Dehydriding reaction of Mg(NH2)2-LiH system under hydrogen pressure

The dehydriding and structural properties of the 3Mg(NH2)(2) + 12LiH system under hydrogen pressure were investigated using the pressure-composition (p-c) isotherm measurement and X-ray diffraction (XRD) analysis. Two distinct regions, a plateau region and a sloping region, can be seen on the p-c isotherms and the amount of the desorbed hydrogen at 523 K was 4.9 mass%. The enthalpy of hydrogenation calculated using a van't Hoff plot was -46 kJ/mol H-2. The dehydriding reaction was proposed for the 3Mg(NH2)(2) + 12LiH system based on the obtained p-c isotherms and XRD profiles and chemical valences of Li, Mg, N, and H. In the plateau region on the p-c isotherm, Mg(NH2)(2), Li4Mg3(NH2)(2)(NH)(4) (tetragonal), and LiH phases coexist and the molar ratio of the Li4Mg3(NH2)(2)(NH)(4) phase increases (while those of Mg(NH,)2 and LiH phases decrease) with the amount of the desorbed hydrogen. On the other hand, the mixture of Li4+xMg3(NH2)(2-x)(NH)(4+x) + (8-x)LiH (0 <= x <= 2) is formed and the lattice volume of the Li4+xMg3(NH2)(2-x)(NH)(4+x) phase continuously increases with the amount of the desorbed hydrogen in the sloping region on the p-c isotherm. (c) 2006 Elsevier B.V. All rights reserved.