii) the calculated and measured heats of formation of A2 Ni7 H8 hydrides are found in good agreement, around -25∼-30 kJ/mol-H2 according to the system.

Introduction Some intermetallics can react with hydrogen to form interstitial hydrides near room temperature and ambient pressure. According to these favorable thermodynamic conditions, the reaction is reversible and yields these compounds usable for energy storage. 1 Indeed, hydrogen can be loaded either by a solid-gas route 2 or an electrochemical one 3 to develop fuel tank or batteries, respectively. Because of their high density, such hydrides may have a very high volumetric energy capacity. Due to their attractive magnetic properties, rare-earths (like praseodymium and neodymium) are over-exploited to produce magnets and subjected to strong price variations. Other rare earth such as samarium, gadolinium, and yttrium having a lower sensitivity to cost and availability, are worth to be investigated in the eld of alloys for electrochemical application 2


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or solid-gas storage as they are not commercially used at present for these technologies. According to the dierent compound families, the binary ANiy systems (A=rare earths) can be smartly dened as stacking structures along one crystallographic axis of two sub-units [A2 Ni4 ] and [ANi5 ]. 4,5 Indeed, all phases reported in the binary LaNi phases diagram for

xNi > 69 %

6,7

can be described by the sequence [A2 Ni4 ]+m[ANi5 ] for which m is an integer

leading to ANi2 (m = 0), ANi3 (m = 1), A2 Ni7 (m = 2), A5 Ni19 (m = 3), ANi4 (m = 4),

ANi5 (m = +∞). The compound stoichiometry can then be written as ANiy with y =

5m+4 . m+2

From a crystallographic point of view, excepted for m = 0 and +∞, there are two possible stacking sequences along the c axis leading either to rhombohedral or hexagonal symmetry. Indeed, there are three stacking periods for the rhombohedral structure (3R), and two for the hexagonal one (2H ). However, the two structures remain very close from each other, their formation energies are comparable 5 and dierences between the thermodynamic sorption properties of the two polymorphs have never been reported. It is generally admitted that the occurrence of either the 3R or 2H phase depends on the thermal synthesis history but also of geometrical factors such as the atomic radius of A, rA . 8 Consequently, the nature of A plays an important role on the hydrogenation properties. It inuences the molar mass of the compounds as Z increases from 39 (Y) then 57 to 71 (lanthanides). It also aects the phase stability since the [A2 Ni4 ] sub-unit is very sensitive to packing geometry such as the rA /rNi atomic radius ratio. It is reported that hydrogenation of ANiy compounds is poorly reversible since hydrogen induces amorphization due to the instability of the [A2 Ni4 ] slabs, 9,10 whereas Y2 Ni7 presents a good reversibility for hydrogenation. 11 The geometric criterion may play a role in the stability of the phase, since yttrium presents a smaller atomic radius than most rare earths. Nevertheless, Gd2 Ni7 behaves as other A2 Ni7 compounds 12 with a poor reversibility even if the atomic radius of Gd is almost the same of Y. It is therefore fundamental to better understand the parameters governing the phase stability to develop materials with high reversible sorption properties and long-term cycling lives. For the present work, the stability of ANiy compounds, A = {Y, Sm, Gd}, y = {2, 3, 3



7 19 , , 2 5

4, 5} and A2 Ni7 H8 hydrides have been investigated by rst principle calculations in the

frame of the Density Functional Theory (DFT). The theoretical results are compared with published and new experimental data regarding the heat of formation of some compounds and their hydrides. The paper is supported by a discussion to understand the role of A and

y parameters in the stability of the intermetallics and the relative hydride.

Experiments The intermetallic compounds were synthesized from high purity elements: Gd (Alfa Aesar 99.9%), Sm (Alfa Aesar 99.9%), Y (Santoku 99.9%) and Ni (Praxair 99.95%). Compounds A2 Ni7 have been synthesized by melting and Pressure-Composition-Isotherm (PCI) curves have been measured at dierent temperatures. For Gd2 Ni7 and Y2 Ni7 , the elements were melted stoichiometrically in an induction furnace. This step was repeated ve times. The obtained ingots were wrapped in tantalum foil and annealed for one week under argon atmosphere in a silica tube at 1273 K. Sm2 Ni7 was synthesized with a small excess of samarium. After induction melting, the ingot was crushed into powder ( 2, associated to a more negative ∆f H .

