Abstract
The magnesium-rich composition Gd13Ni9Mg78 was synthesized from its constituent elements in sealed tantalum tubes in an induction furnace. X-ray diffraction, electron probe microanalysis and dark-field transmission electron microscopy (TEM) images revealed a new compound with a composition ranging from Gd10–15Ni8–12Mg72–78 and low crystallinity. In order to increase the crystallinity, different experimental conditions were investigated for numerous compounds with the initial composition Gd13Ni9Mg78. In addition, several heat treatments (from 573 to 823K) and cooling rates (from room temperature quenched down to 2Kh−1) have been tested. The best crystallinity was obtained for the slower cooling rates ranging from 2 to 6Kh−1. From the more crystallized compounds, the structure was partially deduced using TEM and an average cubic structure with lattice parameter a=4.55Å could be assumed. A modulation along both a∗ and b∗ axis with vectors of modulation q1=0.42a∗ and q2=0.42b∗ was observed. This compound, so-called Gd13Ni9Mg78, absorbs around 3wt.% of hydrogen at 603K, 30bars and a reasonable degree of reversibility is possible, because after the first hydrogenation, irreversible decomposition into MgH2, GdH2 and NiMg2H4 has been shown. The pathway of the reaction is described herein. The powder mixture after decomposition shows an interesting kinetics for magnesium without ball milling.
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