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Effect of Rare Earth Elements and Alloy Composition on Hydrogenation Properties and Crystal Structures of Hydrides in Mg2−xRExNi4 K. Sakaki,*,† N. Terashita,‡ S. Tsunokake,‡ Y. Nakamura,† and E. Akiba†,§ †

National Institute of Advanced Industrial Science and Technology, AIST Central-5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-0035, Japan Japan Metals & Chemicals Co., Ltd., 232 Oguni, Nishiokitama, Yamagata 999-1351, Japan § Department of Mechanical Engineering, Faculty of Engineering and International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ‡

ABSTRACT: The effect of the rare earth elements and alloy composition on the hydrogenation properties and crystal structures of hydrides in Mg2−xRExNi4 (RE = La, Pr, Nd, Sm, and Gd; x = 0.6 and 1.0) was investigated. All Mg2−xRExNi4 alloys had a C15b Laves phase before hydrogenation. Mg1.4RE0.6Ni4 (RE = Pr, Sm, and Gd) alloys were hydrogenated through one plateau to form Mg1.4RE0.6Ni4H∼3.6 while maintaining the C15b structure. Mg1.0RE1.0Ni4 (RE = La, Pr, and Nd) alloys were hydrogenated to ∼1.0 H/M proceeding through two plateaus, and Mg1.0RE1.0Ni4 (RE = Sm and Gd) alloys were hydrogenated to 0.6− 0.7 H/M through one plateau. Mg1.0RE1.0Ni4 alloys initially transformed into Mg1.0RE1.0Ni4H∼4 with an orthorhombic structure. In addition it was experimentally confirmed that Mg1.0RE1.0Ni4H∼4 with La, Pr, and Nd transformed into Mg1.0RE1.0Ni4H∼6 with a C15b structure, while no formation of Mg1.0RE1.0Ni4H∼6 (RE = Sm and Gd) was observed at 40 MPa at 250 K. Theoretical calculations suggest that Mg1.0RE1.0Ni4H∼4 with Sm and Gd also transform to Mg1.0RE1.0Ni4H∼6 at higher pressures than those used in our experiments (264 MPa for Mg1.0Sm1.0Ni4 and 8.5 GPa for Mg1.0Gd1.0Ni4 at 253 K). It was found that the hydrogenation properties and crystal structure of the hydrides in Mg2−xRExNi4 are dependent on the alloy composition, i.e., the ratio of Mg to RE in the alloy phase, but independent of the choice of rare earth element.

1. INTRODUCTION

orthorhombic structure at a lower hydrogen content but changes to a C15b structure at a higher hydrogen content.4 The hydrogenation properties of the related Mg1.0RE1.0Ni4 materials with RE = Y, La, Ce, Nd, and Gd have also been reported.5−12 Although most of the reported hydrides have similar hydrogen contents of ∼0.6 H/M, two types of crystal structures, the C15b and orthorhombic structures, have been reported. The difference in the crystal structure cannot be directly explained by the choice of rare earth element or composition; for example, two different structures were reported even for the same composition of Mg1.0RE1.0Ni4.5−12 This crystal structure difference may arise from the different synthesis methods, such as ball-milling or melting, and/or a small deviation from the stoichiometric composition. The latter hypothesis is supported by the clear dependence of the crystal structure on the ratio of Mg to Pr found in Mg2−xPrxNi4, as described above.4 In this study, in order to systematically understand the effect of the alloy composition and the rare earth elements on the hydrogenation properties and the crystal structure of the

Mg-containing intermetallic compounds are some of the most promising candidates for hydrogen storage because they have a high volumetric hydrogen density. Since practical mobile applications such as fuel cell vehicles require not only a high volumetric hydrogen density but also a high gravimetric hydrogen density, new hydrogen storage materials with higher gravimetric hydrogen densities need to be developed. We have developed different Mg-containing hydrogen storage materials such as (Mg 0.67 Ca 0.33 )Ni 2 with a C15 structure and Mg1.4RE0.6Ni4 (where RE represents a rare earth element) with a C15b structure.1,2 These materials can reversibly absorb and desorb hydrogen up to 0.5−0.7 H/M (H/M is the ratio of the number of hydrogen to metal atoms) at room temperature.1,2 Recently, two distinct plateaus in the pressure− composition (P−C) isotherms were found in the Mg1.0Pr1.0Ni4 system. It absorbed hydrogen up to ∼1.0 H/M, while Mg1.4Pr0.6Ni4 absorbs ∼0.6 H/M through one plateau.3 Our results indicate that the ratio of Mg to Pr in the Mg2−xPrxNi4 alloy significantly affects the hydrogenation properties.3 The ratio of Mg to Pr was also found to affect the crystal structure of their hydrides: the hydride of Mg2−xPrxNi4 with x < 1.0 has a C15b structure, while the hydride of Mg1.4Pr0.6Ni4 has an © 2012 American Chemical Society

