- Email: [email protected]
Synthesis of FCC Mg–Zr and Mg–Hf hydrides using GPa hydrogen pressure method and their hydrogen-desorption properties
Journal of Alloys and Compounds 463 (2008) 311–316
Synthesis of FCC Mg–Zr and Mg–Hf hydrides using GPa hydrogen pressure method and their hydrogen-desorption properties Daisuke Kyoi, Tetsuo Sakai ∗ , Naoyuki Kitamura, Atsushi Ueda, Shigeo Tanase Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Received 20 March 2007; received in revised form 30 August 2007; accepted 2 September 2007 Available online 6 September 2007
Abstract Magnesium-based hydrides with Zr or Hf have been synthesized by means of an ultra-high pressure technique. Powder mixtures of MgH2 and ZrH2 or HfH2 have been heated up to 873 K under 4–8 GPa in a multi-anvil cell. New ternary phases (Mg–Zr–H and Mg–Hf–H) with a face centered cubic (FCC) structure have been formed. In the Mg–Zr hydride, the FCC phase with disordered metal–atom occupancy was observed, while a Ca7 Ge-type super-lattice structure was observed in the Mg–Hf hydride. By the temperature programmed desorption (TPD) measurements, these new hydrides exhibit the hydrogen-desorption at around 543–583 K, which were 130–70 K lower than that of MgH2 at a heating rate of 10 K/min under vacuum. Desorbed hydrogen contents were estimated to be 4.2 and 3.0 mass% for Mg–Zr and Mg–Hf hydrides, respectively. © 2007 Elsevier B.V. All rights reserved. Keywords: High pressure; Hydrogen; Metal hydride; Magnesium; Zirconium; Hafnium
1. Introduction Magnesium hydride (MgH2 ) is a promising hydrogen storage material because of its high hydrogen content of 7.6 mass%. However, its hydrogenation and dehydrogenation kinetics is too slow, and its desorption temperatures above 700 K is too high for wide practical application. The thermodynamic stability should be improved without reducing the high hydrogen storage capacity. In order to surpass these drawbacks, various attempts were carried out to synthesize Mg-based alloys with transition metals (TMs). In a Mg2 NiH4 , notable improvement of absorption kinetics was reported, although the hydrogen storage capacity was reduced to 3.6 wt% [1]. New hydrides, such as Mg2 CoH5 and Mg2 FeH6 were prepared [2]. But, these hydrides decomposed into Mg and Fe or Co after dehydrogenation. In 1990s, ballmilling MgH2 with TMs has been attempted and consequently led to better performance [3–7]. In these cases, the added metals are assumed to work as catalysts. Nevertheless, their desorption temperature are still too high for practical use.
∗
Corresponding author. Tel.: +81 72 751 9611; fax: +81 72 751 9623. E-mail address: [email protected] (T. Sakai).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.09.004
In order to create new Mg–TM hydrides with lower desorption temperature for many kinds of TMs, we have tried to use a high-pressure technique. Using hydrogen pressure up to 8 GPa, we have succeeded to prepare a series of new ternary hydrides “Mg7 TMHx ” (TM = Ti, V, Nb and Ta) with the Ca7 Ge-type FCC super-lattice structure [8–11]. Detailed crystal structure analysis was performed for TM = Ti [12] and Nb [13]. This indicates that one of the Mg sites (4b) in the Mg7 TMHx structure was almost filled for TM = Ti, while only the half is occupied for TM = Nb. It was revealed that hydrogen atoms occupy tetrahedral interstitial sites (32f), providing estimated unit formulas of Mg7 TiH16 (6.9 mass% H) [12] and Mg6.5 NbH14 (5.7 mass% H) [13]. An average Mg–H bonding distance in each FCC hydride is longer than that of MgH2 , explaining the observed lower dehydrogenation temperature. It was reported for TM = Zr that the new hydride phase Mg2 Zr3 Hy with a monoclinic structure was prepared under 2–5 GPa and 1173 K [14]. In this study, we have succeeded to prepare new ternary hydrides Mg6 ZrHx and Mg6.7 HfHx with FCC unit cells by reacting MgH2 with ZrH2 or HfH2 at 873 K in the hydrogen-pressure range of 4–8 GPa. Detailed preparation condition and hydrogendesorption properties for the FCC hydrides will be discussed.
312
D. Kyoi et al. / Journal of Alloys and Compounds 463 (2008) 311–316
Table 1 Preparation conditions and phase components for the Mg–Zr–H specimens Specimen
#z − 1 #z − 2 #z − 3 *
Conditions
Phase component
Mixing molar ratio MgH2 :ZrH2
Pressure (GPa)
Temperature (K)
Duration (h)
6:1 6:1 9:1
4 8 8
873 873 873
1 1 1
Monoclinic* , ␣-MgH2 , ␥-MgH2 , MgO FCC* , MgO FCC* , MgO
The dominant phase.
Table 2 Preparation conditions and phase components for the Mg–Hf–H specimens Specimen
#h − 1 #h − 2 #h − 3 #h − 4 *
Conditions
Phase component
Mixing molar ratio MgH2 :HfH2
Pressure (GPa)
Temperature (K)
Duration (h)
6:1 6:1 7:1 3:1
4 8 4 4
873 873 873 873
1 1 1 1
FCC* , MgO FCC* , MgO FCC* , MgO FCC* , monoclinic, MgO
The dominant phase.
