Crystal structures of La-Mg-Ni_x (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 AB₅-type alloys. We investigated the crystal structures of the La_0.7Mg_0.3Ni _2.5CO_0.5 (alloy T1) and the La_0.75Mg _0.25Ni_3.0CO_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 La ₂Mg(Ni, Co)₉ phases, and alloy T2 consisted of Ce ₂Ni₇-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 [MgZn ₂]-type layer along the c-axis. The crystal structure of La ₃Mg(Ni, Co)₁₄ was of a hexagonal 2H-Ce₂Ni ₇-type with a = 0.5052(1)nm, and c = 2.4245(3)nm. La ₂Mg(Ni, Co)₉ had a trigonal 3R-PuNi₃-type structure with a = 0.5062(1)nm, and c = 2.4500(2)nm. La₄Mg(Ni, Co)₁₉ 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 La_n+1MgNi_5n+4, where n = 0,1,2,3,4,....