STRUCTURAL-ANALYSIS OF THE DECAGONAL QUASI-CRYSTAL AL7ONI15CO15 USINGSYMMETRY-ADAPTED FUNCTIONS

The decagonal quasicrystalline structure of Al70Ni15CO15 is analyzed w ithin the superspace formalism. Symmetry-adapted functions have been u sed for the parametrization of the limits of the atomic surfaces that represent the atoms in superspace. As diffraction data, 253 independen t reflections from [Steurer W., Haibach T., Zhang B., Kek S. and Luck R. Acta Cryst. B49 (1993) 661.] have been considered. Starting from a circular approximation for the atomic surfaces, their boundaries were re fined with the program an QUASI. A fit comparable to that reported in the above reference was attained with 15% fewer adjustable paramete rs. The main difference is the non-inclusion of Debye-Waller-type fact ors in internal space. The boundaries of the resulting atomic surfaces are rather wavy or circular in contrast with the polygonal forms of t he previous model. It could be verified that these polygons evolve in a free refinement towards the obtained wavy forms. The effect on the d iffraction intensities of Debye-Waller factors along internal space is rather equivalent to that of wavy boundaries for the atomic surfaces. An open question is, then, whether the polygonal forms considered in previous models are physically significant or model-forced. The result ing differences in physical space between the two models are subtle, t he main features being essentially identical; however, both of them pr esent a significant. proportion of non-physical atomic distances that concern fully occupied atomic positions. Previous quantitative diffrac tion analyses of other quasicrystalline structures have also shown thi s tendency to introduce a significant number of unphysical interatomic distances; its cause remains unclear, but could be related with the r ecently conjectured existence of nondense atomic surfaces in real quas icrystals.