2. Experimental procedures Commercially available powder of MgH2 (Aldrich Chemical Co.; purity 90%) was used as a starting material. Both ZrH2 and HfH2 were prepared by hydrogenation of powdery metals: Zr (Rare Metallic Co. Ltd.; purity 99%; −100 mesh) and Hf (Aldrich Chemical Co.; purity 99.5%; −325 mesh), respectively. X-ray powder diffraction (XRD) patterns for the dihydrides prepared were consistent with the data of tetragonal ZrH2 (JCPDS #17-0314) and tetragonal HfH1.98 (JCPDS #28-0443). Material handling was done in a glove box filled with dry Ar gas. Powder mixtures of MgH2 with ZrH2 or HfH2 were pressed into pellets. Mixing molar ratios were MgH2 :ZrH2 = 9:1 and 6:1 for Mg–Zr–H, and MgH2 :HfH2 = 7:1, 6:1 and 3:1 for Mg–Hf–H, respectively. As internal hydrogen sources NaBH4 (Aldrich Chemical Co.; purity 98%) and Ca(OH)2 (Wako Pure Chemical Industries, Ltd., purity 99.9%) were used. The sample pellet composed of MgH2 and ZrH2 or HfH2 was sandwiched between BN thin discs, and put between the pellets of the hydrogen source, and then sealed in a NaCl capsule. The NaCl capsule was inserted in a graphite heater tube, which was placed at the center of a set of pyrophyllite octahedral cell. Details of this setup are illustrated in our previous papers [12]. The octahedral cell was compressed up to a desired pressure (4–8 GPa) in 6–8 multi anvils by use of an ultra-high pressure generating apparatus (Sumitomo Heavy Industries Ltd., UHP-2000). The sample mixture was heated up to 873 K and the temperature was held during 1 h at the settled pressure. Phase component in the resultant specimen was charactarized on the basis of XRD data obtained with a Bruker aXS M06XCE powder diffractometer. After
correcting the XRD data by use of a program Pfilm [15], the diffraction patterns were indexed by using programs TREOR97 [16] and PIRUM [17]. Elemental analysis of each phase composition was done with a JEOL JXA-8800RL electron probe micro analyzer (EPMA), of which results were based on the Mg (K), Zr (L) and Hf (M) lines in the spectra. Hydrogen-desorption properties were evaluated by use of a temperature programmed desorption (TPD) gas analysis system (TPD-1-AT manufactured by BEL Japan Co., Ltd.) with a heating rate of 10 K/min under a vacuum of 10−2 Pa. Hydrogen gas released from hydride phases in the specimen was detected from the TPD spectrum for molecular weight = 2.
3. Results and discussions 3.1. High-pressure synthesis Preparation conditions and phase components for specimens obtained in this work are summarized in Tables 1 and 2. The dominant phase is marked with asterisk (*) for each of the specimens. Analyzed cell dimensions of ternary phases with the FCC or monoclinic structure formed in each of the specimens are also summarized in Table 3. In the following sections, we will demonstrate occurrence of the Mg–Zr–H and Mg–Hf–H ternary phases in accordance with the preparation condition.
Table 3 Cell parameters of the Mg–TM hydrides (TM = Zr, Hf and Nb) FCCa
Monoclinicb
a (nm)
a (nm)
b (nm)
c (nm)
β (◦ )
Mg–Zr–H (#z − 1) Mg–Zr–H (#z − 2) Mg–Zr–H (#z − 3)
– 0.4870(1) 0.4868(3)
0.5821(3) – –
0.3350(2) – –
0.8607(5) – –
103.10(4) – –
Mg–Hf–H (#h − 1) Mg–Hf–H (#h − 2) Mg–Hf–H (#h − 3) Mg–Hf–H (#h − 4)
0.9711(1) 0.9717(3) 0.9717(3) 0.9707(1)
– – – 0.5994(3)
– – – 0.3274(3)
– – – 0.8532(4)
– – – 103.87(4)
MgNb2 Hy [10]
–
0.5685(4)
0.32914(6)
0.7924(2)
103.82(3)
Specimen
a b
¯ (No. 225), Z = 4 Space group:Fm3m Space group: C2/m (No. 12), Z = 2
D. Kyoi et al. / Journal of Alloys and Compounds 463 (2008) 311–316
313
diffraction patterns. This is interpreted that the observed “simple” FCC phase results from the disordering of Mg and Zr. Since the atomic radius of zirconium is close to that of magnesium, the Mg and Zr atoms tend to be replaced randomly. A chemical composition analysis of the FCC phase in the specimen Mg–Zr–H (#z − 2) was carried out by EPMA. The metal–atom composition was determined to be Mg0.82 Zr0.18 , which corresponds to the composition Mg4.5 ZrHx . 3.1.2. Mg–Hf–H system Fig. 2 shows the XRD patterns of the original mixture composed of MgH2 :HfH2 = 6:1, the specimens Mg–Hf–H (#h − 1) and Mg–Hf–H (#h − 2). In both specimens, the FCC hydride phase with super-lattice structure was formed, while the both starting materials MgH2 and HfH2 completely disappeared. The diffraction peaks of the FCC hydride phase (marked with solid circles) were indexed with cell dimensions of a = 0.9711(1) nm for the specimen Mg–Hf–H (#h − 1), and a = 0.9717(3) nm for the specimen Mg–Hf–H (#h − 2), respectively. The cell volume of the FCC phase was almost the same between the prepared specimens. The specimen Mg–Hf–H (#h − 2), however, showed broader diffraction peaks than the specimen Mg–Hf–H (#h − 1),
Fig. 1. XRD patterns for Mg–Zr–H: (a) the original mixture (MgH2 :ZrH2 = 6:1), (b) the specimen (#z − 1), (c) the specimen (#z − 2) and (d) the specimen (#z − 3).
3.1.1. Mg–Zr–H system Fig. 1 shows the XRD patterns of the original mixture composed of MgH2 :ZrH2 = 6:1, the specimens Mg–Zr–H (#z − 1), Mg–Zr–H (#z − 2) and Mg–Zr–H (#z − 3). As shown in Fig. 1(b), unreacted ␣-MgH2 and ␥-MgH2 , and an impurity of MgO were found in the specimen Mg–Zr–H (#z − 1). Substantial diffraction peaks (marked with open inverted triangles) were indexed as a monoclinic structure with cell dimensions of a = 0.5821(3) nm, b = 0.3350(2) nm, c = 0.8607(5) nm, β = 103.10(4)◦ . The cell dimensions are close to those of MgNb2 Hy [10,13], which indicate that a similar monoclinic hydride phase would be formed in the specimen Mg–Zr–H (#z − 1). As shown in Fig. 1(c) and (d), the FCC hydride phase appeared in the specimens Mg–Zr–H (#z − 2) and Mg–Zr–H (#z − 3) which were prepared at 8 GPa. Both specimens also contained a small amount of MgH2 and MgO, where the monoclinic phase was not formed. The diffraction peaks for the FCC phase (marked with solid circles) were indexed with cell dimensions of a = 0.4870(1) nm for the specimen Mg–Zr–H (#z − 2), and a = 0.4868(3) nm for the specimen Mg–Zr–H (#z − 3), respectively. No super-lattice structure peaks were observed on both
Fig. 2. XRD patterns for Mg–Hf–H (MgH2 :HfH2 = 6:1): (a) the original mixture, (b) the specimens (#h − 1) and (c) the specimen Mg–Hf–H (#h − 2).
