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Synthesis and structural study of Ti-rich Mg–Ti hydrides
Journal of Alloys and Compounds 593 (2014) 132–136
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Letter
Synthesis and structural study of Ti-rich Mg–Ti hydrides Kohta Asano a,⇑, Hyunjeong Kim a, Kouji Sakaki a, Katharine Page b, Shigenobu Hayashi a, Yumiko Nakamura a, Etsuo Akiba c a
National Institute of Advanced Industrial Science and Technology (AIST), AIST Central-5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Lujan Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, NM 87545, United States c Department of Mechanical Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan b
a r t i c l e
i n f o
Article history: Received 10 December 2013 Received in revised form 8 January 2014 Accepted 8 January 2014 Available online 18 January 2014 Keywords: Hydrogen storage Magnesium–titanium hydrides Mechanical alloying Synchrotron X-ray total scattering Nuclear magnetic resonance (NMR)
a b s t r a c t MgxTi1x (x = 0.15, 0.25, 0.35) alloys were synthesized by means of ball milling. Under a hydrogen pressure of 8 MPa at 423 K these Mg–Ti alloys formed a hydride phase with a face centered cubic (FCC) structure. The hydride for x = 0.25 consisted of single Mg0.25Ti0.75H1.62 FCC phase but TiH2 and MgH2 phases were also formed in the hydrides for x = 0.15 and 0.35, respectively. X-ray diffraction patterns and the atomic pair distribution function indicated that numbers of stacking faults were introduced. There was no sign of segregation between Mg and Ti in Mg0.25Ti0.75H1.62. Electronic structure of Mg0.25Ti0.75H1.62 was different from those of MgH2 and TiH2, which was demonstrated by 1H nuclear magnetic resonance. This strongly suggested that stable Mg–Ti hydride phase was formed in the metal composition of Mg0.25Ti0.75 without disproportion into MgH2 and TiH2. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Synthesis of Mg–Ti hydrides has been often reported lately [1–21] in expectation of the high gravimetric hydrogen capacity because hydrides of MgH2 and TiH2 contain 7.6 mass% and 4.0 mass% of hydrogen, respectively. In the reported binary phase diagram of the Mg–Ti system [22] Mg and Ti metals have a hexagonal close packed (HCP) structure and they are immiscible at moderate temperatures. Solubility of each metal to the other is less than 2 at.% and intermetallic compounds are not found. Mg–Ti alloys and hydrides have been synthesized by means of the ball milling [1–11], sputtering [12–18], high pressure synthesis [19,20] and spark discharge [21] methods other than the melting method. For the last decade we have successfully synthesized MgxTi1x alloys which have a body centered cubic (BCC) structure in the region of 0.25 6 x 6 0.65, a face centered cubic (FCC) structure in the region of 0.35 6 x 6 0.80 and an HCP structure in the region of 0.65 6 x 6 0.80 by ball milling of Mg and Ti powders [5–9]. Those alloys absorbed hydrogen and formed a Mg–Ti hydride with an FCC structure and MgH2 with a body centered tetragonal (BCT) structure [6,7]. For instance, hydrogenation of Mg0.50Ti0.50 BCC alloy led to the formation of the FCC Mg–Ti–H and MgH2 phases under a hydrogen pressure of 8 MPa at 423 K [6]. Based on the phase fractions obtained from the Rietveld refinement of the ⇑ Corresponding author. Tel.: +81 29 861 4485; fax: +81 29 861 4476. E-mail address: [email protected] (K. Asano). 0925-8388/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2014.01.061
X-ray diffraction (XRD) pattern the chemical formula of the FCC hydride phase was estimated to be Mg0.42Ti0.58H1.77. Synthesis of the Mg–Ti FCC hydride phase was also tried by ball milling of MgH2 and Ti powders [8]. However, excess Mg or MgH2 remained in the milled hydrides. Those results indicated that the stability of the Ti-rich Mg–Ti hydride phase was higher than that of the MgH2 phase, which was based on the difference in the enthalpies for hydride formation; 74 kJ mol1H2 for MgH2 and 130 kJ mol1H2 for TiH2 [23]. Dam and Notten et al. have synthesized MgxTi1x thin films which have an HCP structure in the region of x P 0.50 by magnetron co-sputtering of Mg and Ti targets [12–16]. They reported that the crystal structure of MgxTi1x hydrides resembled that of the fluorite type TiH2 FCC phase in the region of x 6 0.87 and the rutile type MgH2 BCT phase in the region of x P 0.90 [13]. These two phases coexisted in the region of 0.87 6 x 6 0.90. Extended X-ray absorption fine structure (EXAFS) spectroscopy study on MgxTi1 x–H in the region of 0.53 6 x 6 0.90 showed the local chemical segregation into Mg-rich and Ti-rich nanometer-sized domains within a large coherent FCC grain [16]. Ti-rich domain seemed to be more favorable to form hydride than Mg-rich one because Ti has a more negative enthalpy for hydride formation than Mg. They have also synthesized Mg0.65Ti0.35D1.1 FCC deuteride from Mg0.65Ti0.35 HCP thin film [24]. Deuterium atoms occupied tetrahedral (T) sites and Mg-rich and Ti-rich domains were also found using 2H magic angle spinning nuclear magnetic resonance (MAS NMR). In addition, they have synthesized MgxTi1x alloys in the region of 0.65 6 x 6 0.85 by means of ball milling [10,11]. Mg0.65Ti0.35
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consisted of two FCC phases. MgD2 and TiD2 nanometer-sized domains were also observed by 2H MAS NMR from the deuterides of Mg0.65Ti0.35D0.65 and Mg0.65Ti0.35D1.12 [25,26]. Kyoi et al. have synthesized Mg7TiH12.7 (Mg0.875Ti0.125H1.59) hydride by reacting a mixture of MgH2 and TiH1.9 in an anvil cell at a pressure in the order of GPa [19]. It had the Ca7Ge type superstructure which consisted of eight (2 2 2) FCC substructures. They reported that hydrogen atoms in Mg6.9TiH13.2 (Mg0.874Ti0.126H1.67) occupied the T sites with Mg4 and Mg3Ti coordination based on the result of synchrotron X-ray diffraction [20]. Er et al. have shown the structure and stability of MgxTi1xH2 in the region of 0 6 x 6 1 obtained from the density functional theory calculations [27]. For x = 1 MgH2 had a rutile type BCT structure and MgxTi1xH2 became less stable with decreasing x. However, in the region of x 6 0.83 MgxTi1xH2 had a fluorite type FCC structure and the stability increased with decreasing x. Finally for x = 0, TiH2 was the most stable. Jensen et al. have calculated the mixing enthalpies for MgxTi1x in the region of 0.0156 6 x 6 0.9844 on both the quasi-random and the nano-cluster models [28–30]. Ti was segregated into nanometer-clusters in the Mg-rich compositions and Mg was segregated in the Ti-rich ones. Introducing hydrogen into Mg0.8125Ti0.1875 hydrogen atoms occupied octahedral (O) sites on the interface between Ti nanometer-clusters and the Mg matrix. Increasing the hydrogen content, hydrogen atoms occupied the T sites inside the Ti nanometer-clusters forming the fluorite type FCC hydride phase. Hydrogen came into the Mg matrix also with a higher hydrogen content, and as a consequent, the HCP metal phase transformed to the FCC hydride phase. As summarized above, numbers of experimental and computational studies have been reported on Mg–Ti hydrides. The composition of the stable hydride phase varies by synthesis and calculation methods. Our previous studies on synthesis of Mg–Ti hydride powder [6,8] suggested that the Ti-rich metal compositions would form a more stable hydride. In the present work, we synthesized Ti-rich Mg–Ti alloys by means of ball milling in order to obtain single Mg–Ti FCC hydride phase. The structure of the hydride was investigated using the atomic pair distribution function (PDF) analysis on synchrotron X-ray total scattering data and 1H NMR.
