The crystal structure of λ-Al 4Mn

Abstract

The crystal structure of the λ-Al 4Mn phase, considered to be closely related to icosahedral quasicrystalline i-AlMn was determined by X-ray analysis. Metallic, hexagonal needles of the λ-phase were obtained by thermal decomposition of a mixture of Al and MnS. The structure crystallizes in P6 3/m with a=28.382(9) A ̊ and c=12.389(2) A ̊ . The volume of the cell is 8643 Å 3 and contains an average of 568.1 atoms. 106.6 atoms (18.8 at.%) are Mn, corresponding to a calculated density of 3.517(6) g cm −3. The final R-values were R u=9.2% and R w=5.9% for 2508 reflections with I>3 σ. There are 66 atoms in the asymmetric unit. 104 positions in the unit cell are occupied with Mn and 482 with Al. 102 Mn have icosahedral coordination and two Mn a tri-capped trigonal prismatic coordination (CN 9). 36 Al atoms have icosahedral coordination, 138 Al have a polar bi-capped pentagonal prismatic, 72 Al have a bi-equatorial capped pentagonal prismatic and 168 Al a mixed capped pentagonal prismatic with CN 13 or 14, 4 Al a CN 16 Frank-Kasper polyhedron, and 52 Al an irregular coordination polyhedron. Six Al positions are partly occupied and five Al positions show disorder with small amounts of Mn. The distance ranges are MnMn 2.689–2.784 Å, AlMn 2.362–2.949 Å and AlAl 2.424–3.414 Å. Additionally, one short distance of 2.180 Å is between two partly occupied Al sites. The structure of the λ-phase is a member of the I3-family, containing the I3-cluster, a fragment of three vertex connected icosahedra. The structure is composed of six chains of fused truncated 1(2)-tetrahedra in the unit cell, aligned along c. The chains are B-type connected via primitive stacked pseudo I13-clusters. A similar arrangement is found in the structure of μ-Al 4.12Mn.