Hydrogen Release from Mg(NH2) - American Chemical Society

Hydrogen Release from Mg(NH2)2-MgH2 through Mechanochemical Reaction. Jianjiang Hu, Guotao Wu, Yongfeng Liu, Zhitao Xiong, and Ping Chen*...
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J. Phys. Chem. B 2006, 110, 14688-14692

Hydrogen Release from Mg(NH2)2-MgH2 through Mechanochemical Reaction Jianjiang Hu, Guotao Wu, Yongfeng Liu, Zhitao Xiong, and Ping Chen* Department of Physics, National UniVersity of Singapore, Singapore 117542

Kenji Murata and Ko Sakata The Institute of Applied Energy, Tokyo, Japan

Gerd Wolf Institut fu¨r Physikalische Chemie, Leipziger Strasse 29, 09596 Freiberg, Germany ReceiVed: March 1, 2006; In Final Form: May 26, 2006

A total of 7.4 wt % of hydrogen was released from the mixture of magnesium amide and magnesium hydride at a molar ratio of 1:2 by mechanical ball milling. Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Diffraction (XRD) characterizations along with the amount of hydrogen released at different stages of ball milling reveal that magnesium imide was first formed in the reaction. The imide then reacted continuously with magnesium hydride and was converted to magnesium nitride and hydrogen. Thermodynamic calculation shows that the hydrogen desorption is a mild endothermic reaction with the standard enthalpy change of about 3.5 kJ/mol of H2.

1. Introduction Hydrogen is the most abundant chemical element on earth. When reacting with oxygen, it produces environment benign energy with water as byproduct. Efficient use of this energy carrier can therefore reduce the dependence on conventional fossil fuel and ultimately lessen the burden of greenhouse gas to the environment. Nevertheless, the storage of hydrogen gas in a safe and economical way proves to be a bottleneck in the application of an on-board power supply.1 In contrast to the physical approaches such as compression and liquefaction, hydrogen storage in the solid state has among others the merits of high volumetric and gravimetric hydrogen density.2 In the solid-state hydrogen storage, metal or alloy hydrides have been studied for several decades.3,4 Improvements have to be made to increase the deliverable hydrogen capacity and/ or lower hydrogen release temperature, to meet the on-board requirements. Sodium alanate is a complex hydride that has been intensively investigated recently. Encouraging improvements in kinetics and cyclization stability have been achieved by doping the analate with Ti compounds.5,6 In 2002, the interaction of metal nitrides with hydrogen was discovered, which implied that metal-N-H compounds could be the potential candidates for the hydrogen storage.7,8 Improvements have been achieved in lowering H2 desorption temperature from 270 to 180 °C by the compositional variation.9,10 An approach in designing a metal-N-H system is by probing the chemical interaction between metal amides and hydrides.8-11 An interesting feature of metal amide-hydride pairs is that the hydrogen atom in hydride is negatively charged, while in the amide it is positively charged. As shown with (NH4+)(BH4-) by Grochala et al.,3 an attractive interaction between Hδ+ and Hδ- is responsible for the hydrogen molecules being desorbed at lower temperatures. * Address correspondence to this author. Phone: +65 68745100. Fax: +65 67776126. E-mail: [email protected].

For a chemical reaction to occur, contact or collision of reactant molecules is indispensable. For the case of the solidsolid reaction, difficulty in bringing the reactants together at the molecular level is obvious. Ball milling is one of the effective methods for conducting solid-state reactions. During ball milling, the solid reactants, usually crystalline, become amorphous liquids which facilitate reactions.12 On the other hand, high pressure in the order of GPa is generated in the solid by colliding balls, which can initialize the reaction.13 Magnesium amide Mg(NH2)2, a lightweight amide, decomposes to magnesium imide (MgNH) and ammonia at elevated temperatures.11 The generation of ammonia starts at about 200 °C. Nakamori et al. investigated the thermal behavior of the mixture of Mg(NH2)2 with MgH2 by means of thermogravimetry-differential thermal analysis (TG-DTA). Instead of the expected interaction between amide and hydride, they only observed the self-decomposition of Mg(NH2)2.14 However, we found in our previous work that the mixture of Mg(NH2)2MgH2 at the molar ratio of 1:1 released a considerable amount of hydrogen without NH3 generation during a ball milling treatment,15 which implies that chemical reaction between Mg(NH2)2 and MgH2 at a molar ratio of 1:1 could happen under the mechanochemical condition. Therefore, the NH3 generation during the thermal treatment could be suppressed. In this study, we mixed equimolar Hδ+ in amide and Hδ- in hydride by means of ball milling to investigate the detailed process in the mechanochemical reaction. Thus, mixtures of Mg(NH2)2-MgH2 were prepared at a molar ratio of 1:2 by ball milling. Structural characterizations of the mixture have been carried out at different milling stages to monitor the reaction process. A stoichiometric amount of hydrogen, as expected, was released, after 72 h of ball milling. 2. Experimental Section 2.1. Materials. MgH2 was a product of Alfa Aesar with 98% purity and was used as received. Mg(NH2)2 was synthesized in

