REFINEMENT OF THE CRYSTAL-STRUCTURE PI-AL9FEMGSI5

The crystal structure of the quaternary so-called si-phase. which was originally solved from photographic single crystal data, has been refi ned from single crystal automatic four circle counter data. Single cry stals were obtained from the residue of a slowly cooled melt of nomina l composition (in at %) Al74.1Fe 0.5Mg8.4Si17.0, which was dissolved i n butanol. The composition of the pi-phase crystal was Al9Fe1Mg3.5Si50 . The pi-phase crystallizes in a unique structure type (Al9FeMg3Si5-ty pe) with the noncentrosymmetric hexagonal space group P (6) over bar 2 m-D-36(3) No, 189. Th lattice parameters obtained from a Gandolfi came ra were a = 0.6625(2) nm and c = 0.7910(4) nm, Z - 1. For 366 reflecti ons (parallel to F-o parallel to greater than or equal to 3 sigma) the residual values are R = Sigma(parallel to Delta F parallel to/Sigma p arallel to F-o parallel to) = 0.064, R-w, = 0.075. Whereas previous st ructure determinations claimed full atom order, the present refinement clearly indicates that the crystal structure is partially ordered wit h random distribution of 0.94 Al + 0.06 Mg In the 6i-sites. Furthermor e, whilst lattice and atom parameter essentially correspond to the val ues derived earlier, the new composition as a result of the refinement (Al8.7Fe1Mg3.3Si5) is in dose agreement with the experimental composi tion Al9FeMg3.5Si5. (C) 1998 Elsevier Science S.A.