Structural Properties of La(NiMMn)_5.61 Hydrogen Storage Alloy

All of the over-stoichiometric La(NiMMn)_5.61(M=Ni,Cu,Al,Fe,Sn) hydrogen storage alloys were studied by neutron powder diffraction. The result shows that the crystal structures of all hydrogen storage alloys are CaCu₅ type with space group of P6/mmm, but the occupancy site is different from AB₅ compounds with the CaCu₅ structure. In AB₅-type alloys, A atoms occupy 1a site in P6/mmm space group, B atoms occupy 2c and 3g sites. In the over-stoichiometric AB_5+x alloys, B atoms occupy site 2e in P6/mmm space group, orient along the c-axis and replace A atoms. Moreover, the B atoms forming a hexagon in the z=0 plane around the dumbbell are no longer on site 2c but shrink into position 6l. The occupation number of B atoms on site 2e and 6l goes up while that of B atoms on site 2c goes down with increasing the degree of over-stoichiometric, and 3g site is fully occupied by B atoms from beginning to end. As compare with AB₅-type stoichiometric alloys, the induction of the dumbbells decreases a-axis while increases c-axis, ultimately decreases unit cell volume.