DIFFUSE-SCATTERING OF DECAGONAL AL70NI15CO15 MEASURED WITH SYNCHROTRON-RADIATION

Diffuse scattering of the thermodynamically stable decagonal quasicrys tal (QC) Al70Ni15Co15 has been measured at a synchrotron radiation sou rce. Perpendicular to the unique 'periodic' direction there is an alte rnating sequence of 'Bragg' and (modulated) diffuse layers. Within the zeroth Bragg layer, strong diffuse streaks and diffuse satellite refl ections, as well as very weak. and slowly varying diffuse scattering, were measured up to high values of Q. Satellites and diffuse streaks, already known from precession photographs, were explored in the q-scan mode with a step width of 0.01 in h and k (pseudo-monoclinic setting with a = 0.081 08 Angstrom(-1), b* = 0.081 09 Angstrom(-1) in the QC plane, c = 0.245 16 Angstrom(-1) in the periodic direction, gamma = 7 2 degrees). The positions, but not the intensities of all registered s atellite reflections obey an overall tenfold symmetry around the Bragg reflection. The satellite vector magnitude \q(s)\ = 3.3 x 10(-2) Angs trom(-1) corresponds to a translational superperiod of the QC substruc ture with a 30 Angstrom period. First- and second-order satellites wer e always observed only in pairs around the Bragg reflections, superimp osed on the diffuse streaks through the Bragg reflection and the satel lites. These observations may be interpreted by lamellar domains exten ded parallel to the unique axis. The full widths at half-maximum of th e satellites are between 1.4 x 10(-3) and 2.0 x 10(-3) Angstrom(-1), c orresponding to correlation lengths between 700 and 500 A of the domai n ordering. The diffuse streaks indicate faults in the regular sequenc e of the lamellar domains. An additional uniform diffuse background is observed, reflecting a high degree of disorder in the (projected) QC arrangement of the basic columnar-like structural elements.