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Study of Structural and Hydrogen Storage Properties of YNi Deuterides by Means of Neutron Powder Diffraction 2

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Véronique Charbonnier, Junxian Zhang, Judith Monnier, Lionel Goubault, Patrick Bernard, Cesar Magen, Virginie Serin, and Michel Latroche J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03096 • Publication Date (Web): 12 May 2015 Downloaded from http://pubs.acs.org on May 20, 2015

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The Journal of Physical Chemistry

Structural and Hydrogen Storage Properties of Y2Ni7 Deuterides Studied by Neutron Powder Diffraction Véronique Charbonnier,† Junxian Zhang,† Judith Monnier,† Lionel Goubault,‡ Patrick Bernard,‡ César Magén,¶ Virginie Serin,§ and Michel Latroche∗,† ICMPE, CNRS-UPEC, UMR7182, 2-8 rue Henri Dunant 94320 Thiais France, SAFT, Direction de la Recherche, 111-113 Bd. Alfred Daney, 33074 Bordeaux, France, Transpyrenean Associated Laboratory for Electron Microscopy (TALEM), CEMES-INA, CNRS-Universidad de Zaragoza, Spain, and CEMES, CNRS UPR8011 and Université de Toulouse, 29 rue Jeanne Marvig, 31055 Toulouse, France E-mail: [email protected] Phone: +33 (0)1 49 78 12 10

Abstract Crystal structure and hydrogenation properties of Gd2 Co7 -type Y2 Ni7 were investigated by X-ray diffraction (XRD), Sievert’s method and neutron powder diffraction. Unlike Nd2 Ni7 and La2 Ni7 , Y2 Ni7 exhibits three plateau pressures and forms three hydrides at room temperature. The maximum hydrogen capacity reaches 8.9 H/f.u at ∗

To whom correspondence should be addressed ICMPE, CNRS-UPEC, UMR7182, 2-8 rue Henri Dunant 94320 Thiais France ‡ SAFT, Direction de la Recherche, 111-113 Bd. Alfred Daney, 33074 Bordeaux, France ¶ Transpyrenean Associated Laboratory for Electron Microscopy (TALEM), CEMES-INA, CNRSUniversidad de Zaragoza, Spain § CEMES, CNRS UPR8011 and Université de Toulouse, 29 rue Jeanne Marvig, 31055 Toulouse, France †

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10 MPa at the first absorption cycle. The compound can be cycled by solid-gas route and structural properties are fully recovered after dehydrogenation. For the first time, structural properties of rhombohedral A2 B 7 Dx deuterides were studied. They were investigated by neutron powder diffraction at three different states of charge (x=2.1, 4.1 and 8.8). Deuterium positions as well as variation of unit cell parameters are given and discussed in this paper.

Keywords: A2 B 7 alloys, neutron powder diffraction, pressure-composition-temperature relationships, structural properties

Introduction Metallic hydrides (MH) are widely studied and used as materials for energy storage. On the one hand, they are able to store reversibly large amount of hydrogen in practical conditions and, on the other hand, they are suitable materials to develop anodes in Ni-MH or Liion batteries. For alkaline Ni-MH batteries, LaNi5 -type hydrogen storage materials are commonly used 1 but more efficient materials are now foreseen. In the binary system AB x (A: rare earth, B : transition metal), a capacity increase can be achieved by using compounds richer in rare earth. Indeed, beside the AB 5 line compound, alloys with stoichiometry A2 B 7 or A5 B 19 exist and adopt a structure either hexagonal H (space group P 63 /mmc) or rhombohedral R (space group R ¯3m). They can be described by the stacking along the c axis of two different subunits, [A2 B 4 ] and [AB 5 ] 2 and their properties are closely related to those of their subunits. In addition, the subunits, [A2 B 4 ] can host lighter elements like Mg and research on ternary A-Mg-Ni alloys 3 are promising as both Mg and A are good hydride-forming compounds and replacing part of the heavy rare earth by light Mg decreases the molar mass and increases the electrochemical capacities of such electrode materials. 4 Indeed, previous works have demonstrated that this type of ternary compounds 5 provide electrochemical capacities close


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to 400 mAh·g−1 , a value much larger than that of the LaNi5 -based compounds. The nature of the A element is also an important factor for alloy optimization and using a rare earth-like element such as yttrium allows further gain in molar mass. In this work, we investigate the single-phase rhombohedral binary compound Y2 Ni7 . Its structural characteristics, as well as its hydrogenation properties are presented. For the first time, structural properties of Y2 Ni7 Dx compounds (x= 2.1, 4.1 and 8.8) are investigated using neutron powder diffraction.

