Structural, magnetic, electrochemical and hydrogen absorption properties of GdyMg2−yNi4−xAlx compounds with 0.4 < y < 2 and 0 < x < 1.2

A solid solution with the C15b structure (MgCu4Sn-type) exists as a series from GdNi2 to Gd0.4Mg1.6Ni4 (Gd0.2Mg0.8Ni2). These compounds were successfully elaborated by (i) Mechanical Alloying (MA) and (ii) melting followed by a subsequent annealing. It was also possible to synthesise the solid solution GdMgNi4−xAlx with x up to 1.2. The product is highly dependent on the elaboration route which induces a drastic change in both chemical and physical properties (i.e. magnetism, electrochemistry, and structural disorder). A direct relationship between all these property modifications was established. Finally, it is shown that different compounds have different behaviours towards hydrogen sorption regarding both the hydrogen uptake and the thermodynamic results

Crystal Structure and Cyclic Hydrogenation Property of Pr₄MgNi₁₉

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

Crystal Structure and Cyclic Hydrogenation Property of Pr₄MgNi₁₉

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