314
D. Kyoi et al. / Journal of Alloys and Compounds 463 (2008) 311–316
[18] reveals that reducing occupancy of Hf or increasing Mg at the 4a atomic site leads to weak super-lattice-structure peaks. This simulation also implies that the super-lattice structure in the Mg–Hf FCC hydride would tend to disappear with increase of the Mg content. Actually, no excess Mg or MgH2 phases were detected in Fig. 3(a) even though the specimen Mg–Hf–H (#h − 3) had higher Mg content than the specimens Mg–Hf–H (#h − 1). The FCC phase in the specimen Mg–Hf–H (#h − 3) is, therefore, considered to have a higher Mg content than that in the specimen Mg–Hf–H (#h − 1). Being focused on the specimen Mg–Hf–H (#h − 4), the FCC super-lattice structure peaks are very small, as can be seen in Fig. 3(c). From the above structural simulation, it is guessed that the FCC phase in the specimen Mg–Hf–H (#h − 4) would have very high Mg content. Nonetheless, the initial Mg content (MgH2 :HfH2 = 3:1) was the lowest among the specimens. Note that another phase with a monoclinic structure was formed in the specimen Mg–Hf–H (#h − 4), as shown in Fig. 3(c). The corresponding diffraction peaks (marked with open inverted triangles) were indexed with cell dimensions of a = 0.5994(3) nm, b = 0.3274(3) nm, c = 0.8532(4) nm, β = 103.87(4)◦ similar to those of MgNb2 Hy [10,13]. For the specimen Mg–Hf–H (#h − 4), it is considered that the starting material of HfH2 was spent to form not only the FCC hydride but also the monoclinic compound, consequently the FCC hydride would have relatively high Mg content. These results indicate that the Mg–Hf FCC hydrides would have various metal–atom compositions in dependence on the preparation condition. Also, the Mg–Hf FCC hydride could have metal–atom arrangements with/without super-lattice structure, i.e. the Ca7 Ge-type or Mg–Zr hydride-type structure. 3.2. Hydrogen-desorption properties Fig. 3. XRD patterns for Mg–Hf–H: (a) the specimen (#h − 3), (b) the specimen (#h − 1) and (c) the specimen (#h − 4).
indicating that the crystal lattice of the FCC hydride phase would be distorted at the higher pressure. No other phase except for a small amount of MgO phase was confirmed from XRD in Fig. 2(b) and (c). According to the EPMA analysis, the metal atomic ratio of the FCC hydride phase in the specimen Mg–Hf–H (#h − 1) was Mg0.87 Hf0.13 , which corresponds to a nominal composition of Mg6.7 HfHx . Fig. 3 shows the XRD patterns of three specimens Mg–Hf–H (a) #h − 3, (b) #h − 1 and (c) #h − 4 which were prepared at 4 GPa and 873 K with the initial mixing molar ratios of MgH2 :HfH2 = 7:1, 6:1 and 3:1, respectively. In the specimens Mg–Hf–H (#h − 3) and (#h − 1), almost a single phase of the FCC hydride with super-lattice structure was formed, as shown in Fig. 3(a) and (b). Note that intensities of the diffraction peaks due to the FCC super-lattice structure, especially in the low diffraction-angle range, were different among the specimens. Miller indices for the reflections are denoted in Fig. 3(b). Compared to the specimen Mg–Hf–H (#h − 1), the specimen Mg–Hf–H (#h − 3) exhibits weaker super-lattice-structure peaks such as 111, 200, 220 and 311, as shown in Fig. 3(a) and (b). A structural simulation for the FCC super-lattice structure (i.e. the ideal Mg7 HfHx ) by using LAZY PULVERIX program
In order to evaluate hydrogen-desorption properties of the FCC Mg–Zr and Mg–Hf hydrides, the specimens Mg–Zr–H (#z − 2) and Mg–Hf–H (#h − 4) were selected as the representative for TPD measurements. The reagent of MgH2 used in this study was also subjected to TPD. Fig. 4(a) shows that the onset hydrogen-desorption temperature of MgH2 was 713 K.
Fig. 4. TPD spectra of H2 released from (a) MgH2 , (b) the specimen Mg–Hf–H (#h − 4) and (c) the specimen Mg–Zr–H (#z − 2). Arrows indicate temperatures focused in the following XRD investigation (Figs. 5 and 6).
D. Kyoi et al. / Journal of Alloys and Compounds 463 (2008) 311–316
This temperature is in good agreement with the one reported by Fern´andez et al. [19] using the same heating rate (10 K/min) under vacuum. For the specimen Mg–Hf–H (#h − 4), H2 releasing peaks are detected in the two temperature ranges, as shown in Fig. 4(b). The larger peak is at the onset temperature of 583 K, while the smaller one is at relatively high temperature over 833 K. The specimen Mg–Zr–H (#z − 2) also exhibited two H2 releasing peaks, as shown in Fig. 4(c). The peak with the onset temperature of 543 K is dominant, whereas the broad peak at 793 K is very weak. It is noteworthy that the onset temperature of the principal hydrogen-desorption for Mg–Zr–H and Mg–Hf–H are significantly low, as compared to that for MgH2 . Fig. 5 explains phase transition in the specimen Mg–Hf–H (#h − 4) before/after the hydrogen-desorption. A sample of the specimen Mg–Hf–H (#h − 4) was heated up to 673 K, just after the large H2 releasing peak, then cooled down and subjected to the XRD measurement. The as-prepared specimen Mg–Hf–H (#h − 4) was mainly composed of the two Mg–Hf–H phases: (1) the FCC hydride with the super-lattice structure, (2) monoclinic compound, as shown in Fig. 5(a). When the sample was heated up to 673 K, the FCC phase with the super-lattice structure has disappeared and the monoclinic phase has remained, as shown in Fig. 5(b). This reveals that the dominant hydrogen-desorption at 583 K in Fig. 4(b) would be ascribed to the decomposition of the FCC Mg–Hf hydride phase. Fig. 5(b) also shows that several phases have appeared in the heated samples. Diffraction peaks marked with solid triangles were indexed as a tetragonal lattice with cell dimensions similar to those of HfH1.98 (JCPDS #280443). Broad diffraction peaks marked with double circles could be indexed on an FCC lattice. Compared with the original FCC Mg–Hf hydride phase, this phase would have a small FCC lattice without super-lattice structure. These results indicate that the FCC Mg–Hf hydride would decompose into another FCC phase and HfH1.98 after releasing hydrogen. The H2 releasing peak over 833 K in Fig. 4(b) would be ascribed to the decomposition of HfH1.98 and/or the Mg–Hf–H monoclinic compound. Fig. 6 describes phase transition in the specimen Mg–Zr–H (#z − 2) under the hydrogen-desorption process. A sample of the specimen Mg–Zr–H (#z − 2) was heated up to 623 K, just after the dominant H2 releasing peak, then cooled down and sub-
315
Fig. 6. XRD patterns of the specimen Mg–Zr–H (#z − 2): (a) as-prepared and (b) annealed at 623 K.
