Structural and In Situ X-ray Diffraction Study of Hydrogenation of Ca_x Mg_1−x Ni₂ (0 ≤ x ≤ 1)

In the quasi-binary system CaNi₂-MgNi₂ solid-solutions Ca_x Mg_1−x Ni₂ (0 ≤ x ≤ 1) were prepared from the elements. They crystallize in the hexagonal Laves phase type (MgNi₂, C36) for x ≤ 0.33 (P6₃ /mmc, a = 482.51(7) pm, c = 1582.1(3) pm for x = 0, a = 482.59 (3), c = 1583.1(1) for x = 0.33) and in the cubic Laves phase type (MgCu₂, C15) for 0.33 < x (Fd⁻ 3m, a = 697.12(3) pm for x = 0.5, a = 705.11(2) pm for x = 0.67, a = 724.80(2) pm for x = 1). After hydrogenation in an autoclave the X-ray diffraction patterns changed completely. Reflections assigned to CaNiH_3, and Ni and Rietveld refinement confirmed this. The hydrogenation properties of Ca_x Mg_1−x Ni₂ (0 ≤ x ≤ 1) compounds were also studied in situ by X-ray powder diffraction. In situ X-ray powder diffraction of Ca_x Mg_1−x Ni₂ (0 ≤ x ≤ 1) compounds under 0.3 MPa hydrogen gas flow (15 sccm), data collected on a Rigaku SmartLab diffractometer in an Anton Paar XRK 900 Reactor Chamber using Cu-Kα1 radiation. Scanning electron microscopy and EDX spectroscopy confirmed the entitled materials and elemental composition, respectively. From the Transmission electron microscopy and Selected area electron diffraction concluded that the Ca_x Mg_1−x Ni₂ (0 ≤ x ≤ 1) compounds were crystalline.