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Hydrogenation Reaction Pathway in Li2Mg(NH)2 Eveline Weidner,*,† Francesco Dolci,‡ Jianjiang Hu,‡ Wiebke Lohstroh,‡ Thomas Hansen,§ Daniel J. Bull,| and Maximilian Fichtner‡ JRC Institute for Energy, Cleaner Energy Unit, Petten, The Netherlands, Institute for Nanotechnology, FZ Karlsruhe, Germany, Institute Laue-LangeVin, Grenoble, France, and Institute for Materials Research, UniVersity of Salford, Salford, U.K. ReceiVed: April 16, 2009; ReVised Manuscript ReceiVed: July 17, 2009
Results of the first time-resolved in situ neutron diffraction measurements during absorption of deuterium in Li2Mg(ND)2 reveal the occurrence of a two-stage reaction. The first stage involves the reaction to LiND2, LiD, and Li2Mg2(ND)3, with the second stage leading to full deuteration to Mg(ND2)2 and LiD. Since no structural model for Li2Mg2(NH)3 exists in the literature, its structure has been determined from refinement of X-ray and neutron diffraction data. These new data provide key information toward the clarification of the hydrogenation mechanism in this system. Introduction Safe and efficient storage is a key technological challenge to be met in order to enable the extensive use of hydrogen as an effective energy carrier. Complex hydrides have been the focus of many research activities due to their high volumetric and improved gravimetric capacities over their metallic hydride counterparts accessible, with a number of systems having been identified that allow hydrogen desorption at moderate temperatures (200 °C is still impractical. Contrary to the above-mentioned reactions, the appearance of nonstoichiometric intermediate phases has been noted by several authors.4-7 Numerous recent studies have been devoted to the thermodynamic destabilization of the Li-N-H system by the addition of alkaline or alkaline earth metals. Among these, the Li-Mg-N-H system shows the greatest promise, as there is a significant decrease in the reaction enthalpy compared to the Li-N-H system. Hydrogen desorption reactions from mechanical mixtures of Mg(NH2)2 and LiH in the ratio 1:2, 3:8, and 1:4 have been proposed by several groups.8-12 The most promising of these appears to be the 1:2 mix* To whom correspondence should
[email protected]. † JRC Institute for Energy. ‡ Institute for Nanotechnology. § Institute Laue-Langevin. | University of Salford.
be
addressed.
E-mail:
ture, for which a reversible desorption reaction has been suggested as
Mg(NH2)2 + 2LiH a Li2Mg(NH)2 + 2H9,10 2
(3)
The reaction enthalpy (calculated from the desorption plateau equilibrium pressure) has been reported to be 39 kJ/mol H2,10 yielding an equilibrium pressure of 1 bar at 90 °C.13 However, there is a significant kinetic barrier, reported as 102 kJ/mol from TPD with the Kissinger method13 and recently as 104.3 kJ/mol from modeling the diffusion equation to isothermal desorption curves at different temperatures.14 This kinetic barrier means that temperatures as high as 200 °C are necessary to obtain practical kinetic performance. Several authors have observed that the isothermal pressure composition (PCI) curves of the Mg(NH2)2/LiH system are composed of two regions,15-19 a steep increase upon initial hydrogenation, followed by a plateau region up to close to completion of the reaction. For the present system, the sloping part ranges from 0 to about 1.5 wt %, with the second from about 1.5 to 5.6 wt %, leading to the complete hydrogenation to Mg(NH2)2 + 2LiH.15-17 This observation has been controversially discussed. In the 1:4 and 3:8 systems, a number of possible nonstoichiometric compounds such as Li4-2xMgx(NH)2,18 LixMg(NH2)2-x(NH)x,20 and Li1+xMgN2H3-x21 have been proposed, based on in situ X-ray and ex-situ neutron and X-ray data. In the present system, a tentative reaction Li2Mg(NH)2 + 0.6H a Li2MgN2H3.215,22 has been suggested for the first part of the reaction, followed by Li2MgN2H3.2 + 1.4H2 a Mg(NH2)2 + 2LiH for the plateau region. The PCI curve has also been interpreted as containing two plateaus indicating the formation of an intermediate compound with the composition Li2MgN2H3.16 On the basis of X-ray diffraction, Leng et al.11 propose the reaction Mg(NH2)2 + 2LiH f Li2NH + MgNH + 2H2, with the two imides subsequently undergoing a solid-state reaction to Li2Mg(NH)2. Hu et al.,19 on the basis of ex-situ X-ray measurements, proposed the following intermediate reaction for the sloping part of the PCI curve:
