Hydrogen absorption in CeNiAl: Formation of crystalline and amorphous hydride phases

Hydriding studies on mixed valent intermetallic compound, CeNiAl, have been carried out. Absorption as well as desorption routes have been employed to find the composition range of various hydride phases. Stable hydride composition, CeNiAlHx, with a maximum value of x=3.2, is obtained. No single phase crystalline hydride could be stabilized for x<1.9. Further, it is found that any attempt to stabilize composition(s) with x<1.9, by desorbing hydrogen from higher hydrides, results in the formation of an amorphous hydride phase. Hydrogen induced amorphization is being reported for the first time in Fe2P structure. X-ray diffraction studies of the hydride with x=3.2, as well as neutron diffraction studies on a deuteride of the same composition show that the hydriding (deuteriding) results in transformation from the Fe2P type hexagonal (space group P6¯2m) to AlB2 type hexagonal (space group P6/mmm) structure. Structural parameters and D atom occupancies are derived from the neutron diffraction measurements on CeNiAlD3.2. Magnetization measurements show that the structural transformation is also accompanied by a transformation from the mixed valent cerium in the parent compound to trivalent Ce in the hydride phases.