10.1021/jp061279u CCC: $33.50 © 2006 American Chemical Society Published on Web 07/07/2006

Hydrogen Release from Mg(NH2)2-MgH2 house by reacting Mg powder (97%, Riedel-De Haen) with 120 psi of ammonia at about 300 °C. The structure of Mg(NH2)2 was confirmed by X-ray diffraction and the purity was 98.1%, determined by thermogravimetry. 2.2. Ball Milling Treatment. The ball milling of Mg(NH2)2 with MgH2 at a molar ratio of Mg(NH2)2:MgH2 ) 1:2 was conducted on a Retsch PM400 planetary ball mill at 200 rpm. A 1.4 g (0.025 mol) sample of Mg(NH2)2 and 1.3 g (0.05 mol) of MgH2 were loaded into the milling jar in a glovebox (MBraun) filled with Ar gas. The milling jar was equipped with gas valves which can be connected to a pressure gauge and enable the measurement of pressure increase in the jar caused by gas release during ball milling. Six stainless steel balls 13 mm in diameter and 4 balls 10 mm in diameter were used. The ball-to-sample weight ratio was 26:1. To ensure even mixing and milling and reduce the heat effect by milling, the mill was set to revolve for 60 s in one direction and pause 15 s, and then revolve in the reverse direction. No obvious temperature increase was noticed, thanks to the ventilation device inside the compartment of the mill. The gas generated during ball milling was analyzed by mass spectrometer and by an ammonia-selective electrode. 2.3. Methods. FTIR measurements were conducted on a Perkin Elemer FTIR-3000 unit in DRIFT mode (Diffuse Reflectance Fourier Transform Infrared). The powdery samples were loaded into the DRIFT in situ cell in the glovebox and measured at a resolution of 4 cm-1. For the measurement of X-ray diffraction profiles (Bruker D8-advance X-ray diffractometer with Cu KR radiation), a ca. 50-70 mg sample was pressed into pellets 13 mm in diameter in the glovebox and fixed to the XRD sample holder with double-sided tape. The sample holder was connected to a vacuum during the data acquisition to protect the air contamination. The period for data acquisition was about 60 min. Diffrential Scanning Calorimetry (DSC) measurements were performed on a Netsch DSC 204 HP housed inside the glovebox. Samples were heated at 2 deg/ min under the flow of purified Ar. Calorimetric investigations were carried out on a Thermal Activity Monitor (TAM) calorimeter (Thermometric, Sweden). The quantitative analysis of ammonia gas was conducted on a Metrohm 781 pH/ion meter (Switzerland) equipped with an NH3-selective electrode. The measuring range of the model is 0.1 × 104 to 1.7 × 104 ppm (NH3). Aqueous solutions with known concentrations of NH4Cl were used for the calibration. For the determination of NH3 amount, the gaseous products were slowly introduced from the milling jar to 30 mL of distilled water. NH3 gas will be absorbed in water upon contacting with water and its concentration in water was measured by the NH3-selective electrode. 3. Results and Discussion 3.1. Milling Investigation. First, the mixture of 0.025 mol of Mg(NH2)2 with 0.050 mol of MgH2 was milled for 72 h. The pressure in the milling jar increased gradually with milling time. Figure 1 shows the time dependence of the pressure increase in the milling jar. Mass spectrometer analysis revealed that the gaseous product was pure hydrogen. By using the ideal gas equation of state, the pressure was converted to the respective H atoms per [Mg(NH2)2-2MgH2] (Table 1). It can be seen from Figure 1 and Table 1 that hydrogen release began after milling for 2 h and was accelerated after 5 h of ball milling. The hydrogen release rate slowed after about 20 h of milling. The two components in the mixture, Mg(NH2)2 and MgH2, are stable in the milling jar if ball milled alone, indicating that the hydrogen generation is due to the chemical reaction between

J. Phys. Chem. B, Vol. 110, No. 30, 2006 14689

Figure 1. Time dependence of the hydrogen pressure increase in the milling jar.

