In-situ X-ray diffraction study on LaNi₅ and LaNi_4.75Al_0.25 in the initial activation process

We investigated the hydriding mechanism of LaNi₅ and LaNi_4.75Al_0.25 by means of in-situ X-ray diffraction (XRD) measurements during their activation process. The XRD profiles were analyzed by the Rietveld method to evaluate the lattice strain and the crystallite size for both the solid solution phase and the hydride phase. The change of the crystallite size and the formation of lattice strain in the initial hydrogen absorption and desorption were studied. In LaNi₅, the lattice strain of the solid solution phase did not increase through the plateau region while large anisotropic lattice strain was introduced in the hydride phase formed. The crystallite size of both phases was around 100 nm, which did not change significantly during the first cycle. These results indicate that lattice strain introduced in the hydride phase did not influence the coexisting solid solution phase in the activation process. The hydride phase is produced within a coherent domain of about 100 nm independently of neighboring domains, forming anisotropic lattice strain only in itself. In LaNi_4.75Al_0.25, anisotropic lattice strain of the same type as in LaNi₅ was not observed over the first absorption and desorption processes. However, a lattice strain that forms only in the hydride phase in the plateau region was observed. The two phases have some contact and exert elastic lattice strain on each other.