Rich magnesium ternary compound so-called LaCuMg8 derived from La2Mg17. Structure and hydrogenation behavior

The magnesium metal rich composition La10TM10Mg80 (with TM = Ni and Cu) were synthesized from the elements in sealed tantalum tubes in an induction furnace. The results show differences depending on the transition metal used. The sample with Ni demixed into ternary and binary compounds with composition close to LaMgNi2, La2Mg17 and Mg2Ni. Only the sample with copper forms a ternary compound. Crystal structure of this new compound was determined by X-ray diffraction on single crystals with an exact formulation La1.744(5)Cu1.53(5)Mg15.73(4). It crystallizes with the La2Mg17 structure type, space group P63/mmc, a = 10.1254(2) Å and c = 10.0751(2) Å. The Mg atoms form hexagonal tubes along the c-axis filled by La and Mg. The Cu atoms are located in these tubes on or around the La and Mg positions, inducing disorder. This compound so-called LaCuMg8 absorbs around 2 wt% of hydrogen at 603 K, 30 bar and reversibility is possible. Nevertheless, after the first hydrogenation, decomposition into MgH2, LaH3 and MgCu2 have been shown. The pathway of the reaction is described herein.

Figure optionsDownload as PowerPoint slideResearch highlights
► Study of the Mg rich region of the ternary diagram La–Cu–Mg.
► The new compound La1.74Cu1.53Mg15.73, so-called LaCuMg8, was characterized.
► La1.74Cu1.53Mg15.73 crystallizes with a disordered variant of the La2Mg17 structure.
► Absorption of hydrogen induces decomposition after the first cycle.
► After decomposition, hydrogen absorption/desorption cycling is possible.