Lattice expansion for MmNi_{4.30 - x}Co_xAl_0.30Mn_0.40(x = 0, 0.75) studied by in situ X-ray diffraction

Hydrogenation properties of MmNi_{4.30 - x} Co_x Al_0.30 Mn_0.40 (x = 0, 0.75) were investigated by means of in situ X-ray diffraction (XRD). The XRD profiles were analyzed by the Rietveld method to evaluate the change of lattice parameters and lattice volume. Hydrogenation-dehydrogenation of these alloys proceeded through two phases between the solid solution phase and the hydride phase. Both the phases of MmNi_4.30Al_0.30Mn_0.40 had the hexagonal lattice with a space group of P6/mmm and those of MmNi_3.55Co_0.75Al_0.30Mn_0.40 had the orthorhombic lattice with Cmmm after initial activation. In the two-phase co-existing region, lattice volumetric ratio of both the phases showed 19% for MmNi_4.30Al_0.30Mn_0.40, while that became 10% by partial substitution of Co for Ni with anisotropic lattice expansion. Anisotropic lattice strain related with high dislocation density was not introduced through formation of the hydride phase.