jected to XRD. The as-prepared specimen Mg–Zr–H (#z − 2) contained the FCC hydride phase with several impurity phases such as MgO, as shown in Fig. 6(a). When the sample was heated up to 623 K, the original FCC hydride phase has disappeared, as shown in Fig. 6(b). This reveals that the dominant hydrogen-desorption at 543 K in Fig. 4(c) would be ascribed to the decomposition of the FCC Mg–Zr hydride phase. Besides the impurity phases, two FCC phases are confirmed in Fig. 6(b). Sharp diffraction peaks (marked with solid triangles) were indexed with a cell dimension of a = 0.477(1) nm, which corresponds to that of cubic ZrH1.66 (JCPDS #34-0649). Broad diffraction peaks (marked with double circles) were also indexed with a cell dimension of a = 0.4625(3) nm. The cell dimension of the unknown phase is smaller than that of the original FCC Mg–Zr hydride (a = 0.4868(3) nm) or ZrH1.66 . It is suggested that this unknown phase would be a Mg-based compound because no Mg containing phase has newly appeared in the heated specimen. Further investigation is required to clarify this suggestion. These results, thus, indicate that the FCC Mg–Zr hydride would decompose into another FCC phase and ZrH1.66 after releasing hydrogen. The weak hydrogen-desorption at 793 K in Fig. 4(c) would be also ascribed to the dehydrogenation of ZrH1.66 . From the peak area on the TPD spectra, amounts of hydrogen released from the FCC Mg–Zr and Mg–Hf hydride phases were evaluated to be 4.2 and 3.0 mass%, respectively. The hydrogendesorption temperatures of these FCC hydrides were lower by about 130–170 K than that of MgH2 . As indicated in the XRD results (Figs. 5 and 6), the Mg–Zr and Mg–Hf hydrides are considered to keep their FCC lattice after the hydrogen-desorption. 4. Conclusion
Fig. 5. XRD patterns of the specimen Mg–Hf–H (#h − 4): (a) as-prepared and (b) annealed at 673 K.
In this paper, the preparation conditions for the FCC Mg–Zr and Mg–Hf hydrides by means of the ultra-high pressure synthesis method and their dehydrogenation properties have been investigated The FCC hydrides were obtained by heating the mixture of MgH2 with ZrH2 or HfH2 under the hydrogen pressure of 4–8 GPa. The metal–atom arrangement of the FCC Mg–Hf hydride would depend on its metal–atom composition,
316
D. Kyoi et al. / Journal of Alloys and Compounds 463 (2008) 311–316
that is, the higher Mg content would lead the similar disordering of Mg and Hf as observed for the Mg–Zr FCC hydride which has no super-lattice structure. In both Mg–Zr–H and Mg–Hf–H systems, another compound similar to monoclinic MgNb2 Hy was formed at 4 GPa. The TPD analysis showed that the FCC Mg–Zr and Mg–Hf hydrides release hydrogen of ca. 4.2 and 3.0 mass%, respectively. The hydrogen-desorption temperatures of these hydrides were by 130–170 K lower than that of MgH2 at a heating rate of 10 K/min under vacuum. The Mg–Zr and Mg–Hf hydrides could keep their FCC metal–atom lattice after releasing hydrogen. From this result, it is expected that such FCC hydrides may have reversibility on hydrogen absorption/desorption. For a series of the Mg–TM FCC hydrides, further investigation on the reversibility (under hydrogen atmosphere) is in progress. Acknowledgement This work was supported by the New Energy and Industrial Technology Development Organization (NEDO), “Development for Safe Utilization and Infrastructure of Hydrogen”. We would appreciate to Professor Dr. Dag Nor´eus and Dr. Toyoto Sato of Stockholm University for valuable discussion on structural analysis. We also thank to Dr. Tomoaki Takasaki of AIST for his helpful advice. References [1] J.J. Reilly, R.H. Wiswall, Inorg. Chem. 7 (1968) 2254.
[2] P. Selvam, K. Yvon, Int. J. Hydrogen Energy 16 (1991) 615. [3] M.Y. Song, Int. J. Hydrogen Energy 20 (1995) 221. [4] K.J. Gross, P. Spatz, A. Zuttel, L. Schlapbach, J. Alloys Compd. 240 (1996) 206. [5] J. Huot, S. Boily, E. Akiba, R. Schulz, J. Alloys Compd. 280 (1998) 306. [6] G. Liang, J. Huot, S. Boily, A. Van Neste, R. Schulz, J. Alloys Compd. 292 (1999) 247. [7] J.F. Pelletier, J. Huot, M. Sutton, R. Schulz, A.R. Sandy, L.B. Lurio, S.G. Mochrie, Phys. Rev. B 63 (2000) 052103. [8] D. Kyoi, T. Sato, E. R¨onnebro, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Nor´eus, T. Sakai, J. Alloys Compd. 372 (2004) 213. [9] D. Kyoi, T. Sato, E. R¨onnebro, Y. Tsuji, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Nor´eus, T. Sakai, J. Alloys Compd. 375 (2004) 253. [10] D. Kyoi, N. Kitamura, H. Tanaka, A. Ueda, S. Tanase, T. Sakai, J. Alloys Compd. 428 (2007) 268. [11] D. Kyoi, T. Sakai, N. Kitamura, H. Tanaka, A. Ueda, S. Tanase, J. Alloys Compd. 463 (2008) 306. [12] E. R¨onnebro, D. Kyoi, A. Kitano, Y. Kitano, T. Sakai, J. Alloys Compd. 404–406 (2005) 68. [13] T. Sato, D. Kyoi, E. R¨onnebro, N. Kitamura, T. Sakai, D. Nor´eus, J. Alloys Compd. 417 (2006) 230. [14] Y. Goto, T. Hayashi, R. Kataoka, H. Kakuta, A. Kamegawa, H. Takamura, M. Okada, Mater. Trans. 46 (2005) 1798. [15] L. Eriksson, Stockholm University, Personal communication. [16] P.-E. Werner, L. Eriksson, M. Westdahl, J. Appl. Crystallogr. 18 (1985) 367. [17] P.-E. Werner, Ark. Kemi 31 (1969) 513. [18] K. Yvon, W. Jeitschko, E. Parthe, LAZY PULVERIX, a program to calculate theoretical X-ray and neutron diffraction powder patterns, 1977. [19] J.F. Fernandez, C.R. S´anchez, J. Alloys Compd. 356–357 (2003) 348.
