Crystal Structure and Cyclic Hydrogenation Property of Pr₄MgNi₁₉

Abstract

The hydrogen absorption–desorption property and the crystal structure of Pr₄MgNi₁₉ was investigated by pressure–composition isotherm measurement and X-ray diffraction (XRD). Pr₄MgNi₁₉ consisted of two phases: 52.9% Ce₅Co₁₉-type structure (3R) and 47.0% Gd₂Co₇-type structure (3R). Sm₅Co₁₉-type structure (2H) and Ce₂Ni₇-type structure (2H) were not observed in the XRD profile. The Mg atoms substituted at the Pr sites in a MgZn₂-type cell. The maximum hydrogen capacity reached 1.14 H/M (1.6 mass%) at 2 MPa. The hysteresis factor, Hf = ln­(<i>P</i>_abs/<i>P</i>_des), was 1.50. The cyclic hydrogenation property of Pr₄MgNi₁₉ was investigated up to 1000 absorption–desorption cycles. After 250, 500, 750, and 1000 cycles, the retention rates of hydrogen were reduced to 94%, 92%, 91%, and 90%, respectively. These properties were superior to those of Pr₂MgNi₉ and Pr₃MgNi₁₄