10.1021/jp9034997 CCC: $40.75 2009 American Chemical Society Published on Web 08/10/2009
Intermediate Reaction in Hydrogenation of Li-Mg-N System
2Li2Mg(NH)2 + H2 a LiNH2 + LiH + Li2Mg2(NH)3 (4) Li2Mg2(NH)3 has been proposed as a desorption product from Mg-amide and Li-hydride:
Mg(NH2)2 + 2LiH f Li2Mg2(NH)3 + NH3 + H13 2
(5) The structure of Li2Mg(NH)2 has been investigated as a function of temperature using combined synchrotron in situ X-ray diffraction and neutron diffraction by Rijssenbeek et al.23 and is reported to undergo a structural transition from the room temperature orthorhombic R-modification to a primitive cubic β-modification at a temperature of 300 °C. At a temperature of 500 °C, a further transition to the fcc γ-polymorph is observed. The clarification of the reaction pathway in the absorption of deuterium in Li2Mg(ND)2 via in situ neutron diffraction is the focus of the present article. A structure determination of the tetragonal mixed imide phase Li2Mg2(NH)3 is presented for the first time. Experimental Section For the in situ neutron measurements, the starting material of Li2Mg(ND)2 was prepared from a ball-milled 2:1 molar mixture of LiNH2 and MgH2 (Sigma-Aldrich). The powder was milled for 12 h under an argon atmosphere in a Fritsch P6 planetary ball mill with silicon nitride vial and balls (powderto-ball ratio of 1:20) at a revolution speed of 600 rpm Hydrogen was desorbed from the resulting powder at 220 °C in a Sieverts’ apparatus, and subsequently deuterium absorption was effected at 95 bar. This procedure was repeated two times in order to obtain the deuterated compound Li2Mg(ND)2. Details of the Sievert equipment used have been published previously.24 Pure Li2Mg2(NH)3 phase was obtained according to the method proposed by Hu et al.19 A 1:1 mixture of Mg(NH2)2 and LiH was ball-milled for 12 h and subjected to a temperatureprogrammed reaction following eq 5, where both hydrogen and ammonia were released at temperatures up to 374 °C. A singlephase material was obtained in this manner. X-ray data were recorded on a Bruker D8 Advance diffractometer with Cu KR radiation; about 50-70 mg of sample was pressed into pellets and fixed to the XRD sample holder. Neutron diffraction measurements were performed on the D20 instrument at the Institute Laue Langevin reactor source, Grenoble, France. This instrument provides high flux and a curved linear position sensitive detector (PSD) resulting in rapid data acquisition over the full angular coverage. With the selected wavelength of 2.42 Å, a d-spacing range of about 1.4-10 Å is obtained. Time-resolved diffraction with a resolution of 1 min was possible. Data sets were corrected for detector efficiency against a vanadium standard, using the standard ILL data processing software LAMP. With regards to the sample cell, null coherent neutron scattering materials such as Vanadium of Ti-Zr alloys are not suitable for high-pressure measurements with hydrogen. The cell used here is an Inconel 600 tube, providing the required mechanical properties while minimizing the number of additional Bragg reflections. The resulting two strong reflections (111) and (200) at a d-spacing of 2.05 and 1.78 Å of the main phase of the sample holder were excluded from the refinement. Heating was effected via the instrument furnace; the actual sample temperature was determined from refinement of the
