Effect of Mg substitution on crystal structure and hydrogenation of Ce₂Ni₇-type Pr₂Ni₇

The effect of Pr being substituted by Mg in Pr₂Ni₇ with a Ce₂Ni₇-type structure was investigated by X-ray diffraction (XRD) and pressure−composition (P−C) isotherm measurements. The maximum hydrogen capacity of Pr₂Ni₇ reached 1.24 H/M in the first absorption process. However, 0.61 H/M hydrogen remained in the sample after the first desorption and the reversible hydrogen capacity decreased to 0.63 H/M. Severe peak broadening was observed in the XRD profile of Pr₂Ni₇H_5.4 after the first P−C isotherm cycle. The metal sublattice of Pr₂Ni₇H_5.4 is deformed and changes from the Ce₂Ni₇-type structure to a lower symmetry during hydrogenation, with no detection of an amorphous phase. Pr_1.5Mg_0.5Ni₇ consists of two phases: 80% Gd₂Co₇-type and 20% PuNi₃-type phases. Mg substitution leads to the relative stability of the Gd₂Co₇-type and PuNi₃-type structures. The Gd₂Co₇-type and PuNi₃-type structures are retained after the P-C isotherm. The reversible hydrogen capacity reached 1.05 H/M. The structural change during the hydrogen absorption−desorption cycle and the hydrogenation characteristics are changed by Mg atoms replacing Pr in the MgZn₂-type cell. - Graphical abstract: The maximum hydrogen capacity is 1.2 H/M in the first absorption process and the reversible capacity is 0.63 H/M.