Received: May 30, 2012 Revised: August 14, 2012 Published: August 15, 2012 19156

dx.doi.org/10.1021/jp3052856 | J. Phys. Chem. C 2012, 116, 19156−19163

The Journal of Physical Chemistry C

Article

Table 1. Annealing Temperatures of Mg2−xRExNi4 Alloys sample

Mg1.0La1.0Ni4

Mg1.0Pr1.0Ni4 Mg1.4Pr0.6Ni4

Mg1.0Nd1.0Ni4

Mg1.0Sm1.0Ni4 Mg1.4Sm0.6Ni4

Mg1.0Gd1.0Ni4 Mg1.4Gd0.6Ni4

temperature (K)

1123

1323

1313

1273

1323

Table 2. Results of Chemical Analyses of Mg1.0RE1.0Ni4 and Mg1.4RE0.6Ni4 target composition analyzed composition target composition analyzed composition

La

Pr

Nd

Sm

Gd

Mg1.0La1.0Ni4 Mg1.0La1.0Ni4.1

Mg1.0Pr1.0Ni4 Mg1.0Pr1.0Ni4.0 Mg1.4Pr0.6Ni4 Mg1.4Pr0.6Ni4.1

Mg1.0Nd1.0Ni4 Mg1.0Nd1.0Ni4.2

Mg1.0Sm1.0Ni4 Mg1.0Sm1.0Ni4.1 Mg1.4Sm0.6Ni4 Mg1.5Sm0.5Ni3.7

Mg1.0Gd1.0Ni4 Mg1.1Gd0.9Ni4.3 Mg1.4Gd0.6Ni4 Mg1.4Gd0.6Ni4.1

hydrides in several Mg2‑xRExNi4 alloys, X-ray diffraction measurements, measurements of the P−C isotherms in Mg1.0 RE 1.0 Ni4 (RE = La, Pr, Nd, Sm, and Gd) and Mg1.4RE0.6Ni4 (RE = Pr, Sm, and Gd), and theoretical calculations were carried out. The factors that determine the crystal structures of the hydrides, the C15b or orthorhombic structures, are discussed from the viewpoint of the local coordination of the hydrogen occupation sites.

Mg1.0RE1.0Ni4, was synthesized by keeping the sample at 8 MPa at 243 K for 1 day and then at 193 K for 2 days. Before the samples were removed from a pressure vessel, the vessel was put into liquid nitrogen (77 K) for several hours at 8 MPa of inner pressure. The XRD patterns of these hydrides were repeatedly measured in air until the Bragg peaks of the initial phase disappeared. It took around 30 min to collect one pattern. All XRD data obtained in this work were analyzed using the Rietveld refinement program RIETAN-2000.13−15 Silicon powder (NIST 640c) was used as an external standard to calibrate the diffractometer. 2.2. Computational Method. Structure optimization and total energy calculations were performed using the Vienna ab initio simulation package (VASP)16,17 with the generalized gradient approximation (GGA) proposed by Perdew et al.18 Except for La, we used the special GGA potentials supplied for Ce−Lu. In these potentials, the f electrons are kept frozen in the core. This is a standard model for the treatment of localized f electrons.18 Potentials based on the all-electron projector augmented wave (PAW) method were used. 19,20 Full optimizations in which the atomic position, cell volume, and cell shape were all allowed to change were performed until the maximum force dropped below 0.001 eV/Å, and the selfconsistent field (SCF) convergence criterion was set at 10−7 eV. The plane wave cutoff energy was chosen to be 425 eV. The requested k-spacing was 0.1 Å−1. Total energy calculations were performed for three different phases, i.e., Mg 1 . 0 RE 1 . 0 Ni 4 , Mg 1 . 0 RE 1 . 0 Ni 4 H 4 , and Mg1.0RE1.0Ni4H7. A C15b structure model with space group F-43m was used for Mg1.0RE1.0Ni4 and Mg1.0RE1.0Ni4H7.7 An orthorhombic structure model with space group Pmn21 was used for Mg1.0RE1.0Ni4H4.6 The formation enthalpy for the hydrides with orthorhombic and C15b structures can be estimated using the following equations

