AN EXAMINATION OF A MULTIPLE-TWINNED PERIODIC APPROXIMANT OF THE DECAGONAL PHASE AL70CO15NI15

High-resolution x-ray scattering was performed on a decaprismatic samp le of the nominal composition Al70Co15Ni15. The overall diffraction pa ttern has tenfold symmetry within experimental resolution, but many lo w-indexed reflections reveal a splitting, which is evidence for twinne d high-order periodic approximants of the decagonal phase, rather than for a truly decagonal quasicrystal. To our knowledge, we report the c learest separation of this kind of splitting to date. For the orthorho mbic approximant, the space group Ccmm with lattice parameters a = 61. 024(5) angstrom, b = 83.99(1) angstrom and c = 4.0799(5) angstrom was derived. Deviations from the diffraction pattern predicted by the twin ning model were observed. We point out the close similarity between th e x-ray diffraction of the real decagonal quasicrystal and that of our twinned sample.