Synthesis of FCC Mg–Zr and Mg–Hf hydrides using GPa hydrogen pressure method and their hydrogen-desorption properties Daisuke Kyoi, Tetsuo Sakai ∗ , Naoyuki Kitamura, Atsushi Ueda, Shigeo Tanase Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Received 20 March 2007; received in revised form 30 August 2007; accepted 2 September 2007 Available online 6 September 2007
Abstract Magnesium-based hydrides with Zr or Hf have been synthesized by means of an ultra-high pressure technique. Powder mixtures of MgH2 and ZrH2 or HfH2 have been heated up to 873 K under 4–8 GPa in a multi-anvil cell. New ternary phases (Mg–Zr–H and Mg–Hf–H) with a face centered cubic (FCC) structure have been formed. In the Mg–Zr hydride, the FCC phase with disordered metal–atom occupancy was observed, while a Ca7 Ge-type super-lattice structure was observed in the Mg–Hf hydride. By the temperature programmed desorption (TPD) measurements, these new hydrides exhibit the hydrogen-desorption at around 543–583 K, which were 130–70 K lower than that of MgH2 at a heating rate of 10 K/min under vacuum. Desorbed hydrogen contents were estimated to be 4.2 and 3.0 mass% for Mg–Zr and Mg–Hf hydrides, respectively. © 2007 Elsevier B.V. All rights reserved. Keywords: High pressure; Hydrogen; Metal hydride; Magnesium; Zirconium; Hafnium
1. Introduction Magnesium hydride (MgH2 ) is a promising hydrogen storage material because of its high hydrogen content of 7.6 mass%. However, its hydrogenation and dehydrogenation kinetics is too slow, and its desorption temperatures above 700 K is too high for wide practical application. The thermodynamic stability should be improved without reducing the high hydrogen storage capacity. In order to surpass these drawbacks, various attempts were carried out to synthesize Mg-based alloys with transition metals (TMs). In a Mg2 NiH4 , notable improvement of absorption kinetics was reported, although the hydrogen storage capacity was reduced to 3.6 wt% [1]. New hydrides, such as Mg2 CoH5 and Mg2 FeH6 were prepared [2]. But, these hydrides decomposed into Mg and Fe or Co after dehydrogenation. In 1990s, ballmilling MgH2 with TMs has been attempted and consequently led to better performance [3–7]. In these cases, the added metals are assumed to work as catalysts. Nevertheless, their desorption temperature are still too high for practical use.
∗
Corresponding author. Tel.: +81 72 751 9611; fax: +81 72 751 9623. E-mail address: [email protected] (T. Sakai).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.09.004
In order to create new Mg–TM hydrides with lower desorption temperature for many kinds of TMs, we have tried to use a high-pressure technique. Using hydrogen pressure up to 8 GPa, we have succeeded to prepare a series of new ternary hydrides “Mg7 TMHx ” (TM = Ti, V, Nb and Ta) with the Ca7 Ge-type FCC super-lattice structure [8–11]. Detailed crystal structure analysis was performed for TM = Ti [12] and Nb [13]. This indicates that one of the Mg sites (4b) in the Mg7 TMHx structure was almost filled for TM = Ti, while only the half is occupied for TM = Nb. It was revealed that hydrogen atoms occupy tetrahedral interstitial sites (32f), providing estimated unit formulas of Mg7 TiH16 (6.9 mass% H) [12] and Mg6.5 NbH14 (5.7 mass% H) [13]. An average Mg–H bonding distance in each FCC hydride is longer than that of MgH2 , explaining the observed lower dehydrogenation temperature. It was reported for TM = Zr that the new hydride phase Mg2 Zr3 Hy with a monoclinic structure was prepared under 2–5 GPa and 1173 K [14]. In this study, we have succeeded to prepare new ternary hydrides Mg6 ZrHx and Mg6.7 HfHx with FCC unit cells by reacting MgH2 with ZrH2 or HfH2 at 873 K in the hydrogen-pressure range of 4–8 GPa. Detailed preparation condition and hydrogendesorption properties for the FCC hydrides will be discussed.
312
D. Kyoi et al. / Journal of Alloys and Compounds 463 (2008) 311–316
Table 1 Preparation conditions and phase components for the Mg–Zr–H specimens Specimen
#z − 1 #z − 2 #z − 3 *
Conditions
Phase component
Mixing molar ratio MgH2 :ZrH2
Pressure (GPa)
Temperature (K)
Duration (h)
6:1 6:1 9:1
4 8 8
873 873 873
1 1 1
Monoclinic* , ␣-MgH2 , ␥-MgH2 , MgO FCC* , MgO FCC* , MgO
The dominant phase.
Table 2 Preparation conditions and phase components for the Mg–Hf–H specimens Specimen
#h − 1 #h − 2 #h − 3 #h − 4 *
Conditions
Phase component
Mixing molar ratio MgH2 :HfH2
Pressure (GPa)
Temperature (K)
Duration (h)
6:1 6:1 7:1 3:1
4 8 4 4
873 873 873 873
1 1 1 1
FCC* , MgO FCC* , MgO FCC* , MgO FCC* , monoclinic, MgO
The dominant phase.