TABLE 1: Hydrogen Release in Dependence of Ball Milling Time exp no.

milling time (h)

H2 pressure (psi)

no. of H atoms per [Mg(NH2)2 + 2MgH2]

1 2 3 4 5 6 7 8 9 10 11 12 13

0 2 4 5 6 8 9 10 12 14 18 22 72

0 3 10 17 23 48 82 101 113 120 133 138 178

0 0.1 0.5 0.8 1.0 2.1 3.7 4.5 5.0 5.3 5.9 6.1 7.9

these two chemicals. Such chemical reaction occurs at an appropriate rate only after the particle sizes of the reactants are small enough. In the acceleration period of hydrogen release, the concentration of the reactants with proper particle size may be high. Afterward, most of the components were consumed and the reaction rate, i.e., the rate of hydrogen release, decreased. After 72 h the pressure reached 178 psi in the milling jar, which corresponds to 0.198 mol of H atoms. As the total amount of H atoms from the starting chemicals was 0.2 mol, an almost stoichiometric amount of hydrogen atoms in the starting chemicals was released. One of the potential problems that should be avoided for the nitrogen-containing system is the coproduction of NH3. The equilibrium pressures of NH3 were about 0.3 and 1.9 psi at 300 and 400 °C, respectively, for the decomposition of LiNH2 to Li2NH and NH3, reported by Juza and Opp.16 This implies that even a small amount of NH3 could suppress the decomposition of LiNH2. In the presence of a hydride, the reaction between the hydride and amide, resulting in H2 generation, competes with the decomposition reaction of the amide. Recently, Hino et al. studied in a closed cell the NH3 partial pressure of the LiNH2-LiH system,17 which is an analogue to the system in this study. NH3 (0.1%, 0.1psi) and H2 (99.9%, 10.0 psi) were detected by Raman spectroscopy. The authors attributed the generation of NH3 to the thermodynamic equilibrium of LiN-H. In our study, the generation of NH3 was measured by NH3-selective electrode (Table 2). The partial pressure of NH3 in the milling jar remained almost constant at an even lower level before 26 h of milling, including the accelerating H2 generation period. In the late stage of ball milling, large amount of amide has been converted to imide and nitride, which accounts for the substantial reduction in the NH3 amount. This

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Figure 2. FTIR spectra of samples milled for 5, 9, 12, 18, and 72 h.

TABLE 2: NH3 Generation during Ball Milling milling time (h)

NH3 concn (ppm)

NH3 pressure in milling jar (psi)

4 5 26 40

1.6 1.7 1.3 0.48

5.1 × 10-3 5.4 × 10-3 4.1 × 10-3 1.5 × 10-3

seems in accordance with the statement of a thermodynamic equilibrium NH3, as long as the amide is available. To investigate the chemical process occurring during the milling treatment, samples at different ball milling stages were collected from the milling jar in separate experiments for the FTIR and XRD analyses. 3.2. FTIR Spectra. The FTIR spectra of samples milled for 5, 9, 12, 18, and 72 h are shown in Figure 2. The sample milled for 5 h (with 0.76 H atoms released per [Mg(NH2)2-2MgH2]) still shows the typical doublet N-H vibration of the amide ions in Mg(NH2)2 at 3272/3326 cm-1.18 In addition, a new weak absorbance is discernible at around 3196 cm-1. As the milling time was extended to 9 h, this absorbance developed to a new broad peak, indicating the formation of a new structure different from that of the starting materials. At the same time the peak intensity of amide at 3272/3326 cm-1 decreased, implying the consumption of Mg(NH2)2. For the sample milled for 12 h, the broad peak is the prevailing absorbance in the spectrum, while the absorbances of amide can hardly be recognized. However, milling the sample further to 18 h resulted in the weakened absorbance at 3196 cm-1. After 72 h of ball milling no absorbance can be detected. The absorbance at 3196 cm-1 represents a new structure that was formed at first and then consumed with the gradual generation of H2 in the milling process. This absorbance was previously observed in our work on [1Mg(NH2)2-1MgH2].15 Since hydrogen was deprived from the mixture, and N-H is the only IR-active species in the mixture in the investigated region, this new structure should contain less hydrogen than Mg(NH2)2, i.e., should be an imide. In ref 18, the IR spectrum of magnesium imide MgNH showed vibrational absorbances at 3251 and 3199 cm-1, with the latter absorbance was close to 3196 cm-1 (it is difficult to obtain an exact readout from the broad absorbing peak). The new structure observed by FTIR is possibly MgNH. The absorbance at 3251 cm-1 is not recognizable in this work, perhaps because of the strong broad absorbance in the vicinity. 3.3. XRD Results. Figure 3 shows the XRD patterns of Mg(NH2)2 as-prepared (upper) and ball milled for 18 h (lower). The typical diffraction peaks of crystalline Mg(NH2)2 disappeared after ball milling, proving that it became an amorphous

