Isothermal hydrogen absorption properties of the ball milled mixture of 3Mg(NH 2 ) 2 and 8LiH after dehydrogenation at 200 • C under high vacuum were investigated at two different temperatures of 150 and 200 • C. The pressure-composition isotherm (PCT) curve at 200 • C revealed a two-plateaus-like behavior, while the PCT curve at 150 • C showed a single-plateau-like behavior. The hydrogenated phases were composed of LiH and Mg(NH 2 ) 2 under 9 MPa at 200 • C, while those were observed as mixed phases of LiH and LiNH 2 at 150 • C without any trace of Mg(NH 2 ) 2 in XRD measurements. These results indicate that there are two-step hydrogenation processes corresponding to high and low pressures at 200 • C, but the kinetics at 150 • C is too slow to proceed with the second hydrogenating step at high pressure region.

Hydrogen storage properties of two ball-milled composites of Ca(NH 2 ) 2 + 2LiH and CaH 2 + 2LiNH 2 were investigated as a series of searching studies of high performance hydrogen storage materials. About 4.5 mass% hydrogen is desorbed from about 100 • C and the thermal desorption profiles show a peak around 200 and 220 • C for the composites of CaH 2 + 2LiNH 2 and Ca(NH 2 ) 2 + 2LiH, respectively, under a helium flow at 5 • C/min heating rate without any NH 3 emission within our experimental accuracy. The powder X-ray diffraction and infrared absorption spectroscopy indicated that the dehydrogenated states of both composites form "an unknown imide phase" after heating up to 400 • C, while the dehydrogenated states after heating up to 200 • C in vacuum are the mixed phases of Li 2 NH and CaNH. The rehydrogenated state for the ball-milled composite of CaH 2 + 2LiNH 2 is transformed into the composite of Ca(NH 2 ) 2 and 2LiH by repeating the dehydrogenation and rehydrogenation cycles at 180-200 • C.

A valence state and a local structure of transition metals (Nb, V, and Ti) in MgH 2 doped with metal oxides (Nb 2 O 5 , V 2 O 5 , and TiO 2 nano ) by ball milling were examined by X-ray absorption spectroscopy (XAS). The main edge regions of the Nb, V, and Ti K-edges in the X-ray absorption near edge structure (XANES) profiles are located between 0 and +5 in the oxidation states. Since these spectra coincide with those of NbO, VO, and Ti 2 O 3 , respectively, the additives are reduced by MgH 2 to the metal oxides, which have lower oxidation states than those of the starting materials. Furthermore, in order to examine the local structures around the transition metal atoms, the extended X-ray absorption fine structure (EXAFS) spectra were analyzed. In the Fourier transformation curves of the EXAFS spectra, all samples doped with the metal oxides show two peaks corresponding to metal-oxygen and metal-metal bonds, being the same as the references of NbO, VO, and Ti 2 O 3 . The local structure formed after ball milling or dehydrogenation is close to that of each of the reduced metal oxides (NbO, VO, and Ti 2 O 3 ) but in a more disarrangement state.

Enthalpy change (ΔH) due to hydrogen desorption (H-desorption) for the lithium amide/imide system was evaluated by differential scanning calorimetry (DSC) measurement. In order to obtain the accurate and precise value of ΔH, we have paid special attention to following two points for correcting the raw experimental data. One is to determine a cell constant of DSC equipment, which was evaluated by using the TiO 2-doped MgH 2 compound as a reference because of its quite similar hydrogen desorption properties to that of the lithium amide/imide system. The other is to estimate the sample amount corresponding to the H-desorption reaction from weight loss in the thermogravimetric analysis (TG). By performing both the corrections, the ΔH value due to the H-desorption reaction from LiNH 2 +LiH to Li 2 NH+H 2 was evaluated to be 67 kJ/mol H 2 .