ABSTRACT
The crystal structure of Pr3MgNi14D18 was determined by neutron diffraction. The determined structure of Pr3MgNi14D18 consisted of 89.0% Gd2Co7-type structure and 11.0% PuNi3-type structure. The lattice parameters of a and c of Gd2Co7-type structure were refined at 0.52903(7) nm and 3.90179(1) nm. The deuterium atoms were distributed among nine deuterium sites in both the CaCu5-type and MgZn2-type cells. The D2 occupancy in the Pr2Ni4 octahedral sites of the CaCu5-type cell was the largest (0.75) when compared with the other deuterium sites (<0.49). The deuterium content of the CaCu5-type cell showed 0.75 D/M, but the D/M value of the MgZn2-type cell was 1.53. The volume expansions during deuteration of the CaCu5-type and MgZn2-type cells were nearly equal. The cyclic hydrogenation property of Pr3MgNi14 is comparable to that of LaNi5. It is inferred that the similar expansion behavior of the CaCu5-type and MgZn2-type cells during deuteration is the origin of this cyclic stability.
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(Pabs/Pdes), 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.
ABSTRACT
Structural parameters of Pr(3)MgNi(14) after a cyclic hydrogen absorption-desorption process were investigated by X-ray diffraction. Pr(3)MgNi(14) consisted of two phases: 80% Gd(2)Co(7)-type structure and 20% PuNi(3)-type structure. The pressure-composition (P-C) isotherm of Pr(3)MgNi(14) indicates a maximum hydrogen capacity of 1.12 H/M (1.61 mass %) at 298 K. The cyclic property of Pr(3)MgNi(14) up to 1000 cycles was measured at 313 K. The retention rate of the sample was 87.5% at 1000 cycles, which compares favorably with that of LaNi(5). After 1000 cycles, the expansions of lattice parameters a and c and the lengths along the c-axes of the PrNi(5) and PrMgNi(4) cells of the Gd(2)Co(7)-type structures were 0.20%, 1.26%, 0.47%, and 3.68%, respectively. The metal sublattice expanded anisotropically after the cyclic test. The isotropic and anisotropic lattice strains can be refined by Rietveld analysis. The anisotropic and isotropic lattice strains were almost saturated at the first activation process and reached values of 0.2% and 0.1%, respectively, after 1000 cycles. These values are smaller by 1 order of magnitude than those of LaNi(5).