J. Phys. Chem. C, Vol. 113, No. 35, 2009 15773 lattice parameter of the Inconel 600 sample holder.25 Deuterium gas is used in preference to hydrogen, owing to its favorable coherent-to-incoherent scattering ratio. In the present work, the amount of deuterium absorbed into the system is given as moles of D added to the starting material of Li2Mg(ND)2 and is calculated from the phase abundances derived from Rietveld refinement. Time-resolved diffraction patterns (resolution of 1 min) were recorded. Measurements were made under constant pressure of deuterium. Owing to the rapid absorption kinetics, measurements were started at room temperature and recorded while the temperature was ramped to the required value. The two regions in the adsorption pressure-composition curves were investigatedsinitially a deuterium pressure of at 55 bar was applied at 245 °C. When this first reaction step was completed (no significant changes occurring in the diffraction patterns), the pressure was raised to 70 bar D2 and temperature lowered to 225 °C (reported equilibrium pressure at 220 °C of ∼45 bar15) to permit full deuteration. Rietveld Refinement Rietveld profile refinement was performed using the GSAS and MAUD software.26,27 The results of the two, independently performed, refinements were comparable within the margin of error. The structural model given by Rijssenbeek et al.23 was used for the R- and β-modification of Li-Mg-imide (Li2Mg(ND)2), and the N and D positions refined for all patterns as long as the phase abundance of this phase was above 50 mol %, to account for minor deviations from the published structure. For Li-amide (LiND2) and Mg-amide (Mg(ND2)2, the recently published structures of Sørby et al. were used.28 The sample holder reflections could be attributed to a main component from the fcc Inconel phase (two reflections which were excluded from refinement) and a minor carbide phase, which was included in the refinement. Profile refinement was significantly complicated due to the fact that the strongest reflections of three phases are almost overlapping. This is due to the structural similarities of the orthorhombic and tetragonal Li-Mg-imides, as well as of Li-amide with respect to the anion lattice. Because of the overlap and neutron absorption from 6Li, refinements were restricted to a few parameters, such as scale factor, lattice, and profile parameters. The isotropic Debye-Waller factor was fixed for all atoms of the phases. With the imposed restrictions reliability factors of weighted agreement factor Rwp of 2.8-3.2% were achieved, for up to 16 variables. Due to the abovementioned difficulties, Rietveld refinement for the in situ experiment was not used primarily to obtain crystallographic information from the diffraction patterns but as a tool to observe relative phase abundances and cell parameters of the different phases present in the sample during absorption of deuterium. Results From the time-resolved neutron diffraction patterns (Figure 1), the phase transitions associated with the two regions in the pressure-composition curve are clearly visible. On the basis of profile refinement of the room temperature neutron diffraction pattern, the synthesized Li2Mg(ND)2 sample comprises a majority phase of the R-polymorph, with a 15 mol % minority phase of the cubic β-modification. The R- and β-imide are consumed during the reaction with deuterium within 40 min. However, no Mg-amide appears until after the pressure has been raised to 70 bar, about 100 min into the reaction.
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Figure 1. Diffraction patterns recorded during deuteration at 55 and 70 bar up to a reaction time of 160 min. The patterns are shown as recorded in 1 min increments. The phase evolution and increase of the background with deuterium content are clearly visible. A temperature overshoot temporarily caused desorption, as values above the intended one of 245 °C were reached; however, the effects proved to be completely reversible.
Figure 2. Two pertinent angular ranges of diffraction patterns recorded at initial stages of deuteration at 55 bar are given. The appearance of LiND2, LiD and Li2Mg2(ND)3, as well as the reduction in intensity of the Li2Mg(ND)2, reflections can be clearly observed in these regions.