Experimental methods High purity elements yttrium (Santoku, 99.9 %) and nickel (Praxair, 99.95 %) were used to synthesize the Y2 Ni7 alloy. The elements were melted several times in an induction furnace under controlled atmosphere to obtain homogeneous 8 g ingots. Subsequent annealing was performed for 7 days at 1000 ◦ C in a silica tube under argon atmosphere before quenching in water to room temperature. X-ray diffraction was performed using a Bruker D8 DAVINCI diffractometer with Cu-Kα radiation, in a 2θ-range from 15 to 80◦ with a step size of 0.01 ◦ . Experimental data were analyzed by Rietveld method using FullProf program. 6 Chemical composition was checked with Electron Probe Microanalysis (EPMA) using a CAMECA SX-100. Transmission Electron Microscopy (TEM) specimen was prepared by mechanical polishing and ion-beam thinning (Gatan PIPS). The sample was observed HAADF-STEM imaging was performed on a probe-corrected FEI Titan 60-300 microscope operating at 300 kV with a probe size of 100 pm. To obtain P-c isotherm measurements, 100 µm size powder was sealed in a sample holder under argon atmosphere and evacuated under primary vacuum. P-c isotherm was measured at 25 ◦ C using Sieverts’ method. To evaluate the cyclability of Y2 Ni7 , a sample holder containing the alloy was connected to



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a hydrogenation rig. It was exposed to the right amount of hydrogen in order to obtain final pressures of 10 MPa, 3.3 MPa, 0.7 MPa or 0.1 MPa once the equilibrium state was reached. Then, the sample was fully desorbed by heating at 150 ◦ C under dynamic primary vacuum for a few hours. This procedure was repeated once for 10 MPa, 50 times for 3.3 MPa, 57 times for 0.7 MPa and 80 times for 0.1 MPa. The obtained powder was characterized by XRD. Deuterated compounds Y2 Ni7 Dx were synthesized by gas-charging on a hydrogenation bench loaded with deuterium gas. Then, they were analyzed by neutron powder diffraction (NPD) at room temperature on beam line 3T2 at LLB (Saclay, France) with λ= 1.225194 ˚ A (x=2.1) or at PSI (Villigen, Switzerland) with λ= 1.494 ˚ A (x=4.1 and 8.8). 100 µm size powder was stored in specially designed stainless steel sample holders closed under deuterium pressure.

Results Phase determination and structural properties Structural properties of the samples were checked by X-ray diffraction. Typical XRD pattern is shown in Figure 1. From Rietveld refinement, it is established that Y2 Ni7 is a single phase and it adopts a Gd2 Co7 -type rhombohedral structure with space group R ¯3m. Structural parameters, given in Table S1 (Supporting Information), are in good agreement with those derived from literature. 7–9 EPMA was used as a complementary compositional analysis. The sample is homogeneous and no impurity was found. The average composition of the alloy derived from forty data points is Y2 Ni6.92(0.02) (Figure S1, Supporting Information). The structural parameters have been further investigated at the atomic scale using transmission electron microscopy. High magnification HAADF STEM image of Y2 Ni7 observed along the [110] axis is shown in Figure 2. The atomic positions for the rhombohedral Gd2 Co7 4 ACS Paragon Plus Environment

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Figure 1: Rietveld analysis of the XRD pattern for Y2 Ni7 . Observed (red dots), calculated (black solid line) and difference curves (blue solid line below) are shown. Vertical bars correspond to (hkl) line positions for the rhombohedral Gd2 Co7 -type structure. type structure projected along the [110] direction are also superposed to the image. The intensity of the atomic columns strongly depends on the element atomic number Z and on the number of atoms perpendicular to the plane. Nickel columns (yellow) can easily be differentiated from yttrium ones (blue) though brighter spots (green) are still related to nickel. This can be explained by the fact that the number of nickel atoms is twice higher for this column. On this basis, [A2 B 4 ] and [AB 5 ] subunits are readily identified and one observes the stacking of 2 [AB 5 ] subunits and one [A2 B 4 ] subunit along the c-axis to form one A4 B 14 block. The stacking of three shifted blocks corresponds to the Gd2 Co7 -type structure, as shown in Figure 2.



