Crystal structures of La-Mg-Nix (x = 3-4) system hydrogen storage alloys

Hiroshi Hayakawa, Etsuo Akiba, Midori Gotoh, Tatsuoki Kohno

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Abstract

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 La0.7Mg0.3Ni 2.5CO0.5 (alloy T1) and the La0.75Mg 0.25Ni3.0CO0.5 (alloy T2) using ICP, SEM-EDX and XRD. We found that alloy T1 consisted of Ce2Ni7-type La3Mg(Ni, CO)14 and PuNi3-type La 2Mg(Ni, Co)9 phases, and alloy T2 consisted of Ce 2Ni7-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 [MgZn 2]-type layer along the c-axis. The crystal structure of La 3Mg(Ni, Co)14 was of a hexagonal 2H-Ce2Ni 7-type with a = 0.5052(1)nm, and c = 2.4245(3)nm. La 2Mg(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 Lan+1MgNi5n+4, where n = 0,1,2,3,4,....

Original languageEnglish
Pages (from-to)1393-1401
Number of pages9
JournalMaterials Transactions
Volume46
Issue number6
DOIs
Publication statusPublished - Jun 1 2005

All Science Journal Classification (ASJC) codes

  • Materials Science(all)
  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering

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