Considering the full diffraction range (Figure 1) and Figure 2, which shows a pertinent angular range of the diffraction patterns during the initial deuteration at 55 bar (first region of a PCI curve), the following observations are made. • The amount of Li-Mg-imide (R- and β-modification) decreases quickly after deuterium is added to the sample. • Li-deuteride is formed at an early stage of the reaction, (i.e., prior to the appearance of Mg-amide). • Two additional phases with tetragonal symmetry appear within a few minutes of reaction time. One of the two new phases appearing concurrently to Li-deuteride is likely to be Li-amide. Refinement of the pattern
Figure 3. Diffraction pattern recorded at 245 °C after 87 min of reaction time. The low-angle reflections have been fitted either assuming the β-modification of Li2Mg(NH)223 or the (for ND) almost structurally identical nonstoichiometric phase reported by Nakamura et al.,22 blue line, or LiND2, dotted red line. The agreement, especially considering the reflection appearing at 31° 2θ (marked by the arrow) is better in the latter case. An offset of 500 counts has been added for clarity. All parameters of the other phases were held fixed for the Rietveld refinement.
with LiND2 gives a high level of agreement with the data, especially a low angle (Figure 3). Other authors have reported the occurrence of LiNH2 under comparable circumstances from IR measurements,15,17,19 therefore Li-amide seems a good candidate for one of the two new phases. The second new phase is identified as the mixed-imide Li2Mg2(ND)3. The evolution of the diffraction pattern, as can be observed in Figures 1 and 2, indicates that the R-modification
Intermediate Reaction in Hydrogenation of Li-Mg-N System
Figure 4. Rietveld refinement of a diffraction pattern collected from Li2Mg(ND)2 sample material dosed with 55 bar D2 and heated up to 245 °C at equilibrium conditions. The three major phases present are 36 mol % Li2Mg2(ND)3, 27 mol % LiD, and 31 mol % LiND2, the balance consists of 5 mol % of the R-phase. The two strongest reflections from the sample holder were excluded from the refinement.
of the Li2Mg(ND)2 phase is replaced by a phase with tetragonal structure and very similar lattice parameters (c, 9.62 Å; a and b, 5.14 Å). As LiND2 and LiD simultaneously appear in the spectra, mass balance suggests that the initial deuterium uptake follows reaction 4, i.e., a tetragonal Li2Mg2(ND)3 phase is formed. A structural model for this phase was developed (to be discussed in the next section) and was used to include Li2Mg2(ND)3 in the Rietveld refinements. Other possible phases could be discounted, such as the primitive cubic nonstoichiometric phase, reported by Nakamura et al.22 (or the structurally similar β-modification). This phase does not give good agreement to the observed diffraction pattern (Figure 3). The temperature in this experiment is too low to consider the γ-modification proposed by Rijssenbeek et al.23 of the mixed Li/Mg amide. In conclusion, four phases, Li2Mg(ND)2, LiD, LiND2, and Li2Mg2(ND)3 are assumed to be involved in the first deuteration step. Figure 4 shows a fit to the data at equilibrium under 55 bar D2, exhibiting a good level of agreement assuming reaction 4. As the deuteration reaction proceeds under 70 bar of pressure, the Mg(ND2)2 phase forms while the reflections of Li2Mg2(ND)3 and LiND2 decrease in intensity (Figures 1,5). It is interesting to note that the refined lattice parameters of Mg(ND2)2 (a and b, 10.574(2)-10.5659(5) Å; c, 19.785(8)-19.884(2) Å at 225 °C) show significant variation, as well as intensity deviations, to the reported structure (a and b, 10.3758(6) Å; c, 20.062(1) Å at RT).28 LiMg(NH2)3, for which no structural model exists to our knowledge, has been reported as having an almost identical X-ray pattern as Mg-amide and indeed forms a series of mixed crystals with Mg-amide.29 The reaction proceeds quite slowly and after 400 min (final time) traces of Li2Mg2(ND)3 and LiND2 are still present in the patterns while the deuterium content only reaches 3.2 mol D/Li2Mg(ND)2 (compared to 4 mol D for full deuteration, eq 3). Figure 6 shows the molar phase fractions obtained from Rietveld analysis. Under 55 bar D2 pressure, the Li2Mg(ND)2 phase is consumed as deuterium is added to the system, and three phases, the tetragonal Li2Mg2(ND)3, LiND2, and LiD, appear in approximately equal molar ratios, thus supporting the proposed reaction 4. At a reaction time of 100 min, the pressure was increased to 70 bar D2 and the temperature reduced to 225
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Figure 5. Time-resolved series of diffraction patterns collected at 70 bar and 225 °C. In the angular region shown the intensity change of three reflections due to absorption of deuterium can be observed. Notable is the relatively much stronger decrease in intensity for the reflection of the Li2Mg2(ND)3 phase as compared to LiND2.
