Crystal structure of hydrogen storage alloys, La-Mg-Ni _x(x = 3∼4) system

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 AB ₅-type alloys. The crystal structures of La _0.7Mg _0.3Ni _2.5Co _0.5 (alloy T1) and La _0.75Mg _0.25 Ni _3.0Co _0.5 (alloy T2) were investigated using ICP, SEM-EDX and XRD. We found that the alloy T1 consisted of Ce ₂Ni ₇-type La ₃Mg (Ni, Co) ₁₄ and PuNi ₃-type La ₂Mg (Ni, Co) ₉ phases and that the alloy T ₂ consisted of Ce ₂Ni ₇-type La ₃Mg (Ni, Co) ₁₄ and Pr ₅ Co ₁₉-type La ₄Mg(Ni, Co) ₁₉ phases. These alloys system has layered structure and shows polytypism that is originated from difference in stacking of some [CaCu ₅]-type layers and one [MgZn ₂]-type layer along c-axis. Crystal structure of La ₃Mg(Ni, Co) ₁₄ is hexagonal 2H-Ce ₂Ni ₇-type, a = 0.5052(1) nm, c = 2.4245(3) nm. La ₂Mg(Ni, Co) ₉ is trigonal 3R-PuNi ₃-type, a = 0.5062 (1) nm, c=2.4500(2) nm. La ₄Mg(Ni, Co) ₁₉ is 2H-Pr ₅Co ₁₉-type, a = 0.5042(2) nm, c = 3.2232(5) nm. In these all structure, La-La distance in the [CaCu ₅] layer was 0.38∼0.40 nm but that in the [MgZn ₂] layer was 0.32 nm. It was also found that Mg occupied the La site in the [MgZn ₂] layer. Selective occupation of Mg at the La site in the [MgZn ₂] 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 La _n+1MgNi _5n+4 where n = 0, 1, 2, 3, 4.