FIVE DIMENSIONAL MAXIMUM ENTROPY METHOD APPLIED TO THE STRUCTURE REFINEMENT OF DECAGONAL Al₇₀Mn₁₇Pd₁₃. S. Weber^AB, A. Yamamoto^B, ^AInstitute of Applied Physics, University Tsukuba, Tsukuba, Ibaraki 305, Japan, ^BNational Institute for Research in Inorganic Materials, Namiki 1- 1, Tsukuba, Ibaraki 305, Japan

A five dimensional structure model for the decagonal phase Al₇₀Mn₁₇Pd₁₃ is proposed, whereby a combination of both the least-squares method and the five dimensional maximum entropy method was applied in the refinement. The calculations are based on a single- crystal X-ray data set using 1428 unique reflections. The superspace group is P10_5/mmc and the structure consists of ten layers (2 flat, 8 puckered), which are stacked along the tenfold screw axis with a ~12 Å period. The model of Hiraga and Sun (1993) was taken as a starting model to obtain an initial 5D-model consisting of four large occupation domains (OD) in. V_I (internal space). Then the size, shape and chemical occupation of the OD's was repeatedly refined to match the 5D-MEM maps. When refining the positional parameters it was found that shifts of atom positions along the tenfold screw axis play a very important role. By doing so the Rw-factor dropped by ~7%. No other refined parameter shows a similar significance. In order to verify the projected structure in V_E(external space) 3D-MEM sections were used to compare the atom positions within the layers with the ones obtained by the refinement of the 5D-model. Since a good agreement was found our refinement then focused on the occupational parameters. Our 5D-model model consists basically of four large and some smaller OD's, which again are subdivided in many smaller ones. The large OD's also split up due to the shifts along the tenfold screw axis so that some parts of them are situated at x₅- coordinates, which are different from the ideal positions that were formerly derived by other groups (eg.:Steurer et al. 1993).