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time, T1, was determined using the inversion recovery pulse sequence followed by the solid echo pulse sequence ð180 s 90x s1 90y s2 echoÞ. The s value denotes the variable delay time. Those of s1 and s2 are the fixed delay times.
3. Results and discussion The XRD patterns of milled MgxTi1x (x = 0 [9], 0.15, 0.25, 0.35, 0.50 [9]) are shown in Fig. 1. Diffraction peaks for pure Ti (x = 0) [9] were significantly broadened by the twinning deformation but structure change was not observed as shown in Fig. 1. In the diffraction patterns for x = 0.15, 0.25 and 0.35 three broadened peaks were observed, which was similar to that for x = 0.50 [9]. As described in Ref. [9], during milling of Mg and Ti at a molar ratio of 1:1 Mg dissolved in Ti and the Mg0.50Ti0.50 BCC phase with the lattice parameter of a = 0.342(1) nm and the grain size of 3 nm was formed. MgxTi1x (x = 0.15, 0.25, 0.35) seem to have a BCC structure. However, the small shoulder at the lower 2h side of the strongest peak at 2h 38° and the small peak at 2h 62° were observed in those patterns. This suggests possibilities that a small fraction of another phase coexists, the BCC phase contains some local parts of Mg-rich and Ti-rich compositions, or the atomic arrangement of the main phase is not a simple BCC structure. MgxTi1x (x = 0.15, 0.25, 0.35) were hydrogenated under a hydrogen pressure of 8 MPa at 423 K. No hydrogen desorption from the hydrides was observed under 1 kPa at the same temperature. The nominal chemical formulae of the hydrides were Mg0.15Ti0.85H1.60 (3.6 mass%), Mg0.25Ti0.75H1.62 (3.9 mass%) and Mg0.35 Ti0.65H1.58 (4.0 mass%), respectively. Fig. 2 shows the XRD patterns of the three hydrides and Mg0.50Ti0.50H1.80 from Ref. [6]. The FCC phase was observed in all the patterns. Mg0.25Ti0.75H1.62 seems to consist of the single FCC hydride phase. In the diffraction patterns of Mg0.35Ti0.65H1.58 and Mg0.50Ti0.50H1.80 [6] not only the hydride phase but also the MgH2 phase were observed. Excess MgH2 was formed in the region of x P 0.35. On the other hand, the secondary FCC phase with a small fraction was observed in the diffraction pattern of Mg0.15Ti0.85H1.60. This phase is possibly Ti hydride, TiHx with x < 1.56, considering that the lattice parameter is smaller than that of TiH1.56 [39]. The crystal structure of the metal lattice of the Mg0.25Ti0.75H1.62 FCC phase was refined by the Rietveld method. The FCC phase was
2. Experimental MgxTi1x (x = 0.15, 0.25, 0.35) alloys were synthesized by ball milling of Mg and Ti powders using stainless steel balls and pots. The detailed procedures for synthesizing the alloys were described in our previous report [5–7,9]. Hydrides were synthesized by hydrogenation of the alloys under a hydrogen pressure of 8 MPa at 423 K. The hydrogen contents of the hydrides were determined by the volumetric method. Hydrogen gas of 7 N purity was used. We also purchased the powders of MgH2 with the purity of 98 mass% and TiH2 with the purity of 99 mass% from Johnson Matthey and Rare Metallic, respectively, for reference. The powder X-ray diffraction (XRD) was measured using a Rigaku 2500 V diffractometer with Cu Ka radiation. The diffraction patterns were analyzed by the Rietveld refinement program RIETAN-2000 [31]. Synchrotron X-ray total scattering experiments were carried out at the 11-ID-B beamline at the advanced photon source at argonne national laboratory. Powder specimens of hydrides were packed in kapton capillaries with a diameter of 1 mm. The powder X-ray diffraction data were collected at room temperature using the rapid acquisition PDF technique [32] with an amorphous silicon-based area detector manufactured by Perkin Elmer. The X-ray energy was 58.291 keV (k = 0.21270 Å). The PDF was obtained by using the program PDFgetX2 [33] and Qmax = 25 Å1 was used. For real space modeling program PDFgui program [34] was used. The detailed procedures for the synchrotron X-ray total scattering experiments were described in the previous report [35–37]. 1 H NMR measurements were performed by a Bruker ASX200 spectrometer with a Larmor frequency of 200.13 MHz. Powder specimens of hydrides were packed in glass tubes with a diameter of 5 mm. The NMR spectra were measured with the solid echo pulse sequence ð90x s1 90y s2 echoÞ. The 90° pulse width was 1.5 ls. The values of s1 and s2 were set at 8.0 ls and 8.8 ls, respectively. The frequency scale of spectra was expressed with respect to neat tetramethylsilane (TMS) by adjusting the signal of water to 4.877 ppm [38]. 1H spin–lattice relaxation
Fig. 1. X-ray diffraction patterns of ball milled MgxTi1x (x = (a) 0.50, (b) 0.35, (c) 0.25, (d) 0.15, and (e) 0). Data for x = 0.50 and 0 is from Ref. [9]. Lattice parameters of a BCC lattice were estimated to be a = 0.342(1) nm for x = 0.50, a = 0.339(1) nm for x = 0.35, a = 0.340(1) nm for x = 0.25 and a = 0.338(1) nm for x = 0.15, respectively.
