STRUCTURE OF THE DECAGONAL QUASICRYSTAL AINiCo

摘要

The decagonal quasicrystalline structure of A^nNiuCojs is analyzed within the superspace formalism. Symmetry-adapted functions are used for the parametrization of the limits of the atomic surfaces. As diffraction data, 253 independent reflections from [Steurer, W., Haibach, T., Zhang, B., Kek, S. & Luck, R.(1993), Acta Cryst. B49, 661-675] have been considered. Starting from a circular approximation for the atomic surfaces, their boundaries have been refined with the program QUASI. A fit comparable to that reported in the above reference was attained with 15% less adjustable parameters. The main difference is the not inclusion of Debye-Waller-type factors in internal space. The boundaries of the resulting atomic surfaces are rather wavy or remain circular, in contrast with the polygonal forms of the previous model. The effect of Debye-Waller factors along internal space on the diffraction intensities seems to be rather equivalent to that of wavy boundaries for some of the atomic surfaces. An open question is, then, whether the polygonal forms considered previously are physically significant or model-forced. The resulting differences in physical space between the two models are subtle, the main features being essentially identical; however, both of them present a significant proportion of non-physical atomic distances that concern fully occupied atomic positions, which cannot be neglected or explained. Previous quantitative diffraction analyses of other quasicrystalline structures have also shown a tendency to introduce a significant number of unphysical interatomic distances; its cause remains unclear, but could be related with the recently conjectured existence of non-dense atomic surfaces in real quasicrystals.