Superordering in Ni-rich and Ni-poor decagonal Al-Co-Ni phases

Single-crystal X-ray diffraction was carried out on a decagonal Ni-rich pha se in the Al-Co-Ni system with a nominal composition of Al72.5CO11Ni16.5. A part from sharp Bragg reflections, satellite scattering was observed in Al7 2.5Co11Ni16.5 which is different from the satellite scattering observed in a Ni-poor phase with nominal composition Al72.5Co16.5Ni11. The two types of superorder scattering are interpreted in an unified way by periodic order superimposed on the quasiperiodic averaged substructure. This periodic regi mes have a limited correlation length (short-range ordering). While the mod ulation period stays almost constant over a wide range of compositions in t hese decagonal alloys, the modulation Vector is rotated by pi/10 if passing from Ni-rich (S satellites) to Ni-poor (C satellites) compositions. Argume nts are given in favour of this interpretation which is also supported by c omputer simulations.