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assumed to be a Mg0.25Ti0.75 disordered phase applied to a cubic structural model of the space group Fm-3m. Here, contribution from hydrogen to the diffraction intensity was assumed to be negligible. The refined XRD pattern is shown in Fig. 3. The calculated curve overall fitted the experimental data but observed 1 1 1 and 2 0 0 diffraction peaks were shifted to the higher and lower angles than the expected positions, respectively. The FCC phase seems to be deformed from cubic symmetry and this is probably due to formation of stacking faults. It has been reported that introduction of stacking faults and twins in FCC metals reduces the crystallite size and changes the shape and position of diffraction peaks [40]. The 1 1 1 and 2 0 0 reflections tend to get closer with increase in stacking faults on {1 1 1} planes. We have reported that the Mg–Ti BCC phase contains high density stacking faults introduced by ball milling [9]. The stacking faults formed in Mg0.25Ti0.75H1.62 were possibly introduced during synthesizing Mg0.25Ti0.75 alloy before hydrogenation. The lattice parameter of Mg0.25Ti0.75H1.62 was evaluated to be a = 0.44565(5) nm from the refinement. This value is larger than a = 0.4400(5) nm of TiH1.56 [39] and rather close to a = 0.446 nm of TiH2 [41] although the hydrogen content H/Mg0.25Ti0.75 = 1.62 is close to 1.56 of TiH1.56. This demonstrates that the observed FCC phase contains Mg which has a larger atomic radius than Ti [42]. Fig. 4 shows X-ray PDFs of Mg0.25Ti0.75H1.62, MgH2 and TiH2. PDF pattern of Mg0.25Ti0.75H1.62 is similar to that of TiH2 rather than MgH2 but the peaks were much broader and quickly decayed with increasing interatomic distance, r. This quick decay indicates that Mg0.25Ti0.75H1.62 has smaller crystallite size or a larger number of lattice defects than TiH2 and MgH2. This is probably because only Mg0.25Ti0.75H1.62 has been synthesized by ball milling followed by hydrogenation. However, the decay seems to be even smaller than that for X-ray PDF of Mg0.50Co0.50D0.75, which has been explained by a two-phase model made of Mg2CoD5 and MgCo2 [36]. It has been reported that hydrogen atoms occupy tetrahedral (T) sites in the FCC Ti–H phase [39]. The PDF of Mg0.25Ti0.75H1.62 was analyzed using a Mg0.25Ti0.75 FCC structural model in which hydrogen atoms occupy T sites with the occupancy of 0.81 that corresponds to H/Mg0.25Ti0.75 = 1.62. The refinement range was 0.15 nm 6 r 6 2 nm. The PDF refinement result is shown in Fig. 5. The model fits the experimental PDF well. The lattice parameter was refined to be a = 0.443957(5) nm. This value agrees well with a = 0.44565(5) nm evaluated by Rietveld refinement of the XRD data. The difference curve between data and calculation suggests that there is no trace of local structure other than the main FCC phase, e.g., secondary phase or local segregation of Ti or Mg. It is
Fig. 2. X-ray diffraction patterns of (a) Mg0.50Ti0.50H1.80, (b) Mg0.35Ti0.65H1.58, (c) Mg0.25Ti0.75H1.62 and (d) Mg0.15Ti0.85H1.60. Data of Mg0.50Ti0.50H1.80 is from Ref. [6].
Fig. 3. Rietveld refinement of X-ray diffraction data for Mg0.25Ti0.75H1.62. The structural model for refinement is Fm-3m (no. 225). The lattice parameter was refined to be a = 0.44565(5) nm.
Fig. 4. X-ray PDFs of (a) MgH2, (b) Mg0.25Ti0.75H1.62 and (c) TiH2.
concluded that Mg0.25Ti0.75H1.62 consists of a single FCC phase without segregation at least from the information of the local structure of metal atoms. The 1H NMR spectra of Mg0.25Ti0.75H1.62, MgH2 and TiH2 are shown in Fig. 6. Signals were significantly broadened, which indicates that hydrogen atoms hardly migrate in the hydrides at room temperature. The line width of the signal in MgH2 was smaller than that of the signals in Mg0.25Ti0.75H1.62 and TiH2. The signal shifted to the negative side with the Ti content. Stable NMR active nuclei of Mg and Ti are 25Mg, 47Ti and 49Ti. Contributions of them to the line width were considered to be negligible because their natural abundances are below 10% and their nuclear dipole moments are lower than 10% of that of 1H. This indicates that the full width at half maximum (FWHM) of the signal is determined mainly by
Fig. 5. Refinement result of X-ray PDF of Mg0.25Ti0.75H1.62. The lattice parameter was refined to be a = 0.443957(5) nm.
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the second moment between 1H spins. The bulk magnetic susceptibility also contributes to the line width. The FWHM of Mg0.25Ti0.75H1.62 was 253 ppm which was equivalent to the value of TiH2 and larger than that of MgH2, as shown in Fig. 6. This might suggest that the distance between hydrogen atoms in Mg0.25Ti0.75 H1.62 is similar to that between hydrogen atoms on T sites in TiH2. However, it is complicated to clarify from this 1H NMR data whether hydrogen atoms occupy T sites or not in Mg0.25Ti0.75H1.62 because the second moment between 1H spins on T sites is estimated to be equivalent to that on octahedral (O) sites for FCC Ti–H [43]. The 1H spin–lattice relaxation time in Mg0.25Ti0.75H1.62 was T1 = 0.229 s. This value was slightly longer than T1 = 0.176 s in TiH2 and significantly shorter than T1 1400 s in MgH2. These results suggest that the density of states at the Fermi energy, N(EF), for Mg0.25Ti0.75H1.62 is close to that for TiH2 and quite different from that for MgH2. It has been reported that N(EF) for FCC Ti–H increases with the hydrogen content in the region of 1.6 6 H/Ti 6 2.0, which results in reduction in T1 [44,45]. One possible reason for the slight difference in T1 between Mg0.25Ti0.75H1.62 and TiH2 is the difference in the hydrogen contents between the two hydrides. The N(EF) relates to the signal position which is called Knight shift especially for transition metals. The signal in MgH2 was observed at around 3.4 ppm. It has been reported that the resonance shift of MgH2 with respect to TMS is tiny, which is close to zero [46,47]. On the other hand, the signal in TiH2 had a large negative Knight shift of 120.3 ppm. The Knight shift of TiH2 has been reported to be 90 200 ppm [48]. The shift of Mg0.25Ti0.75H1.62 was 71.5 ppm, which was between those of MgH2 and TiH2. Mg0.25Ti0.75H1.62 has a different electronic structure from TiH2 and N(EF) is reduced by substitution of a part of Ti by Mg as well as reduction in the hydrogen content. This result from the 1H NMR measurements shows that the synthesized FCC hydride phase surely consists of Mg, Ti and hydrogen without segregation. According to the present work using X-ray PDF and 1H NMR, phase separation with a nanometers scale was not found in the Mg0.25Ti0.75H1.62 FCC hydride phase synthesized under a hydrogen pressure of 8 MPa at 423 K. This fact indicates successful synthesis of the single FCC hydride phase from Mg0.25Ti0.75 but the hydrogen content did not reach to H/Mg0.25Ti0.75 = 2. In contrast, Mg0.15Ti0.85 decomposed to the two FCC phases by hydrogenation under the same condition, although computational studies have estimated that the stability of the hydride phase increases with the Ti content
Fig. 6. 1H NMR spectra of (a) MgH2, (b) Mg0.25Ti0.75H1.62 and (c) TiH2.