2. Experimental procedures Commercially available powder of MgH2 (Aldrich Chemical Co.; purity 90%) was used as a starting material. Both ZrH2 and HfH2 were prepared by hydrogenation of powdery metals: Zr (Rare Metallic Co. Ltd.; purity 99%; −100 mesh) and Hf (Aldrich Chemical Co.; purity 99.5%; −325 mesh), respectively. X-ray powder diffraction (XRD) patterns for the dihydrides prepared were consistent with the data of tetragonal ZrH2 (JCPDS #17-0314) and tetragonal HfH1.98 (JCPDS #28-0443). Material handling was done in a glove box filled with dry Ar gas. Powder mixtures of MgH2 with ZrH2 or HfH2 were pressed into pellets. Mixing molar ratios were MgH2 :ZrH2 = 9:1 and 6:1 for Mg–Zr–H, and MgH2 :HfH2 = 7:1, 6:1 and 3:1 for Mg–Hf–H, respectively. As internal hydrogen sources NaBH4 (Aldrich Chemical Co.; purity 98%) and Ca(OH)2 (Wako Pure Chemical Industries, Ltd., purity 99.9%) were used. The sample pellet composed of MgH2 and ZrH2 or HfH2 was sandwiched between BN thin discs, and put between the pellets of the hydrogen source, and then sealed in a NaCl capsule. The NaCl capsule was inserted in a graphite heater tube, which was placed at the center of a set of pyrophyllite octahedral cell. Details of this setup are illustrated in our previous papers [12]. The octahedral cell was compressed up to a desired pressure (4–8 GPa) in 6–8 multi anvils by use of an ultra-high pressure generating apparatus (Sumitomo Heavy Industries Ltd., UHP-2000). The sample mixture was heated up to 873 K and the temperature was held during 1 h at the settled pressure. Phase component in the resultant specimen was charactarized on the basis of XRD data obtained with a Bruker aXS M06XCE powder diffractometer. After
correcting the XRD data by use of a program Pfilm [15], the diffraction patterns were indexed by using programs TREOR97 [16] and PIRUM [17]. Elemental analysis of each phase composition was done with a JEOL JXA-8800RL electron probe micro analyzer (EPMA), of which results were based on the Mg (K), Zr (L) and Hf (M) lines in the spectra. Hydrogen-desorption properties were evaluated by use of a temperature programmed desorption (TPD) gas analysis system (TPD-1-AT manufactured by BEL Japan Co., Ltd.) with a heating rate of 10 K/min under a vacuum of 10−2 Pa. Hydrogen gas released from hydride phases in the specimen was detected from the TPD spectrum for molecular weight = 2.
3. Results and discussions 3.1. High-pressure synthesis Preparation conditions and phase components for specimens obtained in this work are summarized in Tables 1 and 2. The dominant phase is marked with asterisk (*) for each of the specimens. Analyzed cell dimensions of ternary phases with the FCC or monoclinic structure formed in each of the specimens are also summarized in Table 3. In the following sections, we will demonstrate occurrence of the Mg–Zr–H and Mg–Hf–H ternary phases in accordance with the preparation condition.
Table 3 Cell parameters of the Mg–TM hydrides (TM = Zr, Hf and Nb) FCCa
Monoclinicb
a (nm)
a (nm)
b (nm)
c (nm)
β (◦ )
Mg–Zr–H (#z − 1) Mg–Zr–H (#z − 2) Mg–Zr–H (#z − 3)
– 0.4870(1) 0.4868(3)
0.5821(3) – –
0.3350(2) – –
0.8607(5) – –
103.10(4) – –
Mg–Hf–H (#h − 1) Mg–Hf–H (#h − 2) Mg–Hf–H (#h − 3) Mg–Hf–H (#h − 4)
0.9711(1) 0.9717(3) 0.9717(3) 0.9707(1)
– – – 0.5994(3)
– – – 0.3274(3)
– – – 0.8532(4)
– – – 103.87(4)
MgNb2 Hy [10]
–
0.5685(4)
0.32914(6)
0.7924(2)
103.82(3)
Specimen
a b
¯ (No. 225), Z = 4 Space group:Fm3m Space group: C2/m (No. 12), Z = 2
D. Kyoi et al. / Journal of Alloys and Compounds 463 (2008) 311–316
313
diffraction patterns. This is interpreted that the observed “simple” FCC phase results from the disordering of Mg and Zr. Since the atomic radius of zirconium is close to that of magnesium, the Mg and Zr atoms tend to be replaced randomly. A chemical composition analysis of the FCC phase in the specimen Mg–Zr–H (#z − 2) was carried out by EPMA. The metal–atom composition was determined to be Mg0.82 Zr0.18 , which corresponds to the composition Mg4.5 ZrHx . 3.1.2. Mg–Hf–H system Fig. 2 shows the XRD patterns of the original mixture composed of MgH2 :HfH2 = 6:1, the specimens Mg–Hf–H (#h − 1) and Mg–Hf–H (#h − 2). In both specimens, the FCC hydride phase with super-lattice structure was formed, while the both starting materials MgH2 and HfH2 completely disappeared. The diffraction peaks of the FCC hydride phase (marked with solid circles) were indexed with cell dimensions of a = 0.9711(1) nm for the specimen Mg–Hf–H (#h − 1), and a = 0.9717(3) nm for the specimen Mg–Hf–H (#h − 2), respectively. The cell volume of the FCC phase was almost the same between the prepared specimens. The specimen Mg–Hf–H (#h − 2), however, showed broader diffraction peaks than the specimen Mg–Hf–H (#h − 1),
Fig. 1. XRD patterns for Mg–Zr–H: (a) the original mixture (MgH2 :ZrH2 = 6:1), (b) the specimen (#z − 1), (c) the specimen (#z − 2) and (d) the specimen (#z − 3).
3.1.1. Mg–Zr–H system Fig. 1 shows the XRD patterns of the original mixture composed of MgH2 :ZrH2 = 6:1, the specimens Mg–Zr–H (#z − 1), Mg–Zr–H (#z − 2) and Mg–Zr–H (#z − 3). As shown in Fig. 1(b), unreacted ␣-MgH2 and ␥-MgH2 , and an impurity of MgO were found in the specimen Mg–Zr–H (#z − 1). Substantial diffraction peaks (marked with open inverted triangles) were indexed as a monoclinic structure with cell dimensions of a = 0.5821(3) nm, b = 0.3350(2) nm, c = 0.8607(5) nm, β = 103.10(4)◦ . The cell dimensions are close to those of MgNb2 Hy [10,13], which indicate that a similar monoclinic hydride phase would be formed in the specimen Mg–Zr–H (#z − 1). As shown in Fig. 1(c) and (d), the FCC hydride phase appeared in the specimens Mg–Zr–H (#z − 2) and Mg–Zr–H (#z − 3) which were prepared at 8 GPa. Both specimens also contained a small amount of MgH2 and MgO, where the monoclinic phase was not formed. The diffraction peaks for the FCC phase (marked with solid circles) were indexed with cell dimensions of a = 0.4870(1) nm for the specimen Mg–Zr–H (#z − 2), and a = 0.4868(3) nm for the specimen Mg–Zr–H (#z − 3), respectively. No super-lattice structure peaks were observed on both
Fig. 2. XRD patterns for Mg–Hf–H (MgH2 :HfH2 = 6:1): (a) the original mixture, (b) the specimens (#h − 1) and (c) the specimen Mg–Hf–H (#h − 2).
