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

Hiroshi Hayakawa, Etsuo Akiba, Midori Gotho, Tatsuoki Kohno

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Abstract

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.

Original languageEnglish
Pages (from-to)170-178
Number of pages9
JournalNippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals
Volume69
Issue number1
DOIs
Publication statusPublished - Jan 1 2005

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All Science Journal Classification (ASJC) codes

  • Condensed Matter Physics
  • Mechanics of Materials
  • Metals and Alloys
  • Materials Chemistry

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