An examination of a multiple-twinned periodic approximant of the decagonal phase Al₇₀Co₁₅Ni₁₅

Abstract

High-resolution x-ray scattering was performed on a decaprismatic sample of the nominal composition Al70Co15Ni15. The overall diffraction pattern has tenfold symmetry within experimental resolution. But many low-indexed reflections reveal a splitting, which is evidence for twinned high-order periodic approximants of the decagonal phase, rather than for a truly decagonal quasicrystal. To our knowledge. We report the clearest separation of this kind of splitting to date. For the orthorhombic approximant, the space group Ccmm with lattice parameters a=61.024(5) AA, b=83.99(1) AA and c=4.0799(5) AA was derived. Deviations from the diffraction pattern predicted by the twinning model were observed. We point out the close similarity between the x-ray diffraction of the real decagonal quasicrystal and that of our twinned sample.