Hydrogen-induced phase transitions in RNi 3 and RY 2Ni 9 (R = La, Ce) compounds

Structure stability of RNi ₃ and RY ₂Ni ₉ (R = La, Ce) during cycling of hydrogen absorption/desorption was investigated by both solid-H ₂ reaction and electrochemical measurements, in order to understand the mechanism responsible for the poor reversibility, and to reveal the differences between RNi ₃ and RY ₂Ni ₉ compounds. It has been found that, all of the cycled RNi ₃ and RY ₂Ni ₉ were subject to partial amorphization. This hydrogen-induced amorphization (HIA) occurred immediately in LaNi ₃, but proceeded gradually in CeNi ₃, LaY ₂Ni ₉ and CeY ₂Ni ₉. The HIA in these compounds is attributed to the RM ₂ subunits in their crystal structures.