Figure 6. Evolution of molar phase fractions until the end of the reaction time at 400 min (log scale for clarity). The deuterium content was calculated from Rietveld analysis. The two pressure regimes have been separated with a line.
°C. In this second reaction step, the amount of Li-deuteride increases, Mg-amide is formed, while all other phases decrease. Therefore, approximate stability regions for the phases involved in the hydrogenation reaction can be proposedsMg-amide only appears at pressures over 70 bar at 225 °C, whereas Li-amide and the Li/Mg-imides are not stable at this high pressure. We stress the fact that the appearance of Mg-amide phase only in a second reaction step, whereas LiD, LiND2, and a tetragonal Li-Mg-N-D phase appear in the first step, is not subject to the possible errors in the Rietveld refinements. The general trend of the phase abundances is not affected by the difficulties in the indexing of overlapped reflections. Structure Determination of Li2Mg2(ND)3 The structure of Li2Mg2(NH/D)3 has been determined from X-ray and neutron data in this work. The composition derived from refining cation occupancies with the X-ray data is close to the expected one with Li2.08Mg2.0(NH)3. In order to aid the determination of a structure for Li2Mg2(NH)3, we first noted the close structural similarity between other phases in the Li-Mg-N-H system, namely Li2NH, LiNH2, and Li2Mg(NH)2. With small deviations from cubic symmetry and relevant origin shifts, they can be all be
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Weidner et al. TABLE 1: Atomic Parameters from Rietveld Structure Refinement for Tetragonal Imide for X-ray and Neutron Dataa atom/position
x
y
z
N (8i) (ND) (XRD) D (16j) (ND) Mg (2b) (XRD) Li (4c) (XRD) Li (4e) (XRD) Li (4d) (XRD) Mg (4d)
0.276(1) 0.2691(7) 0.229(3) 0.0000 0.0000 0.0000 0.0000
0.286(1) 0.2707(8) 0.141(2) 0.0000 0.0000 0.5000 0.5000
0.135(1) 0.1371(5) 0.075(1) 0.5000 0.762(2) 0.0000 0.2500
occupancy
Uiso
1
0.12
0.5 1 1 0.224(27) 0.171(7) 0.829(7)
0.08 n.r. 0.033 0.064 0.025 n.r. 0.038
a Lattice parameters at RT (XRD) are a ) 5.130(3)Å and c ) 9.619(1) Å for the space group I4j2m. Some parameters were held fixed, denoted as n.r. Additional information is available in the Supporting Information.