2. METHODS 2.1. Experimental Procedure. Mg1.0RE1.0Ni4 (RE = La, Pr, Nd, Sm, and Gd) and Mg1.4RE0.6Ni4 (RE = Pr, Sm, and Gd) were prepared using high-frequency induction melting. Ingots of Mg, rare earth elements, and Ni, each with a purity higher than 99.9%, were used as the starting materials. The elements were melted in an alumina crucible and were cast into a watercooled board mold under a He atmosphere. The as-cast alloys were annealed for 10 h under an Ar atmosphere. The annealing temperatures determined from the melting points of the as-cast alloys are listed in Table 1. Chemical analyses were performed to identify the compositions of the annealed alloys. The chemical compositions in the annealed alloys were enough close to the target compositions, as shown in Table 2. The annealed alloys were crushed into powders and put into a stainless-steel vessel for P−C isotherm measurements. The vessels were evacuated using a rotary pump at 423 K for more than 2 h. Hydrogenation was carried out in the temperature range from 243 to 373 K. Dehydrogenation was carried out using the rotary pump at 423 K for more than 2 h. These processes were repeated more than three times, and then the P−C isotherms were measured. After several cycles of hydrogenation and dehydrogenation, the dehydrogenated samples were crushed into particles with diameters less than 30 μm. These fine powders were loaded into a sample holder for X-ray diffraction (XRD) measurements. In situ XRD data for the alloys were obtained using a diffractometer (Rigaku, RINT TTR-III) equipped with a highpressure chamber. Data were collected in the 2θ region between 15° and 100° at a power of 50 kV and 300 mA after the pressure in the chamber reached equilibrium. The sample powder was covered with a beryllium plate with a 0.1 mm thickness to keep the surface of the sample layer flat during the hydrogenation and dehydrogenation. Before hydrogen absorption, the sample was kept at 423 K for 1 h in vacuum. In situ XRD measurements were carried out at 273 K. Ex situ XRD measurements were carried out at 298 K for some of the hydrides. Mg1.4Sm0.6Ni4H∼3.6 and Mg1.4Gd0.6Ni4H∼3.6 were prepared by keeping the sample at 8 MPa at 273 K for 1 day. Mg1.0RE1.0Ni4H∼6, which is the higher pressure phase of

ΔHMg

RE1.0 Ni4H4

1.0

=

1⎡ ⎤ ⎣E(Mg1.0RE1.0 Ni4H4) − E(alloy)⎦ 2 − E(H 2)

ΔHMg

RE1.0 Ni4H 7

1.0

=

2⎡ ⎣E(Mg1.0RE1.0 Ni4H 7) 3 − E(Mg1.0RE1.0 Ni4H4)⎤⎦ − E(H 2)

While the first two terms on the right-hand side of these equations are the total energies obtained from the optimized structure models for Mg1.0RE1.0Ni4, Mg1.0RE1.0Ni4H4, and Mg1.0RE1.0Ni4H7, E(H2) was calculated using a hydrogen molecule placed in a 103 Å3 box. 19157



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3. RESULTS 3.1. Crystal Structure of Mg2−xRExNi4 before Hydrogenation. All Mg1.0RE1.0Ni4 alloys synthesized had a single phase of a C15b structure with space group F-43m (No. 216). This agrees with previously reported results.2,21 The lattice constants of the Mg1.0RE1.0Ni4 alloys decreased with increasing atomic number of the rare earth element. This is because the atomic radii of the rare earth elements decrease with increasing atomic number. The refined coordinate of the Ni atom at the 16e site (y, y, y) in Mg1.0RE1.0Ni4 corresponded to y ≈ 0.62. These positions deviated from the ideal symmetric position of the C15 Laves structure (5/8, 5/8, 5/8). All Mg1.4RE0.6Ni4 alloys synthesized had a single phase with a C15b structure like that of Mg1.4Pr0.6Ni4. These results agree with the previous reports for Mg2−xRExNi4.2,3 The XRD patterns for Mg1.4RE0.6Ni4 were refined using a structure model with a C15b structure in which the excess Mg occupied the RE site. Mg1.4RE0.6Ni4 alloys had smaller lattice constants than the Mg1.0RE1.0Ni4 alloys, because Mg has smaller atomic radius than the RE. The lattice constants of Mg1.4RE0.6Ni4 also decreased with increasing atomic number of the rare earth element, as did those of Mg1.0RE1.0Ni4. The refined coordinate of the Ni atom at the 16e site (y, y, y) in Mg1.4RE0.6Ni4 corresponded to y ≈ 0.623. These positions deviated from the ideal symmetric position (5/8, 5/8, 5/8) as did those of Mg1.0RE1.0Ni4. 3.2. Structural Change in Mg1.4RE0.6Ni4 during Hydrogenation and Dehydrogenation. Figure 1 shows the P−C

