Hydrogen storage properties and structure of La1−xMgx(Ni1−yMny)3 intermetallics and their hydrides

‘Hybrid’ RNi3 (R = rare earth metal) crystal structures are built of the slabs of simpler types, CaCu5 and MgZn2. Different affinities of these slabs to hydrogen result in unusual “anisotropic” expansion of the RNi3 and R2Ni7 (R = La, Ce) structures upon hydrogenation. This work focuses on studies of the hydrogenation behaviour of LaNi3 and on the properties of the hydrides of the modified by Mg and Mn La–Ni alloys. The crystal structure of LaNi3D2.8 and the crystal structure and hydrogen storage behaviours of the La1.5Mg0.5Ni7 and La(Ni1−xMnx)3 (x = 0; 0.067; 0.133; 0.2; 0.267; 0.3; 0.333; 0.4) alloys were in focus. The deuteration of LaNi3 with PuNi3 type of structure leads to the formation of LaNi3D2.8 and is accompanied by a deformation of the metal matrix causing a change of the initial rhombohedral symmetry (space group R3¯m) to a monoclinic one (space group C2/m; a = 8.6408(7) Å, b = 4.9281(4) Å, c = 32.774(3) Å; β = 90.850(8)°; V = 1395.5(2) Å3). Similar to the earlier studied CeNi3D2.8, preferential occupation by deuterium atoms of the AB2 layers takes place, leading to the “anisotropic” expansion of the unit cell along [0 0 1] (Δc/c = 30.6%). 14 occupied D crystallographic sites have 4 chemically different types of metal-atom surroundings, including Ni4 (2), La2Ni2 (2), La3Ni (6), and La3Ni3 (4). Modification of the La–Ni alloys by magnesium and manganese leads to the formation of intermetallic compounds crystallising with the PuNi3, CeNi3, and Ce2Ni7-type structures. An ordered substitution of La by Mg in the MgZn2-type slabs was observed, causing a complete alteration of the hydrogenation behaviour of the original LaNi3 alloy. La1.5Mg0.5Ni7D9 isotropically expands upon its formation and leads to a substantial increase of the stability against hydrogenation-induced amorphisation. On the other hand, replacement of Ni by Mn leads to the change in crystal-structure type from PuNi3 to CeNi3 in the LaNi3−xMnx alloys (x > 0.1). An ordered substitution of Ni by Mn proceeds inside the RNi5 slabs only. This decreases the stability of the initial alloy against amorphisation on hydrogenation.