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Figure 2: Wien filtered HAADF image of the (110) plane of Y2 Ni7 . Stacking along the c axis of the rhombohedral Gd2 Co7 -type structure showing the two different subunits, [A2 B 4 ] and [AB 5 ] is superimposed to the image. Blue spheres correspond to yttrium; green and yellow ones to nickel.

Thermodynamic properties The P-c isotherm of Y2 Ni7 measured at 25 ◦ C is shown in Figure 3. Three plateau pressures are observed. The first one appears at 0.055 MPa with hydrogen content between 0.4 and 2 H/f.u. (hydrogen per formula unit), the second one at 0.5 MPa ranges from 3.1 to 4.0 H/f.u. and the third one at 2.9 MPa is between 5.0 and 8.4 H/f.u. The maximal capacity at 10 MPa reaches 8.9 H/f.u.

Cycling stability The sample has been cycled following three different pressure conditions: 0.1, 0.7 and 3.3 MPa for final pressure, which correspond to the first, second end third plateau respectively. Evolution of the reversible capacities as a function of the number of cycles is shown in Figure 4. Under low pressure, capacity remains remarkably constant whereas higher pressure induces a linear decrease of the capacity leading to 45 % lost after 50 cycles. 6 ACS Paragon Plus Environment

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C a p a c ity ( H /f.u .) Figure 3: P-c isotherm curves (absorption and desorption) measured at 25 ◦ C for Y2 Ni7 . The labeled arrows indicate the compositions analyzed by neutron powder diffraction. Figure 5 shows the XRD patterns of Y2 Ni7 before and after different hydrogenationdehydrogenation cycling. Diagrams of Y2 Ni7 performed before cycling (black), after one cycle at 10 MPa (red), after 57 cycles at 0.7 MPa (orange) and after 80 cycles at 0.1 MPa (blue) are identical. They present the same diffraction patterns without shift or broadening of the diffraction peaks. After 50 cycles at 3.3 MPa (green), the diffraction pattern is different showing important peak broadening though the structure looks preserved.

Structural analysis of the deuterides Deuterides Y2 Ni7 Dx were analyzed by mean of neutron powder diffraction under deuterium pressure at different states of charge. Three samples have been investigated at 2.1 (β),



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C y c le n u m b e r Figure 4: Evolution of the reversible capacities of Y2 Ni7 for different hydrogenationdehydrogenation cycling conditions at room temperature. Cycling between vacuum (150◦ C for a few hours) and Peq = 0.1 MPa (up triangles) or 0.7 MPa (squares) or 3.3 MPa (full circles). 4.1 (γ) and, 8.8 (δ) D/f.u. The corresponding compositions are indicated by arrows on the P-c isotherm in Figure 3. The comparison of the neutron diffraction patterns is given in Figure 6. The contribution from the stainless steel sample holder used for neutron diffraction is marked by blue triangles. Beside the sample holder, the diffraction peaks of the main phase are observed and can be indexed keeping the metallic network (Gd2 Co7 -type structure) of Y2 Ni7 with a shift toward larger dhkl when increasing the D content. Neutron diffraction patterns were refined by the Rietveld method. The iron-made sample holder was first refined using the Le Bail method because it was highly textured and its parameters were then kept fixed. Refinement of the three deuterides has been made assuming that the metallic network remains preserved upon deuteration and testing various possible interstitial sites available for hydrogen.


3 .3 M P a 0 .7 M P a 0 .1 M P a 1 0 M P a P r is tin e Y 2 N i7 2 0

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Figure 5: XRD patterns of Y2 Ni7 before and after different hydrogenation-dehydrogenation cycling. From bottom to top: pristine Y2 Ni7 , after PCT measurement at 10 MPa, after 80 cycles at 0.1 MPa, after 57 cycles at 0.7 MPa and after 50 cycles at 3.3 MPa. As an example, result of the Rietveld refinement for the NPD pattern of Y2 Ni7 D2.1 (β) is shown in Figure 7. The crystallographic data of the three deuterides are given in Table 1 and their associated .cif files are provided in Supporting Information. A unit cell expansion is observed upon hydrogenation with ∆a/a = 0.38 %; ∆c/c = 4.9 % for the first deuteride β, ∆a/a = 1.9 %; ∆c/c = 6.0 % for the second one γ and ∆a/a = 6.2 %; ∆c/c = 9.5 % for the third composition δ. Expansion of the unit cell parameters and volume is illustrated in Figure 8. It shows an anisotropic c-oriented expansion for the first composition (β) followed by an isotropic expansion for the two following ones. Atomic positions and occupancy factors have been also determined. Atomic positions of the metallic atoms stay close to that of the intermetallic compounds. Figure 9 shows the crystal structures of Y2 Ni7 Dx at the different states of charge (x=2.1, 4.1 and 8.8). Seven different deuterium sites are involved in the deuteration of the binary compound Y2 Ni7 . 9 ACS Paragon Plus Environment

