Influence of crystallinity on the structural and hydrogenation properties of Mg2X phases (X=Ni, Si, Ge, Sn)

The hydrogen storage properties of nanocrystallized Mg2Ni prepared by ball-milling have been studied. Hydrogenation at 200 °C leads to the formation of the monoclinic low-temperature phase Mg2NiH4 containing a much larger amount of microtwinning than the hydride obtained by hydrogenation of well-crystallized Mg2Ni at the same temperature. The large amount of defects and dislocations present in the ball-milled alloys seems to favour the formation of microtwinning upon hydrogenation. Additionally, Mg2X compounds (X=Si, Ge, Sn) have been prepared by powder metallurgy and ball-milling. Cubic phases with antifluorite structure are obtained, except for Mg2Sn, for which a metastable rhombohedral phase is synthesized by ball-milling. Mg2Si decomposes into MgH2 and Si when ball-milled under hydrogen atmosphere, whereas Mg2Ge and Mg2Sn do not absorb hydrogen and are not decomposed.