314
D. Kyoi et al. / Journal of Alloys and Compounds 463 (2008) 311–316
[18] reveals that reducing occupancy of Hf or increasing Mg at the 4a atomic site leads to weak super-lattice-structure peaks. This simulation also implies that the super-lattice structure in the Mg–Hf FCC hydride would tend to disappear with increase of the Mg content. Actually, no excess Mg or MgH2 phases were detected in Fig. 3(a) even though the specimen Mg–Hf–H (#h − 3) had higher Mg content than the specimens Mg–Hf–H (#h − 1). The FCC phase in the specimen Mg–Hf–H (#h − 3) is, therefore, considered to have a higher Mg content than that in the specimen Mg–Hf–H (#h − 1). Being focused on the specimen Mg–Hf–H (#h − 4), the FCC super-lattice structure peaks are very small, as can be seen in Fig. 3(c). From the above structural simulation, it is guessed that the FCC phase in the specimen Mg–Hf–H (#h − 4) would have very high Mg content. Nonetheless, the initial Mg content (MgH2 :HfH2 = 3:1) was the lowest among the specimens. Note that another phase with a monoclinic structure was formed in the specimen Mg–Hf–H (#h − 4), as shown in Fig. 3(c). The corresponding diffraction peaks (marked with open inverted triangles) were indexed with cell dimensions of a = 0.5994(3) nm, b = 0.3274(3) nm, c = 0.8532(4) nm, β = 103.87(4)◦ similar to those of MgNb2 Hy [10,13]. For the specimen Mg–Hf–H (#h − 4), it is considered that the starting material of HfH2 was spent to form not only the FCC hydride but also the monoclinic compound, consequently the FCC hydride would have relatively high Mg content. These results indicate that the Mg–Hf FCC hydrides would have various metal–atom compositions in dependence on the preparation condition. Also, the Mg–Hf FCC hydride could have metal–atom arrangements with/without super-lattice structure, i.e. the Ca7 Ge-type or Mg–Zr hydride-type structure. 3.2. Hydrogen-desorption properties Fig. 3. XRD patterns for Mg–Hf–H: (a) the specimen (#h − 3), (b) the specimen (#h − 1) and (c) the specimen (#h − 4).
indicating that the crystal lattice of the FCC hydride phase would be distorted at the higher pressure. No other phase except for a small amount of MgO phase was confirmed from XRD in Fig. 2(b) and (c). According to the EPMA analysis, the metal atomic ratio of the FCC hydride phase in the specimen Mg–Hf–H (#h − 1) was Mg0.87 Hf0.13 , which corresponds to a nominal composition of Mg6.7 HfHx . Fig. 3 shows the XRD patterns of three specimens Mg–Hf–H (a) #h − 3, (b) #h − 1 and (c) #h − 4 which were prepared at 4 GPa and 873 K with the initial mixing molar ratios of MgH2 :HfH2 = 7:1, 6:1 and 3:1, respectively. In the specimens Mg–Hf–H (#h − 3) and (#h − 1), almost a single phase of the FCC hydride with super-lattice structure was formed, as shown in Fig. 3(a) and (b). Note that intensities of the diffraction peaks due to the FCC super-lattice structure, especially in the low diffraction-angle range, were different among the specimens. Miller indices for the reflections are denoted in Fig. 3(b). Compared to the specimen Mg–Hf–H (#h − 1), the specimen Mg–Hf–H (#h − 3) exhibits weaker super-lattice-structure peaks such as 111, 200, 220 and 311, as shown in Fig. 3(a) and (b). A structural simulation for the FCC super-lattice structure (i.e. the ideal Mg7 HfHx ) by using LAZY PULVERIX program
In order to evaluate hydrogen-desorption properties of the FCC Mg–Zr and Mg–Hf hydrides, the specimens Mg–Zr–H (#z − 2) and Mg–Hf–H (#h − 4) were selected as the representative for TPD measurements. The reagent of MgH2 used in this study was also subjected to TPD. Fig. 4(a) shows that the onset hydrogen-desorption temperature of MgH2 was 713 K.
Fig. 4. TPD spectra of H2 released from (a) MgH2 , (b) the specimen Mg–Hf–H (#h − 4) and (c) the specimen Mg–Zr–H (#z − 2). Arrows indicate temperatures focused in the following XRD investigation (Figs. 5 and 6).
D. Kyoi et al. / Journal of Alloys and Compounds 463 (2008) 311–316
This temperature is in good agreement with the one reported by Fern´andez et al. [19] using the same heating rate (10 K/min) under vacuum. For the specimen Mg–Hf–H (#h − 4), H2 releasing peaks are detected in the two temperature ranges, as shown in Fig. 4(b). The larger peak is at the onset temperature of 583 K, while the smaller one is at relatively high temperature over 833 K. The specimen Mg–Zr–H (#z − 2) also exhibited two H2 releasing peaks, as shown in Fig. 4(c). The peak with the onset temperature of 543 K is dominant, whereas the broad peak at 793 K is very weak. It is noteworthy that the onset temperature of the principal hydrogen-desorption for Mg–Zr–H and Mg–Hf–H are significantly low, as compared to that for MgH2 . Fig. 5 explains phase transition in the specimen Mg–Hf–H (#h − 4) before/after the hydrogen-desorption. A sample of the specimen Mg–Hf–H (#h − 4) was heated up to 673 K, just after the large H2 releasing peak, then cooled down and subjected to the XRD measurement. The as-prepared specimen Mg–Hf–H (#h − 4) was mainly composed of the two Mg–Hf–H phases: (1) the FCC hydride with the super-lattice structure, (2) monoclinic compound, as shown in Fig. 5(a). When the sample was heated up to 673 K, the FCC phase with the super-lattice structure has disappeared and the monoclinic phase has remained, as shown in Fig. 5(b). This reveals that the dominant hydrogen-desorption at 583 K in Fig. 4(b) would be ascribed to the decomposition of the FCC Mg–Hf hydride phase. Fig. 5(b) also shows that several phases have appeared in the heated samples. Diffraction peaks marked with solid triangles were indexed as a tetragonal lattice with cell dimensions similar to those of HfH1.98 (JCPDS #280443). Broad diffraction peaks marked with double circles could be indexed on an FCC lattice. Compared with the original FCC Mg–Hf hydride phase, this phase would have a small FCC lattice without super-lattice structure. These results indicate that the FCC Mg–Hf hydride would decompose into another FCC phase and HfH1.98 after releasing hydrogen. The H2 releasing peak over 833 K in Fig. 4(b) would be ascribed to the decomposition of HfH1.98 and/or the Mg–Hf–H monoclinic compound. Fig. 6 describes phase transition in the specimen Mg–Zr–H (#z − 2) under the hydrogen-desorption process. A sample of the specimen Mg–Zr–H (#z − 2) was heated up to 623 K, just after the dominant H2 releasing peak, then cooled down and sub-
315
Fig. 6. XRD patterns of the specimen Mg–Zr–H (#z − 2): (a) as-prepared and (b) annealed at 623 K.
