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
0.7Mg
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 Ce
2Ni
7-type La
3Mg(Ni, Co)
14 and PuNi
3-type La
2Mg(Ni, Co)
9 phases, and alloy T2 consisted of Ce
2Ni
7-type La
3Mg(Ni, Co)
14 and Pr
5Co
19-type La
4Mg(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 [CaCu
5]-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-Ce
2Ni
7-type with
a=0.5052(1) nm, and
c=2.4245(3) nm. La
2Mg(Ni, Co)
9 had a trigonal 3R-PuNi
3-type structure with
a=0.5062(1) nm, and
c=2.4500(2) nm. La
4Mg(Ni, Co)
19 had a hexagonal 2H-Pr
5Co
19-type structure with
a=0.5042(2) nm and
c=3.2232(5) nm. In all these structures, the La–La distance of the [CaCu
5] layer was 0.38–0.40 nm and that of the [MgZn
2] layer was 0.32 nm. We also found that Mg occupied the La site in the [MgZn
2] layer. Selective occupation by Mg of the La site in the [MgZn
2] 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,….
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