INVESTIGATION OF A DECAGONAL AL70CO15NI15 SINGLE-CRYSTAL BY MEANS OF HIGH-RESOLUTION SYNCHROTRON X-RAY-DIFFRACTION

X-ray diffraction provides an optimum resolution of about 10(-4) Angstrom(-1) in reciprocal space. Using synchrotron radiation, very weak diffraction phenomena can be studied with this resolution. Careful examination of the prominent Bragg reflections of a decaprismatic Al70Co15Ni15 needle reveals contributions both from a perfect decagonal single crystal and from the orthorhombic (5, 7) approximant of the decagonal phase. The decagonal phase is established by strong quasicrystal reflections perfectly obeying the decagonal metrics. The coexisting approximant state gives rise to a characteristically structured background close to the quasicrystal reflections and to additional weak reflections. Regarding exact peak positions, the weak reflections cannot be described as satellite reflections originating from a long-range superstructure to the decagonal phase.