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2 ( ° ) Figure 7: Rietveld analysis of the NPD pattern for Y2 Ni7 D2.1 . Observed (dots), calculated (red solid line) and difference curves (blue solid line below) are shown. Vertical bars correspond to (hkl) line positions for the sample holder (top) and for Y2 Ni7 D2.1 (bottom). Evolution of the occupancy rates of the seven occupied deuterium sites as a function of the composition is given in Figure 10. A progressive filling of the sites D2, D3 and D5 to D7 is observed whereas D1 and D4 are redistributed.

Discussion Structural characterization of Y2 Ni7 From our XRD analysis, Y2 Ni7 crystallizes in the rhombohedral Gd2 Co7 -type structure with lattice parameters a = 4.9499(2) ˚ A and c = 36.272(2) ˚ A. These results are in line with those reported by Lemaire et al. 10 Atomic positions are also in good agreement with those derived from the literature. 8,11 Due to their relative stabilities, coexistence of the rhombohedral R and hexagonal H polymorphs is often reported for this stoichiometry. Rhombohedral



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C a p a c ity ( D /f.u .) Figure 8: Variations of the unit cell parameters and the cell volume as a function of deuterium content for Y2 Ni7 Dx (x=0, 2.1 (β), 4.1 (γ), 8.8 (δ)). or hexagonal stabilization is a function of the annealing temperature and the R phase is usually the most stable at high temperature. 11–14 Furthermore, according to Buschow et al., 11 the amount of R and H phases in A2 Ni7 alloys is also related to the A-radius. A small radius favors the R form whereas a large one leads to the H one. Indeed, Monnier et al. obtained purely hexagonal La2 Ni7 (RLa = 1.87 ˚ A) by induction melting and 7-day annealing at 1000 ◦ C. 14 Bhattacharyya et al. synthesized Ho2 Ni7 and Dy2 Ni7 (RHo = 1.743 and RDy = 1.752 ˚ A) by arc melting and annealing treatment at 1000 ◦ C for 21 days, they obtained pure Gd2 Co7 -type alloys. 15 In the present work, according to the small atomic radius of yttrium (1.776 ˚ A), the purely rhombohedral Y2 Ni7 is obtained at 1000 ◦ C and confirms the role played by the atomic radius.


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Figure 9: Crystal structures for Y2 Ni7 Dx (x=0, 2.1 (β), 4.1 (γ), 8.8 (δ)) from bottom to top (R ¯3m). Stacking subunits [A2 B 4 ] and [AB 5 ] are also represented.

P-c isotherm curves Y2 Ni7 forms a single phase compound but hydrogenation leads to three different plateaus in our investigated pressure range (0-10 MPa). P-c isotherm of Y2 Ni7 was previously measured by Van Essen and Buschow. 8 They report a hydride Y2 Ni7 H2 with an equilibrium pressure of 2.5 bar (0.25 MPa) at 50◦ C and the curve shows a second plateau-like behavior up to 5 MPa. This is in agreement with the present work on Y2 Ni7 D2.1 showing slightly lower equilibrium pressure (0.055 MPa) at lower temperature 25◦ C. P-c isotherms with multiple plateaus have been previously reported for the ANi5 system. 16–19 Senoh et al. measured P-c isotherms for ANi5 (A=La, Pr, Nd, Sm, and Gd) at different temperatures. 16 For Pr, Nd, Sm, and Gd, they observed two plateaus and noticed that the gap between them diminishes with increasing temperature. For LaNi5 , they observed one single plateau at 25 ◦ C and two plateaus at higher temperature. They concluded that the number of plateaus depends on the temperature and on the nature of the rare earth 13 ACS Paragon Plus Environment