experimental conditions. Their hydrogen storage capacities were 0.6−0.7 H/M, and the equilibrium pressure increased with increasing atomic number of the rare earth element. The increase in equilibrium pressure is related to the decrease in lattice constants because the lattice constants also decreased with increasing atomic number of the rare earth element. This result agrees well with the empirical rule.22,23 Additional P−C isotherm measurements were performed for Mg1.4Pr0.6Ni4 up to 40 MPa at 250 K, but the second plateau was not observed. Figure 2 shows the results of the Rietveld refinement for the XRD patterns of Mg1.4RE0.6Ni4H∼3.6 with RE = Sm and Gd measured at room temperature. All XRD patterns show a single phase with a C15b structure similar to that of the starting alloys. A structure model similar to the alloy phase was used to refine the XRD patterns. Table 3 shows the refined lattice constants and atomic coordinates. The lattice constants of Mg1.4RE0.6Ni4H∼3.6 with RE = Pr, Sm, and Gd decreased with increasing atomic number of the rare earth element as was the case for the alloy phase of Mg1.4RE0.6Ni4. The lattice expansions of Mg1.4Sm0.6Ni4H∼3.6 and Mg1.4Gd0.6Ni4H∼3.6 were 4.8 and 4.5%, respectively, relative to the starting alloys. These values are close to that of Mg1.4Pr0.6Ni4H∼3.6, 4.3%. The volume expansions were 15.1 and 14.1%, respectively. No effect of the rare earth element on the structural and hydrogenation properties, except for the equilibrium pressure, was observed among Mg1.4RE0.6Ni4 alloys. 3.3. Structural Change in Mg1.0RE1.0Ni4 during Hydrogenation and Dehydrogenation. Figure 3 shows the P−C isotherms of Mg1.0RE1.0Ni4 measured at 273 K. The P−C isotherms of Mg1.0La1.0Ni4 and Mg1.0Pr1.0Ni4 showed two absorption plateaus under pressures up to 9 MPa. These results suggest that these compounds form two types of hydrides, Mg1.0RE1.0Ni4H∼4 and Mg1.0RE1.0Ni4H∼6 (RE = La and Pr). This result agrees well with previous reports.4,7 The other compounds absorbed hydrogen up to ∼0.67 H/M at 4 MPa via only one plateau to form Mg1.0RE1.0Ni4H∼4, as shown in Figure 3b. For the first phase transformation to Mg1.0RE1.0Ni4H∼4, the equilibrium pressure increased with increasing atomic number of the rare earth element, as was the case for Mg1.4RE0.6Ni4, and this effect is related to the lattice constants of the alloy phases. Figure 4a shows the result of the Rietveld refinement for the in situ XRD pattern of Mg1.0Nd1.0Ni4H∼4 measured at 273 K and around 1 MPa, and Table 4 shows the structural parameters of Mg1.0Nd1.0Ni4H∼4 and Mg1.0Nd1.0Ni4H∼6 refined using the Rietveld method. The crystal structure of Mg1.0Nd1.0Ni4H∼4 was orthorhombic with space group Pmn21, which is the same as the structure reported for

Figure 1. P−C isotherms of Mg1.4RE0.6Ni4 measured at 273 K.

isotherms of Mg1.4RE0.6Ni4 measured at 273 K. These P−C isotherms have only one flat plateau under the present

Figure 2. Result of Rietveld refinement for XRD patterns of (a) Mg1.4Sm0.6Ni4H∼3.6 and (b) Mg1.4Gd0.6Ni4H∼3.6. 19158



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Table 3. Structural Parameters of (a) Mg1.4Sm0.6Ni4H∼3.6 and (b) Mg1.4Gd0.6Ni4H∼3.6 Refined Using the Rietveld Method (a) Mg1.4Sm0.6Ni4H∼3.6; Space Group F-43m, a = 7.319 (1) Å, Rw = 15.47% x

B/Å2

occupancy

1.8 (1) = Sm = Sm 0.6 (1)

0.6 0.4 1.0 1.0

z

B/Å2

occupancy

0.0 0.0 0.25 0.6205 (1)

0.91 (8) = Gd = Gd = Gd

0.6 0.4 1.0 1.0

y

z

atom

Wyckoff position

Sm Mg Mg Ni

4a 4a 4c 16e

atom

Wyckoff position

x

y

Gd Mg Mg Ni

4a 4a 4c 16e

0.0 0.0 0.25 0.6205 (1)

0.0 0.0 0.25 0.6205 (1)

0.0 0.0 0.0 0.0 0.0 0.0 0.25 0.25 0.25 0.6201 (2) 0.6201 (2) 0.6201 (2) (b) Mg1.4Gd0.6Ni4H∼3.6; Space Group F-43m, a = 7.275 (1) Å, Rw = 14.45%

Figure 3. P−C isotherms of Mg1.0RE1.0Ni4 measured at 273 K: (a) Mg1.0La1.0Ni4 and Mg1.0Pr1.0Ni4 and (b) Mg1.0Nd1.0Ni4, Mg1.0Sm1.0Ni4, and Mg1.0Gd1.0Ni4.

