Structural Evolution and Hydrogen Sorption Properties of YxNi2−yMny (0.825 ≤ x ≤ 0.95, 0.1 ≤ y ≤ 0.3) Laves Phase Compounds

The YxNi2−yMny system was investigated in the region 0.825 ≤ x ≤ 0.95, 0.1 ≤ y ≤ 0.3. The alloys were synthesized by induction melting and corresponding annealing. The substitution of Mn for Ni (y = 0.1) favors the formation of a C15 structure with disordered Y vacancies against the superstructure of Y0.95Ni2. For y = 0.2 and 0.3, Mn can substitute in both Y and Ni sites. Single-phase compounds with a C15 structure can be formed by adjusting both the Y and Mn contents. Their hydrogen absorption–desorption properties were measured by pressure–composition isotherm (PCI) measurements at 150 °C, and the hydrides were characterized at room temperature by X-ray diffraction and TG–DSC experiments. The PCIs show two plateaus corresponding to the formation of crystalline and amorphous hydrides. The heating of the amorphous hydrides leads to an endothermic desorption at first and then a recrystallization into Y(Ni, Mn)3 and YHx phases. At higher temperatures, the Y hydride desorbs, and a recombination into a Y(Ni, Mn)2 Laves phase compound is observed. For y = 0.1, vacancy formation in the Y site and partial Mn substitution in the Ni site enhance the structural stability and suppress the hydrogen-induced amorphization (HIA). However, for a larger Mn content (y ≥ 0.2), Mn substitutes also in the Y sites at the expense of Y vacancies. This yields worse structural stability upon hydrogenation than for y = 0.1, as the mean ratio r(Y, Mn)/r(Ni/Mn) becomes larger than for y = 0.1 r(Y, ☐)/r(Ni/Mn).