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C a p a c ity ( D /f.u .) Figure 10: Occupancy rate of the deuterium sites (D1 to D7) as a function of deuterium content for Y2 Ni7 Dx (x=0, 2.1 (β), 4.1 (γ), 8.8 (δ)). See Table 1 for details. A. As A2 Ni7 compounds are composed of [ANi5 ] subunits, their behavior is probably similar to that of ANi5 compounds. Indeed, same behavior was also observed for the A2 Ni7 system. For Nd2 Ni7 20 and La2 Ni7 , 21 two plateau pressures are observed whereas for Gd2 Ni7 , Iwase et al. published a P-c curve presenting three plateaus. 22

Cycling stability After the first P-c isotherm, Y2 Ni7 was fully desorbed by heating at 150 ◦ C under dynamic primary vacuum for several hours. Figure 5 compares the XRD patterns before and after the P-c isotherm. They are fully identical, indicating that structural properties after the first hydrogenation-dehydrogenation cycle are the same. Moreover, no shift of the diffraction lines in 2θ is observed which means that the unit cell parameters are fully recovered. Iwase et al. studied P-c isotherm for hexagonal Gd2 Ni7 at -15 ◦ C. 22 To desorb the sample, they applied a pressure of 0.0015 MPa. They performed XRD analysis and after Rietveld refinement, 14 ACS Paragon Plus Environment

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they conclude that the Ce2 Ni7 -type structure was conserved. However, they observed some diffraction peak broadening after desorption. This was probably due to decrepitation which induces diminution of the grain size. Furthermore, they noticed that the lattice parameter a did not change whereas c increased of 0.4 %, concluding that Gd2 Ni7 was not completely desorbed. We measured P-c isotherm of Gd2 Ni7 in the same way that for Y2 Ni7 . After hydrogenation under 10 MPa at 25 ◦ C, the maximal capacity was 9.2 H/f.u.. However, the capacity did not decline much upon desorption. Indeed the capacity at 0.2 MPa was still 8 H/f.u.. X-ray diffraction performed after desorption under dynamic vacuum at 150 ◦ C for several hours, showed that the structure was not conserved (Figure S2, Supporting Information). We observed similar behavior for La2 Ni7 . A2 B 7 structures consist of subunits [A2 B 4 ] and [AB 5 ] stacked along c-axis. It is well known that amorphization occurs in A2 B 4 phases when the ratio between atomic radii RA /RB is higher than 1.225. 23 Experimentally, Aoki showed that for a ratio higher than 1.37, hydrogen-induced amorphisation was systematic for A2 B 4 phases. 24 RLa /RNi is equal to 1.500, in the case of gadolinium this ratio is 1.434, which is closed to the ratio calculated for yttrium: 1.425. The fact that the structure of La2 Ni7 is not conserved after dehydrogenation can be explained by the poor stability of the [La2 Ni4 ] subunit related to the high RA /RB ratio. Though gadolinium and yttrium have smaller but similar RA /RB ratios, disproportionation was observed after dehydrogenation of Gd2 Ni7 (Figure S2, Supporting Information), but not after dehydrogenation of Y2 Ni7 . It shows that hydrogen-induced amorphisation is not simply related to the geometrical ration between A and B. The main difference between yttrium and gadolinium is that Y does not possess 4f electrons. Therefore, it would be very interesting to perform Density Functional Theory (DFT) calculations of Gd2 Ni7 , Y2 Ni7 and their hydrides to evaluate the electronic effect on the structure stability.


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Atom Y1 Y2 Ni1 Ni2 Ni3 Ni4 Ni5 D1 D2 D3 D4 D5 D6 D7 Refined

Site Environment 6c 6c 3b 6c 6c 9e 18h 6c Y2Ni53 6c Ni1Ni53 18h Y23 Ni1Ni5 18h Y1Y2Ni52 18h Y1Y2Ni52 18h Y12 Ni42 18h Y12 Ni2Ni3Ni4 Composition

Y2 Ni7 D2.1 (β-phase) a = 4.9686(1) ˚ A c = 38.053(1) ˚ A x y z Occ. 0 0 0.051(3) 1 0 0 0.143(2) 1 1/2 0 0 1 0 0 0.281(2) 1 0 0 0.384(2) 1 1/2 0 0 1 0.498(8) −x 0.1049(9) 1 2/3 1/3 0.5304(9) 0.35(3) 1/3 2/3 0.128(1) 0.31(2) 0.512(3) −x 0.1455(4) 0.39(2) 0.16(1) 2x 0.5618(9) 0.19(2) – – – – – – – – – – – – Y2 Ni7 D2.4(2)