Figure 4. Result of Rietveld refinement for (a) Mg1.0Nd1.0Ni4H∼4 and (b) Mg1.0Nd1.0Ni4H∼4.

Table 4. Structural Parameters of (a) Mg1.0Nd1.0Ni4H∼4 and (b) Mg1.0Nd1.0Ni4H∼6 Refined Using the Rietveld Method (a) Mg1.0Nd1.0Ni4H∼4; Space Group Pmn21, a = 5.077 (4) Å, b = 5.475 (4) Å, c = 7.379 (6) Å, Rw = 14.78% atom

Wyckoff position

Nd Mg Ni Ni Ni

2a 2a 2a 2a 4b

x

y

z

0.0 0.293 (1) 0.0 0.768 (7) 0.0 0.457 (2) 0.0 0.962 (3) 0.741 (2) 0.230 (2) (b) Mg1.0Nd1.0Ni4H∼6; Space Group F-43m, a = 7.603 (4)

0.0 0.221 (4) 0.626 (2) 0.614 (2) 0.383 (2) Å, Rw = 11.98%

B/Å2

occupancy

0.2 0.2 0.2 0.2 0.2

1.0 1.0 1.0 1.0 1.0

atom

Wyckoff position

x

y

z

B/Å2

occupancy

Nd Mg Ni

4a 4c 16e

0.0 0.25 0.6240 (3)

0.0 0.25 0.6240 (3)

0.0 0.25 0.6240 (3)

0.2 (1) = Nd = Nd

1.0 1.0 1.0

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Figure 5. Refined and calculated lattice constants: (a) Mg1.0RE1.0Ni4, (b) Mg1.0RE1.0Ni4H4, and (c) Mg1.0RE1.0Ni4H7.

Mg1.0Nd1.0Ni4D∼4.6 The same results were obtained in the other Mg1.0RE1.0Ni4H∼4 with La, Pr, Sm, and Gd. The lattice constants of the orthorhombic hydrides decreased with increasing atomic number of the rare earth element as did those of Mg1.0RE1.0Ni4 alloy. The volume expansions of the Mg1.0RE1.0Ni4H∼4 with RE = La, Pr, Nd, Sm, and Gd were 14.3, 15.4, 15.1, 15.6, and 15.5%, respectively. These values of the volume expansion are close to those of the Mg1.4RE0.6Ni4H∼3.6 with C15 structures, although the crystal structures were different. The synthesis of Mg1.0Nd1.0Ni4H∼6 was carried out at 8 MPa and 193 K. Figure 4b shows the ex situ XRD pattern of the hydride measured at room temperature in air. This pattern contains a main phase with a C15b structure and a small amount of a secondary phase with an orthorhombic structure. During repeated measurements, the intensity of the C15b phase decreased while that of the orthorhombic phase increased. Finally, the XRD pattern showed a single phase of the orthorhombic hydride. The lattice constant of the hydride with the C15b structure was 7.605 (4) Å. The volume expansion of this hydride from the alloy phase was 23.5%. This value is similar to those for Mg1.0La1.0Ni4 (21.9%) and Mg1.0Pr1.0Ni4 (25.0%). This indicates that Mg1.0Nd1.0Ni4 also forms a hydride phase with a higher hydrogen content, Mg1.0Nd1.0Ni4H∼6, like Mg1.0La1.0Ni4H∼6 and Mg1.0Pr1.0Ni4H∼6. The lattice constants of Mg1.0RE1.0Ni4H∼6 (RE = La, Pr, and Nd) decreased with increasing atomic number of the rare earth element as did those of Mg1.0RE1.0Ni4 and Mg1.0RE1.0Ni4H∼4. The same experiments were done for Mg1.0Sm1.0Ni4 and Mg1.0Gd1.0Ni4. The XRD patterns obtained for the hydrides of Mg1.0Sm1.0Ni4 and Mg1.0Gd1.0Ni4 showed a single phase with an orthorhombic structure, indicating no formation of Mg1.0Sm1.0Ni4H∼6 and Mg1.0Gd1.0Ni4H∼6 under these experimental conditions. 3.4. Hydrogenation Properties of Mg1.0RE1.0Ni4 at High Pressures. Here, the formation enthalpies of Mg1.0RE1.0Ni4H4 and Mg1.0RE1.0Ni4H7 were evaluated using theoretical calculations to help us understand the hydrogenation properties of Mg1.0RE1.0Ni4. The structures for Mg1.0RE1.0Ni4H4 and Mg1.0RE1.0Ni4H7 were modeled based on references 6 and 7. It should be noted that the occupancy of hydrogen at all the sites in both structure models was set to 1 for the theoretical calculation, even though the occupancy reported from experiments was not equal to 1. The calculated lattice constants of each phase after the full geometry optimization are shown in Figure 5 together with the

