Structure of a new ternary compound with high magnesium content, so-called Gd13Ni9Mg78

Abstract The magnesium-rich composition Gd 13 Ni 9 Mg 78 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 Gd 10–15 Ni 8–12 Mg 72–78 and low crystallinity. In order to increase the crystallinity, different experimental conditions were investigated for numerous compounds with the initial composition Gd 13 Ni 9 Mg 78 . In addition, several heat treatments (from 573 to 823 K) and cooling rates (from room temperature quenched down to 2 K h −1 ) have been tested. The best crystallinity was obtained for the slower cooling rates ranging from 2 to 6 K h −1 . From the more crystallized compounds, the structure was partially deduced using TEM and an average cubic structure with lattice parameter a = 4.55 A could be assumed. A modulation along both a ∗ and b ∗ axis with vectors of modulation q 1 = 0.42 a ∗ and q 2 = 0.42 b ∗ was observed. This compound, so-called Gd 13 Ni 9 Mg 78 , absorbs around 3 wt.% of hydrogen at 603 K, 30 bars and a reasonable degree of reversibility is possible, because after the first hydrogenation, irreversible decomposition into MgH 2 , GdH 2 and NiMg 2 H 4 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.