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[27]. Neutron PDF and 2H MAS NMR for Mg–Ti deuterides are in progress to discuss the local structure in a nanometers scale from a viewpoint of distribution of Mg, Ti and hydrogen atoms in the FCC hydride phase. 4. Conclusions A single FCC phase of Mg–Ti hydride was successfully synthesized. The chemical formula was Mg0.25Ti0.75H1.62. The hydride contained numbers of stacking faults introduced by ball milling. Segregation between Mg and Ti was not found by the X-ray PDF method. 1H NMR indicated that the FCC hydride phase consisted of Mg, Ti and hydrogen from the information of the electronic structure. These results have demonstrated that stable Ti-rich Mg–Ti FCC hydride phase was formed from the non-equilibrium Mg–Ti alloy phase. Acknowledgement This work was partly supported by The New Energy and Industrial Technology Development Organization (NEDO) under ‘‘Advanced Fundamental Research on Hydrogen Storage Materials (Hydro-Star)’’. This work was also partly supported by Iketani Science and Technology Foundation. Use of the Advanced Photon Source, an Office of Science User Facility operated for the US Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the US DOE under Contract No. DE-AC02-06CH11357. References [1] C. Suryanarayana, F.H. Froes, J. Mater. Res. 5 (1990) 1880–1886. [2] M. Hida, K. Asai, Y. Takemoto, A. Sakakibara, Mater. Sci. Forum 235–238 (1997) 187–192. [3] G. Liang, R. Schulz, J. Mater. Sci. 38 (2003) 1179–1184. [4] S. Rousselot, M.-P. Bichat, D. Guay, L. Roué, J. Power Source 175 (2008) 621– 624. [5] K. Asano, H. Enoki, E. Akiba, Mater. Trans. 48 (2007) 121–126. [6] K. Asano, H. Enoki, E. Akiba, J. Alloys Comp. 478 (2009) 117–120. [7] K. Asano, H. Enoki, E. Akiba, J. Alloys Comp. 480 (2009) 558–563. [8] K. Asano, E. Akiba, J. Alloys Comp. 481 (2009) L8–L11. [9] K. Asano, H. Enoki, E. Akiba, J. Alloys Comp. 486 (2009) 115–123. [10] W.P. Kalisvaart, H.J. Wondergem, F. Bakker, P.H.L. Notten, J. Mater. Res. 22 (2007) 1640–1649. [11] W.P. Kalisvaart, P.H.L. Notten, J. Mater. Res. 23 (2008) 2179–2187. [12] P. Vermeulen, R.A.H. Niessen, P.H.L. Notten, Electrochem. Commun. 8 (2006) 27–32. [13] D.M. Borsa, R. Gremaud, A. Baldi, H. Schreuders, J.H. Rector, B. Kooi, P. Vermeulen, P.H.L. Notten, B. Dam, R. Griessen, Phys. Rev. B 75 (2007) 205408. [14] R. Gremaud, C.P. Broedersz, D.M. Borsa, A. Borgschulte, P. Mauron, H. Schreuders, J.H. Rector, B. Dam, R. Griessen, Adv. Mater. 19 (2007) 2813–2817. [15] P. Vermeulen, H.J. Wondergem, P.C.J. Graat, D.M. Borsa, H. Schreuders, B. Dam, R. Griessen, P.H.L. Notten, J. Mater. Chem. 18 (2008) 3680–3687. [16] A. Baldi, R. Gremaud, D.M. Borsa, C.P. Baldé, A.M.J. Van der Eerden, G.L. Kruijtzer, P.E. de Jongh, B. Dam, R. Griessen, Int. J. Hydrogen Energy 34 (2009) 1450–1457. [17] S. Bao, K. Tajima, Y. Yamada, M. Okada, K. Yoshimura, Appl. Phys. A 87 (2007) 621–624. [18] S. Bao, K. Tajima, Y. Yamada, M. Okada, K. Yoshimura, Sol. Energy Mater. Sol. Cells 92 (2008) 224–227. [19] D. Kyoi, T. Sato, E. Rönnebro, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Noréus, T. Sakai, J. Alloys Comp. 372 (2004) 213–217. [20] D. Moser, D.J. Bull, T. Sato, D. Noréus, D. Kyoi, T. Sakai, N. Kitamura, H. Yusa, T. Taniguchi, W.P. Kalisvaart, P.H.L. Notten, J. Mater. Chem. 19 (2009) 8150– 8161. [21] A. Anastasopol, T.V. Pfeiffer, J. Middelkoop, U. Lafont, R.J. Canales-Perez, A. Schmidt-Ott, F.M. Mulder, S.W.H. Eijt, J. Am. Chem. Soc. 135 (2013) 7891– 7900. [22] A.A. Nayeb-Hashemi, J.B. Clark, Phase Diagrams Binary Magnesium Alloys (1988) 324. [23] F.D. Manchester, Phase Diagrams Binary Hydrogen Alloys (2000). p. 83 and p. 238. [24] S. Srinivasan, P.C.M.M. Magusin, R.A. van Santen, P.H.L. Notten, H. Schreuders, B. Dam, J. Phys. Chem. C 115 (2011) 288–297. [25] S. Srinivasan, P.C.M.M. Magusin, W.P. Kalisvaart, P.H.L. Notten, F. Cuevas, M. Latroche, R.A. van Santen, Phys. Rev. B 81 (2010) 054107.