• there is no signicant dierence between the [3R] and [2H] stackings at 0 K. The largest dierence is obtained for ANi3 where the ∆f H in PuNi3 structure is 0.05 eV/at lower 8


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Calculated heat of formation of ANiy compounds with A = {Y, Sm, Gd}. For each dierent composition, the most stable compound is indicated with its label, but no signicant dierence between the [3R] and [2H] stackings is observed.

Figure 2:

9



than in CeNi3 . Other ANiy compounds have energy dierence less than 0.05 eV for a given y between [3R] and [2H] stackings. However, theses dierences are below the DFT accuracy which is usually attributed around 0.05 eV/at and did not allow to distinguish the most stable phase between both symmetries. This result is similar for LaNiy . 5 At y = 2, the TmNi2 structure (stable in LaNi system with vacancies in La site) presents an almost equal ∆f H as the C15−MgCu2 structure ( 27 ) might be stabilized in small amount when coexisting with other stacking compounds, probably as residual stacking faults, but this does not allow to observe it as single phase because of a higher energy necessary for relaxing constraints and of a longer time of annealing needed. About the A2 Ni7 H8 hydrides, DFT shows that the formation becomes more exothermic with the order A = Y (-25.6) → Gd (-27.1) → Sm (-30.3 kJ/mol-H2 ). This in agreement with the correlation between the calculated rigidity of the ICMs shown in Fig. 6 and the values of ∆f H , as shown in the previous work on Haucke phase: 43 the most compressible ICMs lead to the most stable hydrides. However, the experimental measurements present a dierent order of hydride stability: A = Sm (-19.7) → Gd (-20.0) 12 → Y (-23.0 kJ/mol-H2 ). First of all, one may notice that the values are of very similar magnitude through measured under dierent conditions. For example, the works of Iwase et al. on Gd2 Ni7 Hx are discutable, 12 since no clear plateau is distinguishable and the van't Ho points are not aligned. Moreover, all compounds form multiple hydrides as shown by the multiple plateaus of PCI. The comparison with DFT calculation is thus skewed since models taken into the theoretical works were chosen for the full hydride with a perfect ordering A2 Ni7 H8 . Moreover, comparison of A2 Ni7 Hx PCI shows that the rst plateau of H-absorption for A = Y is higher that 20


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those for A = Sm or Gd, whereas the ∆f H is lower for the richer-H (Fig. 9). This is in agreement with the larger value of calculated mechanical modulus of Y2 Ni7 yielding to a less compressible host structure for hydrogen at the beginning of the H-absorption. However, the DFT calculation by itself is not sucient to explain the reversible hydrogenation for Y-based compounds whereas Sm-based amorphized after H absorption.

Pressure-Composition-Isotherm (PCI) of hydrogen absorption and desorption of A2 Ni7 hydrides with A = {Y, Sm, Gd}. Figure 9:

11,44

Conclusion A systematic study of ANiy compounds, A = {Y, Sm, Gd}, y = {2, 3,

7 19 , 5, 2

4, 5} and

corresponding A2 Ni7 H8 hydrides has been made by rst principle calculations. In addition, new synthesis and characterizations (XRD, EPMA, PCI) have been done, for which results are compared to the theoretical work. It is shown that:

• the ANi3 and A2 Ni7 are clearly stable by DFT calculation whereas the stability of ANiy 21



for y >

7 2

is not highlighted distinctively. This is in agreement with the diculty to

identify single phases by experimental synthesis for some compounds such as Y5 Ni19 ;

• the ANiy compounds are found more rigid for A=Y and may explained the diculty to observe Y5 Ni19 in comparison with Sm5 Ni19 and Gd5 Ni19 ;

• whereas ANi2 are found Pauli paramagnet, the ferromagnetism is not negligible for y ≥ 3 compounds where magnetism is driven by the Ni 3d-state ling of the charge transfer from A element;

• the calculated and measured heat of formation of A2 Ni7 H8 hydrides are found in good comparison. Small dierences with experimental results are debatable because of the approximations for the DFT calculation (4f frozen core, ordered hydride crystal structure,...) and of the accuracy of measurements since several hydrides are observed in the multiplateau PCI curves.

Supporting Information Available The following SI les are available free of charge. S1: EPMA analysis of dierent samples of YNi system S2: Structural model of the A2 Ni7 H8 hydride S3S20: Density of states of ANiy compounds, A = {Y, Sm, Gd} S21: Calculated elastic constants of ANiy compounds

Acknowledgement DFT calculations were performed using HPC resources from GENCI-CINES (Grant 201896175). The authors are thankful to Dr. E. Leroy for performing the EPMA analysis. 22


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graphical toc 230x160mm (300 x 300 DPI)


Experimental and Theoretical Investigations on the Influence of A on

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