jected to XRD. The as-prepared specimen Mg–Zr–H (#z − 2) contained the FCC hydride phase with several impurity phases such as MgO, as shown in Fig. 6(a). When the sample was heated up to 623 K, the original FCC hydride phase has disappeared, as shown in Fig. 6(b). This reveals that the dominant hydrogen-desorption at 543 K in Fig. 4(c) would be ascribed to the decomposition of the FCC Mg–Zr hydride phase. Besides the impurity phases, two FCC phases are confirmed in Fig. 6(b). Sharp diffraction peaks (marked with solid triangles) were indexed with a cell dimension of a = 0.477(1) nm, which corresponds to that of cubic ZrH1.66 (JCPDS #34-0649). Broad diffraction peaks (marked with double circles) were also indexed with a cell dimension of a = 0.4625(3) nm. The cell dimension of the unknown phase is smaller than that of the original FCC Mg–Zr hydride (a = 0.4868(3) nm) or ZrH1.66 . It is suggested that this unknown phase would be a Mg-based compound because no Mg containing phase has newly appeared in the heated specimen. Further investigation is required to clarify this suggestion. These results, thus, indicate that the FCC Mg–Zr hydride would decompose into another FCC phase and ZrH1.66 after releasing hydrogen. The weak hydrogen-desorption at 793 K in Fig. 4(c) would be also ascribed to the dehydrogenation of ZrH1.66 . From the peak area on the TPD spectra, amounts of hydrogen released from the FCC Mg–Zr and Mg–Hf hydride phases were evaluated to be 4.2 and 3.0 mass%, respectively. The hydrogendesorption temperatures of these FCC hydrides were lower by about 130–170 K than that of MgH2 . As indicated in the XRD results (Figs. 5 and 6), the Mg–Zr and Mg–Hf hydrides are considered to keep their FCC lattice after the hydrogen-desorption. 4. Conclusion
Fig. 5. XRD patterns of the specimen Mg–Hf–H (#h − 4): (a) as-prepared and (b) annealed at 673 K.
In this paper, the preparation conditions for the FCC Mg–Zr and Mg–Hf hydrides by means of the ultra-high pressure synthesis method and their dehydrogenation properties have been investigated The FCC hydrides were obtained by heating the mixture of MgH2 with ZrH2 or HfH2 under the hydrogen pressure of 4–8 GPa. The metal–atom arrangement of the FCC Mg–Hf hydride would depend on its metal–atom composition,
316
D. Kyoi et al. / Journal of Alloys and Compounds 463 (2008) 311–316
that is, the higher Mg content would lead the similar disordering of Mg and Hf as observed for the Mg–Zr FCC hydride which has no super-lattice structure. In both Mg–Zr–H and Mg–Hf–H systems, another compound similar to monoclinic MgNb2 Hy was formed at 4 GPa. The TPD analysis showed that the FCC Mg–Zr and Mg–Hf hydrides release hydrogen of ca. 4.2 and 3.0 mass%, respectively. The hydrogen-desorption temperatures of these hydrides were by 130–170 K lower than that of MgH2 at a heating rate of 10 K/min under vacuum. The Mg–Zr and Mg–Hf hydrides could keep their FCC metal–atom lattice after releasing hydrogen. From this result, it is expected that such FCC hydrides may have reversibility on hydrogen absorption/desorption. For a series of the Mg–TM FCC hydrides, further investigation on the reversibility (under hydrogen atmosphere) is in progress. Acknowledgement This work was supported by the New Energy and Industrial Technology Development Organization (NEDO), “Development for Safe Utilization and Infrastructure of Hydrogen”. We would appreciate to Professor Dr. Dag Nor´eus and Dr. Toyoto Sato of Stockholm University for valuable discussion on structural analysis. We also thank to Dr. Tomoaki Takasaki of AIST for his helpful advice. References [1] J.J. Reilly, R.H. Wiswall, Inorg. Chem. 7 (1968) 2254.
[2] P. Selvam, K. Yvon, Int. J. Hydrogen Energy 16 (1991) 615. [3] M.Y. Song, Int. J. Hydrogen Energy 20 (1995) 221. [4] K.J. Gross, P. Spatz, A. Zuttel, L. Schlapbach, J. Alloys Compd. 240 (1996) 206. [5] J. Huot, S. Boily, E. Akiba, R. Schulz, J. Alloys Compd. 280 (1998) 306. [6] G. Liang, J. Huot, S. Boily, A. Van Neste, R. Schulz, J. Alloys Compd. 292 (1999) 247. [7] J.F. Pelletier, J. Huot, M. Sutton, R. Schulz, A.R. Sandy, L.B. Lurio, S.G. Mochrie, Phys. Rev. B 63 (2000) 052103. [8] D. Kyoi, T. Sato, E. R¨onnebro, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Nor´eus, T. Sakai, J. Alloys Compd. 372 (2004) 213. [9] D. Kyoi, T. Sato, E. R¨onnebro, Y. Tsuji, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Nor´eus, T. Sakai, J. Alloys Compd. 375 (2004) 253. [10] D. Kyoi, N. Kitamura, H. Tanaka, A. Ueda, S. Tanase, T. Sakai, J. Alloys Compd. 428 (2007) 268. [11] D. Kyoi, T. Sakai, N. Kitamura, H. Tanaka, A. Ueda, S. Tanase, J. Alloys Compd. 463 (2008) 306. [12] E. R¨onnebro, D. Kyoi, A. Kitano, Y. Kitano, T. Sakai, J. Alloys Compd. 404–406 (2005) 68. [13] T. Sato, D. Kyoi, E. R¨onnebro, N. Kitamura, T. Sakai, D. Nor´eus, J. Alloys Compd. 417 (2006) 230. [14] Y. Goto, T. Hayashi, R. Kataoka, H. Kakuta, A. Kamegawa, H. Takamura, M. Okada, Mater. Trans. 46 (2005) 1798. [15] L. Eriksson, Stockholm University, Personal communication. [16] P.-E. Werner, L. Eriksson, M. Westdahl, J. Appl. Crystallogr. 18 (1985) 367. [17] P.-E. Werner, Ark. Kemi 31 (1969) 513. [18] K. Yvon, W. Jeitschko, E. Parthe, LAZY PULVERIX, a program to calculate theoretical X-ray and neutron diffraction powder patterns, 1977. [19] J.F. Fernandez, C.R. S´anchez, J. Alloys Compd. 356–357 (2003) 348.