experimental results. The calculated lattice constants for Mg1.0RE1.0Ni4, Mg1.0RE1.0Ni4H4, and Mg1.0RE1.0Ni4H7 decreased with increasing atomic number of the rare earth elements. The calculated lattice constants for Mg1.0RE1.0Ni4 and Mg1.0RE1.0Ni4H4 agree well with our experimental results. The calculated lattice constants of Mg1.0RE1.0Ni4H7 showed a tendency similar to that of the experimental results; although the calculated values were slightly larger than the experimental results. This probably comes from the different hydrogen contents between the calculation and the experiment. The optimized atomic coordinates of M g 1 . 0 Nd 1 . 0 Ni 4 , Mg1.0Nd1.0Ni4H4, and Mg1.0Nd1.0Ni4H7 are shown in Table 5. The optimized atomic coordinates were similar to those obtained using neutron diffraction.6,7 No dependence of the atomic coordinates on the rare earth element was observed in Mg1.0RE1.0Ni4, Mg1.0RE1.0Ni4H4, or Mg1.0RE1.0Ni4H7. These results indicate that the effect of the rare earth element is Table 5. Calculated Structural Parameters of (a) Mg1.0Nd1.0Ni4, (b) Mg1.0Nd1.0Ni4H4, and (c) Mg1.0Nd1.0Ni4H7 (a) Mg1.0Nd1.0Ni4; Space Group F-43m, a = 7.103466 Å atom

Wyckoff position

Nd 4a Mg 4c Ni 16e (b) Mg1.0Nd1.0Ni4H4; Space atom Nd Mg Ni Ni Ni H H H

19160

Wyckoff position

x

y

z

occupancy

0.0 0.0 0.0 1.0 0.25 0.25 0.25 1.0 0.6235 0.6235 0.6235 1.0 Group Pmn21, a = 5.07536 Å, b = 5.48493 Å, c = 7.36522 Å x

y

2a 0.0 0.2997 2a 0.0 0.8176 2a 0.0 0.4485 2a 0.0 0.9919 4b 0.7563 0.2277 4b 0.7479 0.5106 2a 0 0.7201 2a 0 0.9406 (c) Mg1.0Nd1.0Ni4H7; Space group F-43m, a

z

occupancy

0.0017 0.2228 0.6224 0.6037 0.3789 0.7581 0.5105 0.8252 = 7.68744 Å

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

atom

Wyckoff position

x

y

z

occupancy

Nd Mg Ni H H

4a 4c 16e 24g 4b

0.0 0.25 0.6233 0.9908 0.5

0.0 0.25 0.6233 0.75 0.5

0.0 0.25 0.6233 0.75 0.5

1.0 1.0 1.0 1.0 1.0



Article

Figure 6. Formation enthalpies of (a) Mg1.0RE1.0Ni4H4 and (b) Mg1.0RE1.0Ni4H7.

evaluated to be −17.7 and 99.4 J/mol H2 K from the van’t Hoff plot. This formation enthalpy is close to the calculated value (−15.4 kJ/mol H2). The P−C isotherms of Mg1.0Sm1.0Ni4 and Mg1.0Gd1.0Ni4 up to 40 MPa did not show the second plateau at 250 K, suggesting that Mg1.0Sm1.0Ni4H∼6 and Mg1.0Gd1.0Ni4H∼6 were not formed under these experimental conditions. These results are also consistent with our theoretical calculation.

