A study on hydrogen-induced amorphization in the LaNi system

In order to investigate the mechanism of hydrogen-induced amorphization, the structure changes by hydrogen absorption in La2Ni7, LaNi2, LaNi1.5 and LaNi compounds are investigated by X-ray diffraction. Also thermal stabilities of the hydrogenated samples are examined by DSC and hydrogen desorption technique. In case of La2Ni7 compound, the severe lattice distortion is found as hydrogen is absorbed. However, as the hydrogen absorption-desorption cycle is repeated, the amorphization takes place. LaNi2 and LaNi1.5 compounds show the complete absence of diffraction lines when hydrogenated at room temperature, i.e. the hydrogen-induced amorphization has occurred. The DSC curve of the hydrogen-induced amorphous alloy shows a small exothermic peak related to the precipitation of LaH2 and LaNi5. LaNi compound may transform into another crystalline structure by hydrogenation at room temperature, which is different from that of original LaNi compound. It is suggested that the amorphization of a compound is dependent upon the stability of the compound and the decomposition rate to LaH2 and LaNi5. The mechanism of the amorphization is related to the lattice distortion caused by the motion of the metal atoms so that the surroundings of the hydrogen atoms in the amorphous state resemble those of hydrogen atoms in LaH2.