Crystal structure and cyclic properties of hydrogen absorption–desorption in Pr2MgNi9

We investigated the crystal structure and cyclic hydrogen absorption–desorption properties of Pr2MgNi9. The structural model is based on the PuNi3-type structure; the Mg atom is assumed to substitute for the Pr site in an MgZn2-type cell. The refined lattice parameters were determined from X-ray diffraction. A wide plateau region was observed in the P–C (pressure composition) isotherm at 298 K. The maximum hydrogen capacity reached 1.12 H/M (1.62 mass%) under a hydrogen pressure of 2.0 MPa. After 1000 hydrogen absorption–desorption cycles, the hydrogen capacity was superior to that of LaNi5 (82%). Anisotropic lattice strain occurred in the hydriding process. The anisotropic peak-broadening vector was determined to be <001>. The calculated anisotropic lattice strains of the initial cycle and after 1000 cycles were far smaller than those of LaNi5.

► The substituted Mg atom in the Pr2MgNi9 was located only at 6c site in the MgZn2-type cell. ► The maximum hydrogen capacity of the initial cycle of Pr2MgNi9 reached 1.12 H/M at 298 K. ► After 1000 cycles, the hydrogen capacity was kept at 85% of that of the initial cycle.