Crystal Structures of La–Mg–Ni_x (x=3–4) System Hydrogen Storage Alloys

抄録

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_2.5C0_0.5 (alloy T1) and the La_0.75Mg_0.25Ni_3.0C0_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. 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,….