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Letter
Synthesis and structural study of Ti-rich Mg–Ti hydrides Kohta Asano a,⇑, Hyunjeong Kim a, Kouji Sakaki a, Katharine Page b, Shigenobu Hayashi a, Yumiko Nakamura a, Etsuo Akiba c a
National Institute of Advanced Industrial Science and Technology (AIST), AIST Central-5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Lujan Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, NM 87545, United States c Department of Mechanical Engineering, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan b
a r t i c l e
i n f o
Article history: Received 10 December 2013 Received in revised form 8 January 2014 Accepted 8 January 2014 Available online 18 January 2014 Keywords: Hydrogen storage Magnesium–titanium hydrides Mechanical alloying Synchrotron X-ray total scattering Nuclear magnetic resonance (NMR)
a b s t r a c t MgxTi1x (x = 0.15, 0.25, 0.35) alloys were synthesized by means of ball milling. Under a hydrogen pressure of 8 MPa at 423 K these Mg–Ti alloys formed a hydride phase with a face centered cubic (FCC) structure. The hydride for x = 0.25 consisted of single Mg0.25Ti0.75H1.62 FCC phase but TiH2 and MgH2 phases were also formed in the hydrides for x = 0.15 and 0.35, respectively. X-ray diffraction patterns and the atomic pair distribution function indicated that numbers of stacking faults were introduced. There was no sign of segregation between Mg and Ti in Mg0.25Ti0.75H1.62. Electronic structure of Mg0.25Ti0.75H1.62 was different from those of MgH2 and TiH2, which was demonstrated by 1H nuclear magnetic resonance. This strongly suggested that stable Mg–Ti hydride phase was formed in the metal composition of Mg0.25Ti0.75 without disproportion into MgH2 and TiH2. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Synthesis of Mg–Ti hydrides has been often reported lately [1–21] in expectation of the high gravimetric hydrogen capacity because hydrides of MgH2 and TiH2 contain 7.6 mass% and 4.0 mass% of hydrogen, respectively. In the reported binary phase diagram of the Mg–Ti system [22] Mg and Ti metals have a hexagonal close packed (HCP) structure and they are immiscible at moderate temperatures. Solubility of each metal to the other is less than 2 at.% and intermetallic compounds are not found. Mg–Ti alloys and hydrides have been synthesized by means of the ball milling [1–11], sputtering [12–18], high pressure synthesis [19,20] and spark discharge [21] methods other than the melting method. For the last decade we have successfully synthesized MgxTi1x alloys which have a body centered cubic (BCC) structure in the region of 0.25 6 x 6 0.65, a face centered cubic (FCC) structure in the region of 0.35 6 x 6 0.80 and an HCP structure in the region of 0.65 6 x 6 0.80 by ball milling of Mg and Ti powders [5–9]. Those alloys absorbed hydrogen and formed a Mg–Ti hydride with an FCC structure and MgH2 with a body centered tetragonal (BCT) structure [6,7]. For instance, hydrogenation of Mg0.50Ti0.50 BCC alloy led to the formation of the FCC Mg–Ti–H and MgH2 phases under a hydrogen pressure of 8 MPa at 423 K [6]. Based on the phase fractions obtained from the Rietveld refinement of the ⇑ Corresponding author. Tel.: +81 29 861 4485; fax: +81 29 861 4476. E-mail address: [email protected] (K. Asano). 0925-8388/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2014.01.061
X-ray diffraction (XRD) pattern the chemical formula of the FCC hydride phase was estimated to be Mg0.42Ti0.58H1.77. Synthesis of the Mg–Ti FCC hydride phase was also tried by ball milling of MgH2 and Ti powders [8]. However, excess Mg or MgH2 remained in the milled hydrides. Those results indicated that the stability of the Ti-rich Mg–Ti hydride phase was higher than that of the MgH2 phase, which was based on the difference in the enthalpies for hydride formation; 74 kJ mol1H2 for MgH2 and 130 kJ mol1H2 for TiH2 [23]. Dam and Notten et al. have synthesized MgxTi1x thin films which have an HCP structure in the region of x P 0.50 by magnetron co-sputtering of Mg and Ti targets [12–16]. They reported that the crystal structure of MgxTi1x hydrides resembled that of the fluorite type TiH2 FCC phase in the region of x 6 0.87 and the rutile type MgH2 BCT phase in the region of x P 0.90 [13]. These two phases coexisted in the region of 0.87 6 x 6 0.90. Extended X-ray absorption fine structure (EXAFS) spectroscopy study on MgxTi1 x–H in the region of 0.53 6 x 6 0.90 showed the local chemical segregation into Mg-rich and Ti-rich nanometer-sized domains within a large coherent FCC grain [16]. Ti-rich domain seemed to be more favorable to form hydride than Mg-rich one because Ti has a more negative enthalpy for hydride formation than Mg. They have also synthesized Mg0.65Ti0.35D1.1 FCC deuteride from Mg0.65Ti0.35 HCP thin film [24]. Deuterium atoms occupied tetrahedral (T) sites and Mg-rich and Ti-rich domains were also found using 2H magic angle spinning nuclear magnetic resonance (MAS NMR). In addition, they have synthesized MgxTi1x alloys in the region of 0.65 6 x 6 0.85 by means of ball milling [10,11]. Mg0.65Ti0.35
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consisted of two FCC phases. MgD2 and TiD2 nanometer-sized domains were also observed by 2H MAS NMR from the deuterides of Mg0.65Ti0.35D0.65 and Mg0.65Ti0.35D1.12 [25,26]. Kyoi et al. have synthesized Mg7TiH12.7 (Mg0.875Ti0.125H1.59) hydride by reacting a mixture of MgH2 and TiH1.9 in an anvil cell at a pressure in the order of GPa [19]. It had the Ca7Ge type superstructure which consisted of eight (2 2 2) FCC substructures. They reported that hydrogen atoms in Mg6.9TiH13.2 (Mg0.874Ti0.126H1.67) occupied the T sites with Mg4 and Mg3Ti coordination based on the result of synchrotron X-ray diffraction [20]. Er et al. have shown the structure and stability of MgxTi1xH2 in the region of 0 6 x 6 1 obtained from the density functional theory calculations [27]. For x = 1 MgH2 had a rutile type BCT structure and MgxTi1xH2 became less stable with decreasing x. However, in the region of x 6 0.83 MgxTi1xH2 had a fluorite type FCC structure and the stability increased with decreasing x. Finally for x = 0, TiH2 was the most stable. Jensen et al. have calculated the mixing enthalpies for MgxTi1x in the region of 0.0156 6 x 6 0.9844 on both the quasi-random and the nano-cluster models [28–30]. Ti was segregated into nanometer-clusters in the Mg-rich compositions and Mg was segregated in the Ti-rich ones. Introducing hydrogen into Mg0.8125Ti0.1875 hydrogen atoms occupied octahedral (O) sites on the interface between Ti nanometer-clusters and the Mg matrix. Increasing the hydrogen content, hydrogen atoms occupied the T sites inside the Ti nanometer-clusters forming the fluorite type FCC hydride phase. Hydrogen came into the Mg matrix also with a higher hydrogen content, and as a consequent, the HCP metal phase transformed to the FCC hydride phase. As summarized above, numbers of experimental and computational studies have been reported on Mg–Ti hydrides. The composition of the stable hydride phase varies by synthesis and calculation methods. Our previous studies on synthesis of Mg–Ti hydride powder [6,8] suggested that the Ti-rich metal compositions would form a more stable hydride. In the present work, we synthesized Ti-rich Mg–Ti alloys by means of ball milling in order to obtain single Mg–Ti FCC hydride phase. The structure of the hydride was investigated using the atomic pair distribution function (PDF) analysis on synchrotron X-ray total scattering data and 1H NMR.
