In situ XRD for pseudo Laves phases hydrides highlighting the remained cubic structure

The hydrogenation behaviour of the Gd_0.5Y_0.5Ni_3.75Al_0.25Mg compound was studied by in situ X-ray diffraction under hydrogen pressure and at room temperature. The presence of gadolinium induce an exchange between the 4a (occupied by RE) and 4c sites (occupied by Mg). Nevertheless, only the Gd atoms are mixed by Mg atoms and no occurrence of Y/Mg mixing can be observed. The exchange is then directly correlated with the 4f electrons. Moreover, in the compounds without Gd (i.e. YNi_3.75Al_0.25Mg) no exchange was observed. The structure of the metal hydrides was carefully investigated by refining the in situ XRD patterns obtained under various H₂ pressures. It was concluded that the structure remains cubic during the sorption process with an increase of the cell volume close to 13%, in agreement with the maximum sorption capacity (i.e. 3H/formula unit). No variation of the exchange rate between the hydride and the initial intermetallic can be highlighted. The changes of both the crystallite size and lattice strains during the sorption process (i.e. along the PC isotherms) indicated that a decrease of the crystallinity is observed but no HIA process occurred.