Hu et al.

Figure 3. XRD patterns of Mg(NH2)2: (1) as-prepared and (2) ball milled for 18 h.

Figure 4. XRD patterns of samples milled for 5, 9, 12, 18, and 72 h.

form, and therefore is not detectable by means of XRD. The XRD patterns of the samples milled for different periods are shown in Figure 4. It can be seen that the MgH2 phase (symbol ∆) dominates in the XRD profile after 5 h of milling. But a broad peak next to 40° is visible. After milling for 9 h, two broad peaks at 40.8° and 59.3° with considerable intensities emerge in the XRD profile, though the MgH2 phase can be unambiguously identified from the peaks at 27.9°, 35.7°, 39.8°, and 54.7°, with distortion at 35.7° and 39.8°. The XRD pattern for the sample milled for 12 h shows lower intensity of peaks assignable to MgH2 indicating less MgH2 survived after the treatment. In addition, there are three peaks at 35.3°, 40.8°, and 59.3° (symbol x), corresponding to the new structure observed by FTIR. In our previous work on Mg(NH2)2-MgH2 with a 1:1 molar ratio,15 two diffraction peaks at 41.0° and 61.0° were assigned to an imide-like structure, which is possibly the same product as in the present work, i.e., MgNH. The differences in the diffraction angles may be due to the broad XRD peaks which make it difficult for exact readings. On the other hand, it may rise from the different conditions of sample preparation which affect the crystallization of samples. The broadness of the three peaks at 35.3°, 40.8°, and 59.3° also indicates that the new phase MgNH formed in the ball milling process is not well crystallized. It should be pointed out that the three peaks at 35.3°, 40.8°, and 59.3° cannot be matched to a known Mg-N crystal structure from the database (JCPDS-CDD 2000). So the observed MgNH phase has a different crystal structure than that in the database. For the sample milled for 18 h, a clear Mg3N2 profile can be identified (symbol 0). The XRD pattern of the sample milled for 72 h shows unambiguously the formation of Mg3N2.

Hydrogen Release from Mg(NH2)2-MgH2

J. Phys. Chem. B, Vol. 110, No. 30, 2006 14691 TABLE 3: Thermodynamic Data from Literature

Figure 5. DSC curve of the thermal decomposition of Mg(NH2)2 at 2 deg/min.

3.4. Description of the Reaction between Mg(NH2)2 and MgH2. On the basis of the above discussions, we would like to give a description of the chemical process during ball milling of a [1Mg(NH2)2-2MgH2] mixture. MgNH was first formed from the reaction between Mg(NH2)2 and MgH2. From the stoichiometry of Hδ+ in Mg(NH2)2 and Hδ- in MgH2, 1 mol of Mg(NH2)2 may react with 1 mol of MgH2 and yield 2 mol of MgNH and 4 mol of H atoms:

Mg(NH2)2 + MgH2 ) 2MgNH + 2H2

Cp (J K-1 mol-1)

∆fHθ (kJ mol-1)

Mg3N2 NH3 MgH2

105 35 35

-461 -46 -75

an endothermic desorption reaction with the standard enthalpy change in the range between 29 and 46 kJ/mol of H2 is desired.19 A thermodynamic analysis on the reaction between the hydride and amide was performed. The enthalpy of formation of Mg(NH2)2 was not available in the literature to the best of our knowledge. So it was measured both by DSC and by calorimetry. Figure 5 is the DSC curve of Mg(NH2)2, which shows an endothermic nature of decomposition. Integration of the endothermic peak gave a heat effect of 1827 J/g, which can be converted to 102.3 kJ/mol. Hess’s law and the peak temperature (638 K) in the DSC curve were used to calculate the reaction enthalpy change ∆Hθ at standard temperature (25 °C). The thermodynamic parameters used for the calculation are listed in Table 3.20