reproduced well and that these calculation models are reasonable. The formation enthalpies for Mg1.0RE1.0Ni4H4 and Mg1.0RE1.0Ni4H7 were calculated from the total energy of each structure model, as shown in Figure 6. The calculated formation enthalpies for Mg1.0RE1.0Ni4H4 and Mg1.0RE1.0Ni4H7 increased with increasing atomic number of the rare earth element. The calculated formation enthalpies showed a tendency similar to those from our experimental results;3 although the experimental formation enthalpies for Mg1.0RE1.0Ni4H∼4 were scattered. The calculated formation enthalpies of Mg1.0RE1.0Ni4H7 with RE = Nd, Sm, and Gd were −15.4, −8.3, and −1.0 kJ/mol H2, respectively. These calculated results suggest that Mg 1 . 0 Nd 1 . 0 Ni 4 H ∼ 6 , Mg1.0Sm1.0Ni4H∼6, and Mg1.0Gd1.0Ni4H∼6 with C15b structures can be synthesized at much higher hydrogen pressures, for example, at 9 MPa for Mg1.0Nd1.0Ni4, at 264 MPa for Mg1.0Sm1.0Ni4, and at 8.5 GPa for Mg1.0Gd1.0Ni4 at 253 K, if the formation entropy of Mg1.0RE1.0Ni4H∼6 is assumed to be 98.2 J/mol H2 K, which was obtained from the van’t Hoff plot for Mg1.0Pr1.0Ni4. To confirm the results obtained using the theoretical calculations, the P−C isotherms of Mg1.0RE1.0Ni4 with RE = Nd, Sm, and Gd were measured up to 40 MPa at 250 K. Mg1.0Nd1.0Ni4 exhibited a second plateau in its P−C isotherms, as shown in Figure 7. The equilibrium pressure and hydrogen storage capacity were around 10 MPa at 250 K and ∼1.0 H/M. This clearly indicates that Mg1.0Nd1.0Ni4H∼6 with a C15b structure was formed, as our calculation predicted. The formation enthalpy and entropy of Mg1.0Nd1.0Ni4H∼6 were

4. DISCUSSION 4.1. Effect of the Rare Earth Elements and Alloy Composition on Crystal Structures of Hydrides. In this study, the crystal structure and hydrogenation properties in Mg1.0RE1.0Ni4 and Mg1.4RE0.6Ni4 were investigated to understand their dependence on the rare earth elements and the alloy composition. All Mg1.4RE0.6Ni4 alloys were hydrogenated to form Mg1.4RE0.6Ni4H∼3.6 with maintaining the C15b structure, while all Mg1.0RE1.0Ni4 alloys were anisotropically expanded to form the orthorhombic Mg1.0RE1.0Ni4H∼4 upon hydrogenation and Mg1.0RE1.0Ni4H∼4 with La, Pr, and Nd transformed into Mg1.0RE1.0Ni4H∼6 with a C15b structure. Our theoretical calculation suggests that Mg1.0RE1.0Ni4H∼4 with Sm and Gd transform to Mg1.0RE1.0Ni4H∼6 with a C15b structure at higher hydrogen pressures. In summary, the phase transformations and crystal structures of the hydride phases are only dependent on the ratio of Mg to RE and are independent of the rare earth element. Three hydrogen occupation sites have been reported for Mg1.0Nd1.0Ni4D∼4.6 One is located near the center of the RENi3 tetrahedron, and the others are near the center of the RE2MgNi2 triangular bipyramid. The position of the latter site is shifted from the site inside the REMgNi2 tetrahedron reported for hydrides with C15 structures. In Mg-rich Mg2−xRExNi4, some of the RE2MgNi2 and RENi3 structures become REMg2Ni2 and MgNi3, respectively, because the excess Mg atoms occupy the RE sites. The substituted REMg2Ni2 triangular bipyramid is asymmetric with respect to the Mg− Ni−Ni plane, as shown in Figure 8. The hydrogen sites probably shift from on the plane to off the plane, e.g., toward the center of the REMgNi2 tetrahedron. In consequence, the shift of the hydrogen sites would stabilize a cubic structure in Mg-rich Mg2−xRExNi4, as seen in the hydrides with C15 structures. The effect of the ratio of Mg to Pr on the crystal structure of the hydrides in Mg2−xPrxNi4 would be determined by the stability of the hydrogen occupying the RE2MgNi2 triangular bipyramidal site or the REMgNi2 tetrahedral site.

Figure 7. P−C isotherms of Mg1.0Nd1.0Ni4 up to 40 MPa. 19161



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the center of one of the tetrahedrons rather than the center of the REMg2Ni2 triangular bipyramid, which leads to a cubic structure rather than an orthorhombic structure. The (Mg,Ca)Ni2 alloy prepared by melting had a C15 structure. The Mg and Ca are not ordered, which corresponds to a high exchange rate if the C15b model is applied. In fact (Mg,Ca)Ni2 forms a hydride with a cubic C15 structure rather than an orthorhombic one.1 This is also consistent with our assumption that a high exchange rate promotes the formation of a cubic hydride. It can be concluded that the different crystal structures of Mg1.0RE1.0Ni4H∼4 arise from the different exchange rates for the 4a and 4c sites.

Figure 8. Atomic coordinates of triangular bipyramid in Mg2−xRExNi4H∼4 for (a) stoichiometric and (b) Mg-rich compounds.