Figure 7. Fit from XRD Rietveld refinement of Li2Mg2(NH)3 tetragonal phase. An R value of 12% and χ2 of 2.6 are achieved. The diffraction pattern was collected with a wavelength of 1.54 Å.
thought of as comprising the basic ‘building block’ of the antifluorite structure. In this structure, the amide/imide anions occupy the fcc positions, with the Li+/Mg2+ cations occupying the tetrahedral sites at (1/4, 1/4, 1/4), forming a primitive cubic sublattice. In LiNH2 and Li2Mg(NH)2, one-half and one-third of the tetrahedral cation sites are vacant, respectively. The [NH]2- or [NH2]-anions are centered around the fcc positions, with N at the fcc sites and an associated H+ ion on a neighboring site, adjacent to a vacant site in case of LiNH2 and Li2Mg(NH)2. For Li2NH the hydrogen is statistically distributed over a set of neighboring sites at temperatures above 85 °C, allowing a cubic symmetry to be adopted.30 A tetragonal phase with the composition Li2Mg2(NH)3 phase was first synthesized by Juza and Eberius31 and subsequently identified by means of X-ray diffraction as an intermediate in mixed Li-Mg-N-H systems by Hu et al. and Xiong et al.19,32 Rijssenbeek et al.23 found a tetragonal phase appearing during in situ X-ray diffraction experiments at 290 °C and under 13.7 MPa hydrogen. This phase was denoted as δ-Li2Mg(NH)2; however, the fact that this phase was found during partial desorption from a Mg(NH2)2/2LiH mixture makes it likely that this is not a simple modification of the R-phase. The structure of the δ phase has not been published. A structural model for the tetragonal Li2Mg2(NH)3 phase has been developed by indexing the measured reflections from X-ray diffraction (Figure 6). The cell symmetry is identified as tetragonal, with lattice parameters a ) 5.130(3) Å and c ) 9.619(1)Å, the extinction rules indicating a body-centered lattice. The structure of Li2Mg2(NH/D)3 is assumed to be similar to the phases mentioned above. Nitrogen is initially placed on sites corresponding to cubic fcc positions. The space group with the highest symmetry compatible with this structure is I4j2m (121). The other atom positions, except hydrogen, are found by difference Fourier calculation. This preliminary structure is refined further with the X-ray (Figure 6) and neutron data. The neutron data where the highest abundance of the phase occurs is used to determine the D positions. Deuterium atoms are located initially on the hypothesis of an imide composition, with ND bonds, with the orientation of D atoms toward cation vacancies as found in the R-Li2Mg(ND)2 structure.23 A structure with deuterium half occupying the 16e positions gives the best agreement (Figure 7). The proposed structural model is given in table 1.
Figure 8. Structure of Li2Mg2(NH)3. The translucent spheres denote a ∼1/4 filled Li site. The hydrogen sites are half occupied.
Discussion The first hydrogenation step (up to 1.5 wt % H2) of the isotherm has been interpreted either as a slope (and related the existence of a nonstoichiometric intermediate phase)15-18,20-22 or as comprising two plateaux and involving a stoichiometric intermediate compound.16 The in situ diffraction data reveal that the first step of deuteration in the PCI curve proceeds via the formation of Li-amide, Li2Mg2(ND)3 (Figure 8), and LiD according to the reaction proposed by Hu et al.19 and is therefore likely to correspond to a first plateau. In a second step, Liamide and Li2Mg2(ND)3 react with each other and the hydrogenation process ends with the formation of the fully hydrogenated products Mg(ND2)2 and further LiD. The observed intermediate reaction can then be stated