(1) Thus,

Further reaction between MgNH and MgH2 gives rise to the formation of Mg3N2. From the compositional changes in the milling treatment, the mixture with 3.7 H atoms released (Table 1) after milling for 9 h would consist of MgNH and MgH2 with a residual amount of Mg(NH2)2. After milling for 12 h, 5.0 H atoms were released (>4 H) (Table 1). This means that excessive MgH2 reacts further with MgNH. The reaction process surpassed the stage of MgNH formation and entered the step of Mg3N2 formation. This deduction correlates well with the results of FTIR and XRD measurements (Figure 2), which show at first the progressive consumption of Mg(NH2)2 and the formation of new structures, i.e., MgNH, in the milling process until 12 h, and then, the conversion of the newly formed structure to Mg3N2. Thus, the reaction that occurred during the ball milling is suggested to proceed in two steps. In the first step, Mg(NH2)2 reacts with MgH2 to form MgNH with hydrogen release. MgNH then reacts with excessive MgH2 to yield Mg3N2 and H2.

Mg(NH2)2 + 2MgH2 ) 2MgNH + MgH2 + 2H2

(2)

2MgNH + MgH2 ) Mg3N2 + 2H2

(3)

The overall reaction between Mg(NH2)2 and MgH2 at a 1:2 molar ratio is:

Mg(NH2)2 + 2MgH2 ) Mg3N2 + 4H2

substance

∆Hθ ) ∆H1 + ∆H2 + ∆H3 + ∆H4 where ∆H1, ∆H3, and ∆H4 are the enthalpy changes of Mg(NH2)2, Mg3N2, and NH3 due to the temperature change between 298 and 638 K (∆T ) 330 K), respectively. ∆H2 is the reaction enthalpy measured by DSC, i.e., ∆H2 ) 102.3 (kJ/mol). On the other hand, ∆Hθ can be expressed by the enthalpies of formation in the following relationship:

1 4 ∆Hθ ) ∆fHθ(Mg3N2) + ∆fHθ(NH3) - ∆fHθ(Mg(NH2)2) 3 3 Rearranging the above equation, we obtain:

1 4 ∆fHθ(Mg(NH2)2) ) ∆fHθ(Mg3N2) + ∆fHθ(NH3) - ∆Hθ 3 3 ∆H1 ) Cp(Mg(NH2)2)∆T ) 604 J/g ) 34.5 (kJ/mol) ∆H3 ) -0.105 × 330 ) -34.7 (kJ/mol) ∆H4 ) -0.035 × 330 ) -11.6 (kJ/mol)

(4)

The hydrogen release from this reaction amounts to 7.4 wt %, which is high in hydrogen capacity and attractive as hydrogen storage materials. 3.5. Thermodynamic Analysis. Both the high H capacity in the system [1Mg(NH2)2 + 2MgH2] and the readiness of hydrogen release are of interest for the practical application. For a thermodynamically reversible hydrogen storage process,

∆Hθ ) ∆H1 + ∆H2 + ∆H3 + ∆H4 ) 34.5 + 102.3 - 1/3(34.7) - 4/3(11.6) ) 109.8 ≈ 110 (kJ/mol) Thus, the enthalpy of formation of Mg(NH2)2 can be obtained as follows:

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1 4 ∆fHθ(Mg(NH2)2) ) ∆fHθ(Mg3N2) + ∆fHθ(NH3) - ∆Hθ 3 3 ) 1/3(-461) + 4/3(-46) - 110 ) -325 (kJ/mol) It should be pointed out that the decomposition of NH3 into N2 and H2 is not included in the calculation as the decomposition extent is low in the temperature range of DSC measurement.21 With the enthalpy of formation of Mg(NH2)2, the reaction enthalpy of reaction 4 can be calculated as follows:

∆Hdes ) 4∆fHθ(H2) + ∆fHθ(Mg3N2) ∆fHθ(Mg(NH2)2) - 2∆fHθ(MgH2) ) 0 + (-461) - (-325) - 2(-75) ) 14 (kJ/mol) ) 3.5 (kJ/mol of H2) For the calorimetry method, the heat of dissolution of Mg(NH2)2 in water was first measured. The standard enthalpies of formation of the respective ions were obtained from the HSC database (Chemical Reaction and Equilibrium Software, Outokumpu ResearchOy, Finnland). Then, ∆fHθ(Mg(NH2)2) was calculated to be 351 ( 2 kJ/mol. The reaction enthalpy of (4) is estimated to be 40 or 10 kJ/mol of H2, a bit higher than that from the DSC method. According to the above calculation, the hydrogen release from the reaction of 1Mg(NH2)2 with 2MgH2 is mildly endothermic. Although the endothermic reaction enthalpy predicts that the hydrogen release is thermodynamically reversible, it is so low that the equilibrium hydrogen pressure is too high at ambient temperature, which makes the operation impractical. 4. Conclusion The reaction between magnesium amide and hydride at a feed molar ratio of 1:2 by means of ball milling resulted in the release of a stoichiometric amount of H2 (7.4 wt %) and the formation of magnesium nitride. Magnesium imide (MgNH) was found

Hu et al. to be the intermediate of the reaction. Thermodynamic calculations showed that the hydrogen release reaction of [1Mg(NH2)2 + 2MgH2] is mildly endothermic. However, the low reaction enthalpy predicts impractical operation conditions for the hydrogen uptake reaction. Acknowledgment. The authors thank the financial support from the Agency for Science, Technology and Research (A*STAR, Singapore) and the New Energy and Industrial Technology Development Organization (NEDO, Japan). References and Notes (1) National Hydrogen Energy Roadmap; U.S. Department of Energy; 2002, http://www.eere.energy.gov/hydrogenandfuelcells/pdfs/hydrogen_posture_plan.pdf. (2) Schlapbach, L.; Zu¨ttel, A. Nature 2001, 414, 353-358. (3) Grochala, W.; Edwards, P. P. Chem. ReV. 2004, 104, 1283-1315. (4) Schu¨th, F.; Bogdanovic, B.; Felderhoff, M. Chem. Commun. 2004, 2249-2258. (5) Bogdanovic, B.; Schwickardi, M. J. J. Alloys Compd. 1997, 253254, 1-9. (6) Bogdanovic, B.; Brand, R. A.; Marjanovic, A.; Schwickardi, M.; To¨lle, J. J. Alloys Compd. 2000, 302, 36-58. (7) Chen, P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. L. Nature 2002, 420, 302-304. (8) Chen, P.; Xiong, Z.; Luo, J.; Lin, J.; Tan, K. L. J. Phys. Chem. B 2003, 107, 10967-10970. (9) Luo, W. J. Alloys Compd. 2004, 381, 284-287. (10) Xiong, Z. T., Wu, G. T.; Hu, J. J.; Chen, P. AdV. Mater. 2004, 16, 1522-1525. (11) Leng, H. Y.; Ichikawa, T.; Hino, S.; Hanada, N.; Isobe, S.; Fujii, H. J. Phys. Chem. B 2004, 108, 8763-8765. (12) Rothenberg, G.; Downie, A. P.; Raston, C. L.; Scott, J. L. J. Am. Chem. Soc. 2001, 123, 8701-8708. (13) Maurice, D. R.; Courtney, T. H. Metall. Trans. A 1990, 21, 289303. (14) Nakamori, Y.; Kitahara, G.; Orimo, S. J. Power Sources 2004, 138, 309-312. (15) Hu, J.; Xiong, Z.; Wu, G.; Chen, P.; Murata, K.; Sajkata, K. J. Power Sources. In press. (16) Juza, R.; Opp, K. Z. Anorg. Allg. Chem. 1951, 266, 325-330. (17) Hino, S.; Ichikawa, T.; Ogita, N.; Udagawa, M.; Fujii, H. Chem. Commun. 2005, 3038-3040. (18) Linde, G.; Juza, R. Z. Anorg. Allg. Chem. 1974, 409, 199-214. (19) Libowitz, G. G. The Solid-State Chemistry of Binary Metal Hydrides; The Physical inorganic chemistry series; W. A. Benjamin Inc.: New York, 1965. (20) Aylward, G.; Findlay, T. SI Chemical Data, 4th ed.; John Wiley & Sons: Singapore, 1998. (21) Xiong, Z. T.; Hu, J. J.; Wu, G. T.; Chen, P. J. Alloys Compd. 2005, 395, 209-212.