Y2 Ni7 D4.1 (γ-phase) a = 5.0438(3) ˚ A c = 38.455(4) ˚ A x y z Occ. 0 0 0.0485(4) 1 0 0 0.1438(3) 1 1/2 0 0 1 0 0 0.2808(3) 1 0 0 0.3824(3) 1 1/2 0 0 1 0.495(1) −x 0.1054(1) 1 2/3 1/3 0.5348(9) 0.50(4) 1/3 2/3 0.129(1) 0.50(4) 0.516(4) −x 0.1471(5) 0.44(3) 0.16(1) 2x 0.571(1) 0.15(2) 0.512(4) −x 0.4351(6) 0.41(2) 2/3 0.513(7) −x 0.20(2) – – – – Y2 Ni7 D4.6(4)

Y2 Ni7 D8.8 (δ-phase) a = 5.2556(3) ˚ A c = 39.709(6) ˚ A x y z Occ. 0 0 0.0484(4) 1 0 0 0.1390(4) 1 1/2 0 0 1 0 0 0.2784(4) 1 0 0 0.3847(3) 1 1/2 0 0 1 0.492(1) −x 0.1092(2) 1 – – – – 1/3 2/3 0.13(1) 0.52(5) 0.501(3) −x 0.1506(4) 0.48(5) – – – – 0.507(2) −x 0.4326(3) 1 2/3 0.504(3) −x 0.45(5) 1/6 5/6 0.2928(4) 0.62(7) Y2 Ni7 D8.2(5)

Table 1: Structural parameters derived from Rietveld analysis structures of Y2 Ni7 D2.1 (β), Y2 Ni7 D4.1 (γ) and Y2 Ni7 D8.8 (δ). Numbers between brackets stand for standard deviation. Environment of the D sites is described as Ain Bjm where A, B is Y or Ni, i and j the site numbers, m and n the number of neighbors for the same site. The Journal of Physical Chemistry Page 16 of 26

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For further cycling at low pressure (first plateau (0.1 MPa)), observation of the diffraction peaks (Figure 5) does not show any broadening confirming that the crystallinity of the sample is little affected upon cycling up to 2.1 H/f.u.. After cycling at intermediate pressure, corresponding to the end of the second plateau (0.7 MPa and 4.4 H/f.u.), the diffraction pattern remains the same, indicating that cycling at this pressure did not damage the crystallographic structure. However, cycling at higher pressure (50 cycles at 3.3 MPa and 8.7 H/f.u., third plateau) leads to an enlargement of the diffraction linewidth though the structure is still preserved. These results are confirmed by the capacity variation upon cycling (Figure 4). Low and intermediate pressure cycling leads to constant reversible capacity whereas high pressure loading induces a capacity reduction of 45 % after 50 cycles. The amount of hydrogen present in the hydride δ (third plateau) is larger than in the hydride γ (second plateau) and even larger than in the hydride β (first plateau), and in each case hydrogen was introduced in one single step. So Y2 Ni7 underwent heavy stress when converted directly into the δ hydride.

Neutron diffraction NPD analysis allows to conclude that the metallic network is preserved upon hydrogenation up to 10 MPa. Increase of the pressure leads to the formation of three successive deuterides Y2 Ni7 D2.4(2) (β), Y2 Ni7 D4.6(4) (γ), and Y2 Ni7 D8.2(5) (δ). These compositions are in good agreement (within the error bars) with the different phases in equilibrium at the three plateaus observed in the P-c isotherm curve (Figure 3). D loading induces (an)isotropic volume expansion of the crystallographic cell. At the end of the first plateau pressure (β-Y2 Ni7 D2.1 ), c undergoes a large increase whereas a is slightly augmented. This behavior can be understood on the basis of the partial filling of the stacking structure with deuterium. It has been reported that filling of only the units [A2 B 4 ] leads to an anisotropic expansion along the c axis as the D-free [AB 5 ] subunits do not allow cell increase in the basal plan. Such behavior was reported for CeNi3 D2.8 , La2 Ni7 D6.5 , 25 17 ACS Paragon Plus Environment