5. CONCLUSIONS We studied the phase transformation and the structural changes in Mg2−xRExNi4 with a C15b structure upon hydrogenation. Experimental XRD results and P−C isotherms for Mg1.0RE1.0Ni4 and Mg1.4RE0.6Ni4 were obtained along with theoretical results for Mg1.0RE1.0Ni4. From these results and previous reports, the following conclusions were drawn: The hydrogenation properties and the crystal structure of the hydrides of Mg2−xRExNi4 are dependent on the ratio of Mg to RE but independent of the choice of rare earth element: Compounds with x = 1 form orthorhombic Mg1.0RE1.0Ni4H∼4 phases and C15b Mg1.0RE1.0Ni4H∼6 phases, while compounds with x < 1 form Mg2−xRExNi4H∼3.6 phases with a C15b structure. The formation of the cubic Mg2−xRExNi4H∼3.6 phase tends to be promoted by the deviation from the stoichiometry (x < 1) and/or the lower ordering of the Mg and RE caused by some preparation conditions. In both of these mechanisms, one RE atom of the RE2MgNi2 triangular bipyramidal sites created in the orthorhombic Mg2−xRExNi4H∼4 phase is replaced by Mg, which will prevent the formation of orthorhombic hydrides and stabilize the cubic hydrides. Therefore, the crystal structure changes in Mg2−xRExNi4 upon hydrogenation can be classified using the ratio of Mg to RE and the exchange rate at the 4a and 4c sites.

4.2. Crystal Structure of Mg2−xRExNi4H∼4: Orthorhombic or C15b. The hydrogenation properties and the crystal structures of the hydrides have been reported for several Mg1.0RE1.0Ni4 alloys with C15b structures. Different crystal structures of the hydrides, Mg1.0RE1.0Ni4H∼4, were reported by different groups, even though they all prepared hydrides with the same stoichiometric alloy composition.5−12 To our knowledge, hydrides with C15b structures were reported for Mg1.0La1.0Ni4,5 Mg1.0Gd1.0Ni4‑xAlx,10 and Mg1.0Y1.0Ni3.5Al0.5,8 while orthorhombic hydrides were reported for Mg1.0La1.0Ni4,7 Mg1.0Nd1.0Ni4,6 and Mg1.0Y1.0Ni4.9 On the other hand, the theoretical calculation for Y1.0Mg1.0Ni4 performed by Pregent and Gupta24 showed that Y1.0Mg1.0Ni4H4 is more stable with an orthorhombic structure than with a C15b structure. Most of the reported compounds whose hydrides had C15b structures were prepared by ball-milling followed by short-time annealing (e.g., 1 h) at 923 K. Ball-milled samples exhibited a partial mixing of Mg and RE at the 4a and 4c sites in the C15b structure model, although Mg occupies the 4c site and RE occupies the 4a site in the ideal ordered Mg1.0RE1.0Ni4 with the C15b structure. In ref 8, an exchange rate τ is defined as τ=

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occ(Mg in 4a) occ(Mg in 4a) + occ(RE in 4a)

AUTHOR INFORMATION

Corresponding Author

occ(Mg in 4a) = occ(RE in 4c)

*E-mail: [email protected].

occ(Mg in 4a) + occ(Mg in 4c) = 1

Notes

The authors declare no competing financial interest.

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This τ becomes 0 for a C15b structure in which Mg and RE are completely ordered, while τ is 50% for a C15 structure in which Mg and RE are completely disordered. For Mg1.0Gd1.0Ni4 prepared by ball-milling, the exchange rate was about 14%.8 On the other hand, most of the reported compounds whose hydrides had orthorhombic structures were prepared by the melting method; the one such sample prepared by ball-milling was produced using much longer annealing (36 h) at 993 K.7 The site mixing phenomenon has not been reported for these compounds. By consideration of the above results, the factor determining whether the crystal structure of the hydride is C15b or orthorhombic is probably the exchange rate of Mg and RE between the 4a site and the 4c site. As shown in Figure 8, orthorhombic Mg1.0RE1.0Ni4H∼4 is considered to have one RENi3 tetrahedral site and two RE2MgNi2 triangular bipyramidal sites for hydrogen. When the exchange rate increases, some of the RE atoms are replaced by Mg atoms, and some of the Mg atoms are replaced by RE atoms. Because of the asymmetric coordination, hydrogen atoms will tend to occupy

ACKNOWLEDGMENTS This work has been supported by the New Energy and Industrial Technology Development Organization (NEDO) under its “Advanced Fundamental Research Project on Hydrogen Storage Materials” and “Development of technologies for hydrogen production, delivery and storage systems.”

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REFERENCES

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