Synthesis and crystal structure of a Pr₅Ni₁₉ superlattice alloy and its hydrogen absorption-desorption property

The intermetallic compound Pr₅Ni₁₉, which is not shown in the Pr-Ni binary phase diagram, was synthesized, and the crystal structure was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Two superlattice reflections with the Sm₅Co ₁₉-type structure (002 and 004) and the Pr₅Co ₁₉-type structure (003 and 006) were observed in the 2θ region between 2° and 15° in the XRD pattern using Cu Kα radiation. Rietveld refinement provided the goodness-of-fit parameter S = 6.7 for the Pr₅Co₁₉-type (3R) structure model and S = 1.7 for the Sm₅Co₁₉-type (2H) structure model, indicating that the synthesized compound has a Sm₅Co₁₉ structure. The refined lattice parameters were a = 0.50010(9) nm and c = 3.2420(4) nm. The high-resolution TEM image also clearly revealed that the crystal structure of Pr₅Ni₁₉ is of the Sm₅Co₁₉ type, which agrees with the results from Rietveld refinement of the XRD data. The P-C isotherm of Pr₅Ni₁₉ in the first absorption was clearly different from that in the first desorption. A single plateau in absorption and three plateaus in desorption were observed. The maximum hydrogen storage capacity of the first cycle reached 1.1 H/M, and that of the second cycle was 0.8 H/M. The 0.3 H/M of hydrogen remained in the metal lattice after the first desorption process.