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Ce2 Ni7 D4.7 , 26 LaNi3 D2.8 27 and CeY2 Ni9 D7,7 . 28 This hypothesis is confirmed here as only the deuterium sites located in the [A2 B 4 ] subunits (D1, D2, D3) or at its border (D4) are filled at the end of the first plateau (Table 1). For the two following deuterides, Y2 Ni7 D4.1 (γ) and Y2 Ni7 D8.8 (δ), one deuterium site located at the [A2 B 4 ]-[AB 5 ] border (D5) and two sites located in the [AB 5 ] subunits (D6 for γ then D7 for δ) start to be filled whereas D1 and D4 are depopulated (Table 1). Average filling of both [A2 B 4 ] and [AB 5 ] subunits leads to isotropic volume expansion as noticed in Figure 8. The site positions D1, D2, D3 and D4 located in the [A2 B 4 ] subunits (or at its border) at the end of the first plateau are equivalent to the deuterium sites described by Yartys et al. for La2 Ni7 D6.5 . 25 This later composition corresponds to the branch between the first and the second plateau of the La2 Ni7 P-c isotherm. 21 This compound undergoes a large anisotropic expansion upon hydrogenation (∆a/a = −2.1 % and ∆c/c = 19.8 %). The four occupied sites of this deuteride are located in the [A2 B 4 ] subunits. D1 and D2 are found octahedral and D3 and D4 tetrahedral. Yartys et al. explained that hydrogen insertion deforms the [A2 B 4 ] subunits so that new types of positions are created: tetrahedral AB 3 turns into octahedral A3 B 3 . However, in our case, the structure is not distorted that much and D1 and D2 sites remain tetrahedrally coordinated. At the end of the second plateau (Y2 Ni7 D4.1 ), the occupancy rates increase for D1, D2 and D3 whereas it is slightly lower for D4. Two new tetrahedral sites, D5 (located at the [A2 B 4 ]-[AB 5 ] border) and D6 (located in the [AB 5 ] subunits) are occupied. At the end of the third plateau Y2 Ni7 D8.8 , the occupancy rates increase for every site except for D1 and D4 which are depopulated. A new site located in the [AB 5 ] subunit (tetrahedral D7) appears to be populated. According to Switendick, the minimal distance between two hydrogen atoms is 2.1 ˚ A. 29 In Table 2 are presented the most significant interatomic distances. It is worth to notice that some D–D distances are less than 2.1 ˚ A. However, for the first and second deuterides, all sites (D1 to D6) are partially occupied. For the third deuteride, D5 is found fully occupied and,


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˚). Environment of the D sites is described Table 2: Most significant interatomic distances (A as Ain Bjm where A, B is Y or Ni, i and j the site numbers, m and n the number of neighbors for the same site.

D1

Environment Y2Ni53 Tetrahedron AB 3

D2

Ni1Ni53 Tetrahedron B4

D3

Y23 Ni1Ni5 Trigonal bipyramid A3 B 2

D4

Y1Y2Ni52 Tetrahedron A2 B 2

D5

Y1Y2Ni52 Tetrahedron A2 B 2

D6

Y12 Ni42 Tetrahedron A2 B 2 D7 Y12 Ni2Ni3Ni4 Trigonal bipyramid A2 B 3

Neighbor Y2 Ni7 D2.1 3 Ni5 1.88 Y2 2.06 3 D3 1.38 3 D5 – Ni1 1.49 3 Ni5 1.66 3 D4 1.64 3 D3 1.68 3 D4 – Ni5 1.55 Ni1 1.73 Y2 2.17 2 Y2 2.49 D1 1.38 D2 1.68 2 D3 – 2 Ni5 1.40 Y1 2.07 Y2 2.52 2 D5 – D2 1.63 2 Ni5 – Y2 – Y1 – 2 D4 – D1 – 2 Ni4 – 2 Y1 – 2 D6 – Ni4 – Ni3 – Ni2 – 2 Y1 –

Y2 Ni7 D4.1 1.81 2.22 1.44 1.94 1.43 1.68 1.73 1.73 1.96 1.61 1.76 2.10 2.53 1.44 1.73 – 1.49 2.32 2.34 1.42 1.97 1.54 2.10 2.45 1.42 1.94 1.40 2.44 1.57 – – – –


Y2 Ni7 D8.8 – – – – 1.37 1.74 – 1.68 – 1.67 1.65 2.34 2.67 – 1.68 2.00 – – – – – 1.63 2.15 2.44 – – 1.51 2.46 1.52 1.56 1.59 1.66 2.65


