THE CRYSTAL-STRUCTURE OF LAMBDA-AL4MN

The crystal structure of the lambda-Al4Mn phase, considered to be clos ely related to icosahedral quasicrystalline i-AlMn was determined by X -ray analysis. Metallic, hexagonal needles of the lambda-phase were ob tained by thermal decomposition of a mixture of Al and MnS. The struct ure crystallizes in P6(3)/m with a=28.382(9) Angstrom and c=12.389(2) Angstrom. The volume of the cell is 8643 Angstrom(3) and contains an a verage of 568.1 atoms. 106.6 atoms (18.8 at.%) are Mn, corresponding t o 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 1>3 sigma. There are 66 atoms in the asymmetric unit. 104 positions in the unit cell are occu pied with Mn and 482 with Al. 102 Mn have icosahedral coordination and two Mn a tri-capped trigonal prismatic coordination (CN 9). 36 Al ato ms have icosahedral coordination, 138 Al have a polar bi-capped pentag onal 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 Angstrom, Al-Mn 2.362-2.949 Angstrom and Al-Al 2.424-3.41 4 Angstrom. Additionally, one short distance of 2.180 Angstrom is betw een two partly occupied Al sites. The structure of the lambda-phase is a member of the I3-family, containing the I3-cluster, a fragment of t hree vertex connected icosahedra. The structure is composed of six cha ins of fused truncated I(2)-tetrahedra in the unit cell, aligned along c. The chains are B-type connected via primitive stacked pseudo I13-c lusters. A similar arrangement is found in the structure of mu-Al4.12M n. (C) 1997 Elsevier Science S.A.