Formation of lattice strain in MmNi4.30-xCoxAl0.30Mn0.40 (x=0,0.75) during hydrogenation

We investigated the hydrogenation mechanism of MmNi4.30-xCoxAl0.30Mn0.40 (Mm: misch metal, x=0, 0.75) by means of in situ X-ray diffraction (XRD) measurement. Formation of lattice strain and change of crystallite size for both the solid solution phase and the hydride phase were analyzed by the Rietveld refinement of XRD profiles. In MmNi4.30Al0.30Mn0.40, the isotropic lattice strain showed less than 0.8% for both the phases through the whole range of hydrogen content, and anisotropic strain was not observed. The crystallite size for both the phases showed around 100 nm, which did not change significantly during hydrogenation and dehydrogenation. In MmNi3.55Co0.75Al0.30Mn0.40, the isotropic strain for both the phases depended on the fraction of each phase. It reached the maximum of 1.2% around the phase fraction of 0.5, while anisotropic strain was not observed. The crystallite size for both the phases showed around 100 nm through the whole range of hydrogen content. This result indicates that the solid solution phase changes to the hydride phase domain by domain during hydrogenation.