Crystal structures of La-Mg-Nix (x=3-4) system hydrogen storage alloys

The new alloys of the La-Mg-Ni (Ni/(La + Mg) = 3-4) system absorb and desorb hydrogen at room temperature, and their hydrogen storage capacities are greater than those of conventional AB(5)-type alloys. We investigated the crystal structures of the La(0.7)Mg(0.3)Ni(2.5)C0(0.5) (alloy T1) and the La(0.75)Mg(0.25)Ni(3.0)C0(0.5) (alloy T2) using ICP, SEM-EDX and XRD. We found that alloy T1 consisted of Ce2Ni7-type La3Mg(Ni, Co)(14) and PuNi3-type La2Mg(Ni, Co)(9) phases, and alloy T2 consisted of Ce2Ni7-type La3Mg(Ni, Co)(14) and Pr5Co19-type La4Mg(Ni, Co)(19) phases. These alloy systems had layered structures and showed polytypism that originated from differences in the stacking patterns of the units, which were composed of several [CaCu5]-type layers and a single [MgZn2]-type layer along the c-axis. The crystal structure of La3Mg(Ni, Co)(14) was of a hexagonal 2H-Ce2Ni7-type with a = 0.5052(1)nm, and c = 2.4245(3)nm. La2Mg(Ni, Co)(9) had a trigonal 3R-PuNi3-type structure with a = 0.5062(1)nm, and c = 2.4500(2)nm. La2Mg(Ni, Co)(9) had a hexagonal 2H-Pr5Co19-type structure with a = 0.5042(2) nm and c = 3.2232(5) nm. In all these structures, the La-La distance of the [CaCu5] layer was 0.38-0.40 nm and that of the [MgZn2] layer was 0.32 nm. We also found that Mg occupied the La site in the [MgZn2] layer. Selective occupation by Mg of the La site in the [MgZn2] layer makes the alloy stable against repeated reaction cycles with hydrogen. The alloy system that forms this material group can be described by the general formula Lan+1 MgNi5n+4, where n = 0, 1, 2, 3,4,....