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according to Switendick’s rule, D1 and D4 located at less than 2.1 ˚ A, cannot be occupied simultaneously with D5. They are indeed found empty. As the distance dD2–D3 is also shorter than 2.1 ˚ A, the sum of the occupation factors of D2 and D3 sites was fixed to 1. All sites are tetrahedral but D1 is surrounded by one yttrium and three nickel (AB 3 -type) whereas D4 and D5 are surrounded by two yttrium and two nickel (A2 B 2 -type). It is known that tetrahedral sites richer in A-element are more energetically favorable to host deuterium atoms. 30 However, despite the fact that D4 and D5 have the same first neighbors, they are not equivalent. Calculations of energy sites for the D4 and D5 should be done to confirm that D4 is more favorable to host hydrogen in the case of β hydride and D5 is more favorable to host hydrogen in the case of δ.

Conclusions The intermetallic compound Y2 Ni7 was successfully synthesized. Rietveld refinement of Xray diffraction pattern highlighted the single phase character of the alloy which crystallizes in the rhombohedral Gd2 Co7 -type structure. This result was confirmed by EPMA and STEMHAADF analysis. The P-c isotherm performed at 25 ◦ C presents three plateau pressures and the maximum hydrogen capacity at 10 MPa reach 8.9 H/f.u. After complete desorption, the unit cell parameters were fully recovered. Furthermore, the capacity measured during cycles of hydrogenation-dehydrogenation remained constant for at least 80 cycles. Those results emphasize reversible hydrogen absorption-desorption properties of Y2 Ni7 . For the first time, crystallographic structure of Gd2 Co7 -type deuterides were solved. It was shown that at the end of the first plateau deuterium is located exclusively in [A2 B 4 ] subunit. For a higher state of charge, deuterium is located both in [A2 B 4 ] and [AB 5 ] units. The minimal distance of 2.1 ˚ A between two hydrogen atoms is respected and allows to describe the site occupancies for the different deuterides.


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Acknowledgement The research program MALHYCE (ANR-2011-PRGE-006 01) is acknowledged for financial support. The authors are thankful to Dr. E. Leroy, from ICMPE, for technical assistance in the EPMA analysis. Dr. F. Porcher and Dr. D. Sheptyakov are also acknowledged for their help in ND experiments at LLB (France) and PSI (Switzerland) respectively.

Supporting Information Available Table S1. Structural parameters (Gd2 Co7 -type structure; space group R ¯3m) derived from the Rietveld analysis of the XRD pattern of Y2 Ni7 . Numbers between brackets stand for standard deviation. Figure S1. Results from EPMA for Y2 Ni7 showing the homogeneity of the alloy (left image) and the corresponding measures compositions (right curve) from forty data points at the the surface of the left image. Figure S2. XRD patterns of Gd2 Ni7 before and after PCT measurement at maximal pressure: 10 MPa. Crystallographic information (cif) files for the three deuterides structures determined by Rietveld refinement of PND data. This material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Notten, P. H. L. In Interstitial intermetallic alloys; Grandjean, F., Long, G., Buschow, K., Eds.; Dordrecht: Kluwer Academic Publishers, 1995; Chapter Rechargeable Nickel-Metalhydride Batteries: a Succesful New Concept, pp 151–196. (2) Notten, P. H. L.; Latroche, M. In Encyclopedia of Electrochemical Power Sources; Dyer, C., Moseley, P., Ogumi, Z., Rand, D., Scrosati, B., Eds.; Amsterdam: Elsevier,



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Deuterides LaY2 Ni9 D12.8 and CeY2 Ni9 D7.7 Determined by Neutron Powder Diffraction and X-ray Absorption Spectroscopy. J. Solid State Chem. 2004, 177, 2542–2549. (29) Switendick, A. C. Band Structure Calculations for Metal Hydrogen Systems. Z. Phys. Chem. 1979, 117, S. 89–112. (30) Sherstobitova, E. A.; Gubkin, A.; Stashkova, L. A.; Mushnikova, N. V.; Terent’ev, P. B.; Cheptiakov, D.; Teplykh, A. E.; Park, J.; Pirogov, A. N. Crystal Structure of ErFe2 D3.1 and ErFe2 H3.1 at 450 K. J. Less Common Met. 2010, 508, 348–353.



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Colored Wien filtered HAADF image of the (110) plane of Y2 Ni7


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