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time, T1, was determined using the inversion recovery pulse sequence followed by the solid echo pulse sequence ð180 s 90x s1 90y s2 echoÞ. The s value denotes the variable delay time. Those of s1 and s2 are the fixed delay times.
3. Results and discussion The XRD patterns of milled MgxTi1x (x = 0 [9], 0.15, 0.25, 0.35, 0.50 [9]) are shown in Fig. 1. Diffraction peaks for pure Ti (x = 0) [9] were significantly broadened by the twinning deformation but structure change was not observed as shown in Fig. 1. In the diffraction patterns for x = 0.15, 0.25 and 0.35 three broadened peaks were observed, which was similar to that for x = 0.50 [9]. As described in Ref. [9], during milling of Mg and Ti at a molar ratio of 1:1 Mg dissolved in Ti and the Mg0.50Ti0.50 BCC phase with the lattice parameter of a = 0.342(1) nm and the grain size of 3 nm was formed. MgxTi1x (x = 0.15, 0.25, 0.35) seem to have a BCC structure. However, the small shoulder at the lower 2h side of the strongest peak at 2h 38° and the small peak at 2h 62° were observed in those patterns. This suggests possibilities that a small fraction of another phase coexists, the BCC phase contains some local parts of Mg-rich and Ti-rich compositions, or the atomic arrangement of the main phase is not a simple BCC structure. MgxTi1x (x = 0.15, 0.25, 0.35) were hydrogenated under a hydrogen pressure of 8 MPa at 423 K. No hydrogen desorption from the hydrides was observed under 1 kPa at the same temperature. The nominal chemical formulae of the hydrides were Mg0.15Ti0.85H1.60 (3.6 mass%), Mg0.25Ti0.75H1.62 (3.9 mass%) and Mg0.35 Ti0.65H1.58 (4.0 mass%), respectively. Fig. 2 shows the XRD patterns of the three hydrides and Mg0.50Ti0.50H1.80 from Ref. [6]. The FCC phase was observed in all the patterns. Mg0.25Ti0.75H1.62 seems to consist of the single FCC hydride phase. In the diffraction patterns of Mg0.35Ti0.65H1.58 and Mg0.50Ti0.50H1.80 [6] not only the hydride phase but also the MgH2 phase were observed. Excess MgH2 was formed in the region of x P 0.35. On the other hand, the secondary FCC phase with a small fraction was observed in the diffraction pattern of Mg0.15Ti0.85H1.60. This phase is possibly Ti hydride, TiHx with x < 1.56, considering that the lattice parameter is smaller than that of TiH1.56 [39]. The crystal structure of the metal lattice of the Mg0.25Ti0.75H1.62 FCC phase was refined by the Rietveld method. The FCC phase was
2. Experimental MgxTi1x (x = 0.15, 0.25, 0.35) alloys were synthesized by ball milling of Mg and Ti powders using stainless steel balls and pots. The detailed procedures for synthesizing the alloys were described in our previous report [5–7,9]. Hydrides were synthesized by hydrogenation of the alloys under a hydrogen pressure of 8 MPa at 423 K. The hydrogen contents of the hydrides were determined by the volumetric method. Hydrogen gas of 7 N purity was used. We also purchased the powders of MgH2 with the purity of 98 mass% and TiH2 with the purity of 99 mass% from Johnson Matthey and Rare Metallic, respectively, for reference. The powder X-ray diffraction (XRD) was measured using a Rigaku 2500 V diffractometer with Cu Ka radiation. The diffraction patterns were analyzed by the Rietveld refinement program RIETAN-2000 [31]. Synchrotron X-ray total scattering experiments were carried out at the 11-ID-B beamline at the advanced photon source at argonne national laboratory. Powder specimens of hydrides were packed in kapton capillaries with a diameter of 1 mm. The powder X-ray diffraction data were collected at room temperature using the rapid acquisition PDF technique [32] with an amorphous silicon-based area detector manufactured by Perkin Elmer. The X-ray energy was 58.291 keV (k = 0.21270 Å). The PDF was obtained by using the program PDFgetX2 [33] and Qmax = 25 Å1 was used. For real space modeling program PDFgui program [34] was used. The detailed procedures for the synchrotron X-ray total scattering experiments were described in the previous report [35–37]. 1 H NMR measurements were performed by a Bruker ASX200 spectrometer with a Larmor frequency of 200.13 MHz. Powder specimens of hydrides were packed in glass tubes with a diameter of 5 mm. The NMR spectra were measured with the solid echo pulse sequence ð90x s1 90y s2 echoÞ. The 90° pulse width was 1.5 ls. The values of s1 and s2 were set at 8.0 ls and 8.8 ls, respectively. The frequency scale of spectra was expressed with respect to neat tetramethylsilane (TMS) by adjusting the signal of water to 4.877 ppm [38]. 1H spin–lattice relaxation
Fig. 1. X-ray diffraction patterns of ball milled MgxTi1x (x = (a) 0.50, (b) 0.35, (c) 0.25, (d) 0.15, and (e) 0). Data for x = 0.50 and 0 is from Ref. [9]. Lattice parameters of a BCC lattice were estimated to be a = 0.342(1) nm for x = 0.50, a = 0.339(1) nm for x = 0.35, a = 0.340(1) nm for x = 0.25 and a = 0.338(1) nm for x = 0.15, respectively.
