Heat treatment effect on structural evolution and hydrogen sorption properties of Y <sub>0.5</sub> La <sub>0.2</sub> Mg <sub> 0.3 <i>−x</i> </sub> Ni <sub>2</sub> compound
Hao Shen, Lijun Jiang, Ping Li, Huiping Yuan, Zhinian Li, Junxian Zhang
Abstract
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%.