Heat treatment effect on structural evolution and hydrogen sorption properties of Y0.5La0.2Mg0.3−xNi2 compound

The effect of heat treatment on phase occurrence, crystal structures and hydrogen sorption properties of Y 0.5 La 0.2 Mg 0.3− x Ni 2 compounds has been investigated. The targeted compounds were synthesized through induction melting and processed heat treatment at 700 and 900 °C, respectively. Phase occurrence and structural properties were studied by X-ray powder diffraction (XRD). The global compositions and phase compositions have been determined by inductively coupled plasma-optical emission spectrometer (ICP-AES) and electron probe micro-analysis (EPMA) respectively. The hydrogenation properties were characterized by pressure–composition isotherm measurements and cycle curves at 25 °C. The annealing temperature of 700 °C could boost the production of A 2 B 4 -type (Y, La, Mg) 2 Ni 4 (A = rare earth, B = transition metal) phase, the Mg evaporation leads to the formation of La-doped AB 3 phase. The La substitution limit inside the C 15 b Y 0.7− z La z Mg 0.3 Ni 4 phase is estimated around z = 0.16. An exchange phenomenon has been confirmed in Y 0.5 La 0.2 Mg 0.3− x Ni 2 compounds meaning that partial Y atoms share 4c sites with Mg. The substitution of La raised the atomic-size ratio r A / r B , resulting in a hydrogen-induced disproportionation (HID) of C 15 b -type A 2 B 4 and C 15-type AB 2 phases upon hydrogen absorption and desorption. New A 2 B 4 phases with more Mg content (close to YMgNi 4 ) prefer to form upon HID. The annealing process at 900 °C and Mg evaporation stimulate the generation of a non-stoichiometric AB 2.6 Y 0.48 La 0.19 Mg 0.10 Ni 2 compound, which shows a high reversible hydrogen absorption capacity of 1.63 wt%. Graphical abstract