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assumed to be a Mg0.25Ti0.75 disordered phase applied to a cubic structural model of the space group Fm-3m. Here, contribution from hydrogen to the diffraction intensity was assumed to be negligible. The refined XRD pattern is shown in Fig. 3. The calculated curve overall fitted the experimental data but observed 1 1 1 and 2 0 0 diffraction peaks were shifted to the higher and lower angles than the expected positions, respectively. The FCC phase seems to be deformed from cubic symmetry and this is probably due to formation of stacking faults. It has been reported that introduction of stacking faults and twins in FCC metals reduces the crystallite size and changes the shape and position of diffraction peaks [40]. The 1 1 1 and 2 0 0 reflections tend to get closer with increase in stacking faults on {1 1 1} planes. We have reported that the Mg–Ti BCC phase contains high density stacking faults introduced by ball milling [9]. The stacking faults formed in Mg0.25Ti0.75H1.62 were possibly introduced during synthesizing Mg0.25Ti0.75 alloy before hydrogenation. The lattice parameter of Mg0.25Ti0.75H1.62 was evaluated to be a = 0.44565(5) nm from the refinement. This value is larger than a = 0.4400(5) nm of TiH1.56 [39] and rather close to a = 0.446 nm of TiH2 [41] although the hydrogen content H/Mg0.25Ti0.75 = 1.62 is close to 1.56 of TiH1.56. This demonstrates that the observed FCC phase contains Mg which has a larger atomic radius than Ti [42]. Fig. 4 shows X-ray PDFs of Mg0.25Ti0.75H1.62, MgH2 and TiH2. PDF pattern of Mg0.25Ti0.75H1.62 is similar to that of TiH2 rather than MgH2 but the peaks were much broader and quickly decayed with increasing interatomic distance, r. This quick decay indicates that Mg0.25Ti0.75H1.62 has smaller crystallite size or a larger number of lattice defects than TiH2 and MgH2. This is probably because only Mg0.25Ti0.75H1.62 has been synthesized by ball milling followed by hydrogenation. However, the decay seems to be even smaller than that for X-ray PDF of Mg0.50Co0.50D0.75, which has been explained by a two-phase model made of Mg2CoD5 and MgCo2 [36]. It has been reported that hydrogen atoms occupy tetrahedral (T) sites in the FCC Ti–H phase [39]. The PDF of Mg0.25Ti0.75H1.62 was analyzed using a Mg0.25Ti0.75 FCC structural model in which hydrogen atoms occupy T sites with the occupancy of 0.81 that corresponds to H/Mg0.25Ti0.75 = 1.62. The refinement range was 0.15 nm 6 r 6 2 nm. The PDF refinement result is shown in Fig. 5. The model fits the experimental PDF well. The lattice parameter was refined to be a = 0.443957(5) nm. This value agrees well with a = 0.44565(5) nm evaluated by Rietveld refinement of the XRD data. The difference curve between data and calculation suggests that there is no trace of local structure other than the main FCC phase, e.g., secondary phase or local segregation of Ti or Mg. It is
Fig. 2. X-ray diffraction patterns of (a) Mg0.50Ti0.50H1.80, (b) Mg0.35Ti0.65H1.58, (c) Mg0.25Ti0.75H1.62 and (d) Mg0.15Ti0.85H1.60. Data of Mg0.50Ti0.50H1.80 is from Ref. [6].
Fig. 3. Rietveld refinement of X-ray diffraction data for Mg0.25Ti0.75H1.62. The structural model for refinement is Fm-3m (no. 225). The lattice parameter was refined to be a = 0.44565(5) nm.
Fig. 4. X-ray PDFs of (a) MgH2, (b) Mg0.25Ti0.75H1.62 and (c) TiH2.
concluded that Mg0.25Ti0.75H1.62 consists of a single FCC phase without segregation at least from the information of the local structure of metal atoms. The 1H NMR spectra of Mg0.25Ti0.75H1.62, MgH2 and TiH2 are shown in Fig. 6. Signals were significantly broadened, which indicates that hydrogen atoms hardly migrate in the hydrides at room temperature. The line width of the signal in MgH2 was smaller than that of the signals in Mg0.25Ti0.75H1.62 and TiH2. The signal shifted to the negative side with the Ti content. Stable NMR active nuclei of Mg and Ti are 25Mg, 47Ti and 49Ti. Contributions of them to the line width were considered to be negligible because their natural abundances are below 10% and their nuclear dipole moments are lower than 10% of that of 1H. This indicates that the full width at half maximum (FWHM) of the signal is determined mainly by
Fig. 5. Refinement result of X-ray PDF of Mg0.25Ti0.75H1.62. The lattice parameter was refined to be a = 0.443957(5) nm.
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the second moment between 1H spins. The bulk magnetic susceptibility also contributes to the line width. The FWHM of Mg0.25Ti0.75H1.62 was 253 ppm which was equivalent to the value of TiH2 and larger than that of MgH2, as shown in Fig. 6. This might suggest that the distance between hydrogen atoms in Mg0.25Ti0.75 H1.62 is similar to that between hydrogen atoms on T sites in TiH2. However, it is complicated to clarify from this 1H NMR data whether hydrogen atoms occupy T sites or not in Mg0.25Ti0.75H1.62 because the second moment between 1H spins on T sites is estimated to be equivalent to that on octahedral (O) sites for FCC Ti–H [43]. The 1H spin–lattice relaxation time in Mg0.25Ti0.75H1.62 was T1 = 0.229 s. This value was slightly longer than T1 = 0.176 s in TiH2 and significantly shorter than T1 1400 s in MgH2. These results suggest that the density of states at the Fermi energy, N(EF), for Mg0.25Ti0.75H1.62 is close to that for TiH2 and quite different from that for MgH2. It has been reported that N(EF) for FCC Ti–H increases with the hydrogen content in the region of 1.6 6 H/Ti 6 2.0, which results in reduction in T1 [44,45]. One possible reason for the slight difference in T1 between Mg0.25Ti0.75H1.62 and TiH2 is the difference in the hydrogen contents between the two hydrides. The N(EF) relates to the signal position which is called Knight shift especially for transition metals. The signal in MgH2 was observed at around 3.4 ppm. It has been reported that the resonance shift of MgH2 with respect to TMS is tiny, which is close to zero [46,47]. On the other hand, the signal in TiH2 had a large negative Knight shift of 120.3 ppm. The Knight shift of TiH2 has been reported to be 90 200 ppm [48]. The shift of Mg0.25Ti0.75H1.62 was 71.5 ppm, which was between those of MgH2 and TiH2. Mg0.25Ti0.75H1.62 has a different electronic structure from TiH2 and N(EF) is reduced by substitution of a part of Ti by Mg as well as reduction in the hydrogen content. This result from the 1H NMR measurements shows that the synthesized FCC hydride phase surely consists of Mg, Ti and hydrogen without segregation. According to the present work using X-ray PDF and 1H NMR, phase separation with a nanometers scale was not found in the Mg0.25Ti0.75H1.62 FCC hydride phase synthesized under a hydrogen pressure of 8 MPa at 423 K. This fact indicates successful synthesis of the single FCC hydride phase from Mg0.25Ti0.75 but the hydrogen content did not reach to H/Mg0.25Ti0.75 = 2. In contrast, Mg0.15Ti0.85 decomposed to the two FCC phases by hydrogenation under the same condition, although computational studies have estimated that the stability of the hydride phase increases with the Ti content
Fig. 6. 1H NMR spectra of (a) MgH2, (b) Mg0.25Ti0.75H1.62 and (c) TiH2.
