Sn-containing (La,Mm)Ni5-xSnxH5-6 intermetallic hydrides: thermodynamic, structural and kinetic properties

This work focused on studies of the phase composition and crystal structure of LaxMm1-xNi4.7Sn0.3 (x=0, 0.5 and 1.0) alloys, where Mm denotes La-rich mischmetal produced by Norsk Hydro ASA. The alloys have been studied using conventional powder and synchrotron X-ray diffraction (XRD), powder neutron diffraction and scanning electron microscopy (SEM). After annealing at 950 deg. C the alloys were nearly single-phase materials with small precipitations of RENiSn equiatomic intermetallics formed at grain boundaries. Hydrogen absorption-desorption studies at 298, 323 and 348 K showed (a) a 'flat' structure of the pressure-composition isotherms; (b) a rather broad range of working H storage capacities; (c) an increase of the plateau pressures and decrease in both enthalpy and entropy changes following an increase of the Mm content in the alloy; and (d) fast formation-decomposition rate. Desorption isotherms were approximated by a recently developed model of phase equilibria in the metal-hydrogen systems. In situ powder neutron diffraction studies of the system MmNi4.7Sn0.3-D2 show that in the region of α-solid solution deuterium insertion leads to a gradual increase of the unit cell volume reaching 1.9%. In the hexagonal β-MmNi4.7Sn0.3 D4.51 deuteride D atoms occupy two of four available types of tetrahedral interstices including 6m RE2(Ni,Sn)2 and 12n RE(Ni,Sn)3