2Li2Mg(ND)2 + 4D2 f LiD + LiND2 + Li2Mg2(ND)3 f 4LiD + 2Mg(ND2)2 (6) From a structural point of view the Li2Mg(ND)2 imide, identified by Rijssenbeek et al.,23 magnesium amide, Li2Mg2(ND)3, LiNH2, and Li2NH all share a fcc nitrogen lattice; thus, a spatial rearrangement of the Mg/Li-nitrogen tetrahedra together with ion migration7,33 is likely to be involved during the phase transitions driven by the hydrogen uptake. These processes can have a significant energy barrier which has to be overcome. The lithium-magnesium imide/amide system is characterized by the high interchangeability of their ions, with several known mixed cation imides, as well as the above-mentioned LiMg(NH2)3 amide.29 As the latter compound may form a solid solution with Mg(NH2)2 and the lattice parameters are reported to be similar for these two phases,29 they may be difficult to distinguish in the diffraction pattern. The variation in lattice parameter for Mg-amide mentioned above, as well as small changes in reflection intensity, could be attributed to the
Intermediate Reaction in Hydrogenation of Li-Mg-N System formation of the Li-Mg-amide phase or nonstoichiometric variants thereof. Figure 5 shows the evolution of reflection intensity as high deuterium pressure is added. From visual inspection alone it would be tempting to conclude that the much sharper decrease of intensity of the Li2Mg2(ND)3 phase as compared to LiND2 means that this phase initially is converted to LiMg(ND2)3, and then in a further step reacts with Li-amide to form the Mg-amide. This is at the current stage quite speculative, and additional measurements are planned to verify this proposed second intermediate reaction. The formation of a nonstoichiometric amide phase, however, could explain the sloping part of the PCI curves. Conclusions The in situ neutron diffraction monitoring of the reaction of Li2Mg(ND)2 with deuterium clearly shows that the hydrogenation of the system is best described as a two-step process. The first hydrogenation plateau in the low-hydrogen-content region unambiguously involves the formation of LiND2, Li2Mg2(ND)3, and LiD as intermediates to the final hydrogenation process ending with the formation of Mg(ND2)2 and LiD. The structure of Li2Mg2(NH)3 has been solved and reported for the first time. The appearance of LiNH2 as intermediate phase could be detrimental to the overall performance of the system. Theoretical calculations suggest that a -NH2 group in LiNH2 forms strong internal bonds with a covalent character, which can be weakened by Mg substitution.33-35 The breaking of these bonds during the second absorption step may be the rate-limiting step. The sloping part of the PCI curve may in fact contain a first plateau due to an intermediate reaction, as well as a sloping region where a nonstoichiometric mixed-amide phase is formed. Understanding these intermediate steps in the hydrogenation plays an important role in improving the properties of this potentially promising system. Acknowledgment. Partial funding is acknowledged from the European Commission Sixth Framework Programme under the Marie Curie Research Training Network COSY (Contract No. MRTN-CT-2006-035366) and the Integrated Project NESSHY. Supporting Information Available: Crystallographic information files (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Bogdanovich, B.; Schiwickardi, M. J. Alloys Compd. 1997, 253254, 1.