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[27]. Neutron PDF and 2H MAS NMR for Mg–Ti deuterides are in progress to discuss the local structure in a nanometers scale from a viewpoint of distribution of Mg, Ti and hydrogen atoms in the FCC hydride phase. 4. Conclusions A single FCC phase of Mg–Ti hydride was successfully synthesized. The chemical formula was Mg0.25Ti0.75H1.62. The hydride contained numbers of stacking faults introduced by ball milling. Segregation between Mg and Ti was not found by the X-ray PDF method. 1H NMR indicated that the FCC hydride phase consisted of Mg, Ti and hydrogen from the information of the electronic structure. These results have demonstrated that stable Ti-rich Mg–Ti FCC hydride phase was formed from the non-equilibrium Mg–Ti alloy phase. Acknowledgement This work was partly supported by The New Energy and Industrial Technology Development Organization (NEDO) under ‘‘Advanced Fundamental Research on Hydrogen Storage Materials (Hydro-Star)’’. This work was also partly supported by Iketani Science and Technology Foundation. Use of the Advanced Photon Source, an Office of Science User Facility operated for the US Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the US DOE under Contract No. DE-AC02-06CH11357. References [1] C. Suryanarayana, F.H. Froes, J. Mater. Res. 5 (1990) 1880–1886. [2] M. Hida, K. Asai, Y. Takemoto, A. Sakakibara, Mater. Sci. Forum 235–238 (1997) 187–192. [3] G. Liang, R. Schulz, J. Mater. Sci. 38 (2003) 1179–1184. [4] S. Rousselot, M.-P. Bichat, D. Guay, L. Roué, J. Power Source 175 (2008) 621– 624. [5] K. Asano, H. Enoki, E. Akiba, Mater. Trans. 48 (2007) 121–126. [6] K. Asano, H. Enoki, E. Akiba, J. Alloys Comp. 478 (2009) 117–120. [7] K. Asano, H. Enoki, E. Akiba, J. Alloys Comp. 480 (2009) 558–563. [8] K. Asano, E. Akiba, J. Alloys Comp. 481 (2009) L8–L11. [9] K. Asano, H. Enoki, E. Akiba, J. Alloys Comp. 486 (2009) 115–123. [10] W.P. Kalisvaart, H.J. Wondergem, F. Bakker, P.H.L. Notten, J. Mater. Res. 22 (2007) 1640–1649. [11] W.P. Kalisvaart, P.H.L. Notten, J. Mater. Res. 23 (2008) 2179–2187. [12] P. Vermeulen, R.A.H. Niessen, P.H.L. Notten, Electrochem. Commun. 8 (2006) 27–32. [13] D.M. Borsa, R. Gremaud, A. Baldi, H. Schreuders, J.H. Rector, B. Kooi, P. Vermeulen, P.H.L. Notten, B. Dam, R. Griessen, Phys. Rev. B 75 (2007) 205408. [14] R. Gremaud, C.P. Broedersz, D.M. Borsa, A. Borgschulte, P. Mauron, H. Schreuders, J.H. Rector, B. Dam, R. Griessen, Adv. Mater. 19 (2007) 2813–2817. [15] P. Vermeulen, H.J. Wondergem, P.C.J. Graat, D.M. Borsa, H. Schreuders, B. Dam, R. Griessen, P.H.L. Notten, J. Mater. Chem. 18 (2008) 3680–3687. [16] A. Baldi, R. Gremaud, D.M. Borsa, C.P. Baldé, A.M.J. Van der Eerden, G.L. Kruijtzer, P.E. de Jongh, B. Dam, R. Griessen, Int. J. Hydrogen Energy 34 (2009) 1450–1457. [17] S. Bao, K. Tajima, Y. Yamada, M. Okada, K. Yoshimura, Appl. Phys. A 87 (2007) 621–624. [18] S. Bao, K. Tajima, Y. Yamada, M. Okada, K. Yoshimura, Sol. Energy Mater. Sol. Cells 92 (2008) 224–227. [19] D. Kyoi, T. Sato, E. Rönnebro, N. Kitamura, A. Ueda, M. Ito, S. Katsuyama, S. Hara, D. Noréus, T. Sakai, J. Alloys Comp. 372 (2004) 213–217. [20] D. Moser, D.J. Bull, T. Sato, D. Noréus, D. Kyoi, T. Sakai, N. Kitamura, H. Yusa, T. Taniguchi, W.P. Kalisvaart, P.H.L. Notten, J. Mater. Chem. 19 (2009) 8150– 8161. [21] A. Anastasopol, T.V. Pfeiffer, J. Middelkoop, U. Lafont, R.J. Canales-Perez, A. Schmidt-Ott, F.M. Mulder, S.W.H. Eijt, J. Am. Chem. Soc. 135 (2013) 7891– 7900. [22] A.A. Nayeb-Hashemi, J.B. Clark, Phase Diagrams Binary Magnesium Alloys (1988) 324. [23] F.D. Manchester, Phase Diagrams Binary Hydrogen Alloys (2000). p. 83 and p. 238. [24] S. Srinivasan, P.C.M.M. Magusin, R.A. van Santen, P.H.L. Notten, H. Schreuders, B. Dam, J. Phys. Chem. C 115 (2011) 288–297. [25] S. Srinivasan, P.C.M.M. Magusin, W.P. Kalisvaart, P.H.L. Notten, F. Cuevas, M. Latroche, R.A. van Santen, Phys. Rev. B 81 (2010) 054107.
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