The Li-Mg-N-H system has the property of reversible reaction with hydrogen between hydrogenation and dehydrogenation (Mg3N2 + 4Li3N + 12H2 ↔ 3Mg(NH2)2 + 12LiH). At the several dehydrogenation stages of p-c isotherm measurement at 523 K, the structural change was investigated using the synchrotron X-ray diffraction. There are two regions in p-c isotherm of the Li-Mg-N-H system, i.e. plateau and sloping region. In the plateau region, Mg(NH2) 2 and Li3Mg3(NH2)(NH)4 coexist. In the sloping region, the intermediate phase Li3+3yMg 3(NH2)1-y(NH)4+2y changes continuously from Li3Mg3(NH2)(NH)4 to Li2Mg(NH)2. The chemical composition of the intermediate phase was estimated from the amount of desorbed hydrogen by p-c isotherm and the atomic ratio of Mg and N by Rietveld analysis. The crystal structure of the intermediate phase, Li3+3yMg3(NH 2)1-y(NH)4+2y (space group: I222), was determined. Because all these intermediate structures are similar to anti-CaF2-type, it is deduced that the dehydrogenation process are caused by the diffusion of Li+ to cation sites of Mg(NH 2)2. The analysis of structural change clarified the dehydrogenation process that is accomplished by the diffusion of Li+ and Mg2+ without N atom diffusion.
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