La-Mg-Nix(x=3~4)系水素吸蔵合金の結晶構造

New alloys of La-Mg-Ni (Ni/(La+Mg)=3~4) system absorb and desorb hydrogen at room temperature, and the hydrogen capacity is higher than conventional AB5-type alloys. The crystal structures of La0.7Mg0.3Ni2.5Co0.5 (alloy T1) and La0.75Mg0.25Ni3.0Co0.5 (alloy T2) were investigated using ICP, SEM-EDX and XRD. We found that the alloy T1 consisted of Ce2Ni7-type La3Mg(Ni, Co)14 and PuNi3-type La2Mg(Ni, Co)9 phases and that the alloy T2 consisted of Ce2Ni7-type La3Mg(Ni, Co)14 and Pr5Co19-type La4Mg(Ni, Co)19 phases. These alloys system has layered structure and shows polytypism that is originated from difference in stacking of some [CaCu5]-type layers and one [MgZn2]-type layer along c-axis. Crystal structure of La3Mg(Ni, Co)14 is hexagonal 2H-Ce2Ni7-type, a=0.5052(1) nm, c=2.4245(3) nm. La2Mg(Ni, Co)9 is trigonal 3R-PuNi3-type, a=0.5062(1) nm, c=2.4500(2) nm. La4Mg(Ni, Co)19 is 2H-Pr5Co19-type, a=0.5042(2) nm, c=3.2232(5) nm. In these all structure, La-La distance in the [CaCu5] layer was 0.38~0.40 nm but that in the [MgZn2] layer was 0.32 nm. It was also found that Mg occupied the La site in the [MgZn2] layer. Selective occupation of Mg at the La site in the [MgZn2] layer makes the alloy stable in repeated reaction cycles with hydrogen. This alloy system has formed an agent group is described by the general formula Lan+1MgNi5n+4 where n=0, 1, 2, 3, 4…….