J. Phys. Chem. C, Vol. 113, No. 35, 2009 15777 (2) Orimo, S.; Nakamori, Y.; Eliseo, J.; Zuttel, A.; Jensen, C. Chem. ReV. 2007, 107, 4111. (3) Chen, P.; Xiong, Z. T.; Luo, J. Z.; Lin, J. Y.; Tan, K. L. Nature 2002, 420, 302. (4) Weidner, E.; Bull, D. J.; Shabalin, I. L.; Keens, S. G.; Telling, M. F. T.; Ross, D. K. Chem. Phys. Lett. 2007, 444, 76. (5) Bull, D. J.; Weidner, E.; Shabalin, I. L.; Telling, M. F. T.; Jewell, C. M.; Gregory, D. H.; Ross, D. K. Phys. Chem. Chem. Phys., submitted. (6) Chen, P.; Xiong, X.; Yang, L.; Guotao, W.; Luo, W. J. Phys. Chem. B 2003, 107, 10967. (7) David, W. I. F.; Jones, M. O.; Gregory, D. H.; Jewell, C. M.; Johnson, S. R.; Walton, A.; Edwards, P. P. J. Am. Chem. Soc. 2007, 129 (6), 15941. (8) Chen, P.; Xiong, Z.; Lee Tan, K. MRS Fall Meeting, 2003, Boston. (9) Xiong, Z.; Wu, G.; Hu, J.; Chen, P. AdV. Mater. 2004, 16, 1522. (10) (a) Luo, W. J. Alloys Compd. 2004, 381, 284. (b) Luo, W. J. Alloys Compd. 2004, 385, 316. (11) Leng, H. Y.; Ichikawa, T.; Hino, S.; Hanada, N.; Isobe, S.; Fujii, H. J. Phys. Chem. B 2004, 108, 8763. (12) Nakamori, Y.; Kitahara, G.; Miwa, K.; Towata, S.; Orimo, S. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 1. (13) Xiong, Z.; Hu, J.; Wu, G.; Chen, P.; Luo, W.; Gross, K.; Wang, J. J. Alloys Compd. 2005, 398, 235. (14) Wang, J. C.; Li, H. L.; Wang, S. M.; Liu, X. P.; L,i, Y.; Jiang, L. J. Int. J. Hydrogen Energy 2009, 34, 1411. (15) Luo, W.; Sickafoose, S. J. Alloys Compd. 2006, 407, 274. (16) Janot, R.; Eymery, J.-B.; Tarascon, J.-M. J. Power Sources 2007, 164, 496. (17) Barison, S.; Agresti, F.; Lo Russo, S.; Maddalena, A.; Palade, P.; Principi, G.; Torzo, G. J. Alloys Compd. 2008, 459, 343. (18) Aoki, M.; Noritake, T.; Kitahara, G.; Nakamori, Y.; Towata, S.; Orimo, S. J. Alloys Compd. 2007, 428, 307. (19) Hu, J.; Liu, Y.; Wu, G.; Xiong, Z.; Chen, P. J. Phys. Chem. C 2007, 111, 18439. (20) Noritake, T.; Aoki, M.; Towata, S.; Nakamori, Y.; Orimo, S. MRS Proc. 2006, 971, Z07–05. (21) Isobe, S.; Ichikawa, T.; Leng, H.; Fujii, H.; Kojima, Y. J. Phys. Chem. Sol. 2008, 69, 2234. (22) Nakamura, Y.; Hino, S.; Ichikawa, T.; Fujii, H.; Brinks, H. W.; Hauback, B. C. J. Alloys Compd. 2008, 457, 362. (23) Rijssenbeek, J.; Gao, Y.; Hanson, J.; Juang, Q.; Jones, C.; Toby, B. J. Alloys Compd. 2007, 454, 233. (24) Kircher, O.; Fichtner, M. J. Appl. Phys. 2004, 95, 7748. (25) Raju, S.; Sivasubramanian, K.; Divakar, R.; Panneerselvam, G.; Banerjee, A.; Mohandas, E.; Antony, M. P. J. Nucl. Mater. 2004, 325, 18. (26) Larson, A. C.; von Dreele, R. B. Los Alamos National Laboratory Report LAUR 86-748. (27) (a) Lutterotti, L.; Scardi, P. J. Appl. Crystallogr. 1990, 23, 246. (b) Lutterotti, L. The MAUD Program - http://www.ing.unitn.it/∼maud/. (28) Sorby, M.; Nakamura, Y.; Brinks, H. W.; Ichikawa, T.; Hino, S.; Fujii, H.; Hauback, B. C. J. Alloys Compd. 2007, 428, 297. (29) Juza, R. Angew. Chem., Int. Ed. 1963, 3, 471. (30) Ohoyama, K.; Nakamori, K. Y.; Orimo, S.; Yamada, S. K. J. Phys. Soc. Jpn. 2005, 74, 483. (31) Juza, R.; Eberius, E. Naturwissenschaften 1962, 49, 104. (32) Xiong, Z.; Wu, G.; Hu, J.; Chen, P.; Luo, W.; Wang, J. J. Alloys Compd. 2006, 417, 190. (33) Wu, H. J. Am. Chem. Soc. 2008, 130 (20), 6515. (34) Wang, Y.; Chou, M. Y. Phys. ReV. B. 2007, 76, 014116. (35) Miwa, K.; Ohba, N.; Towata, S.; Nakamori, Y.; Orimo, S. Phys. ReV B 2005, 71, 195109.
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