Pentagonal Al-Co-Ni quasicrystal with a superstructure

A one-dimensionally periodic pentagonal quasicrystal with a superstructure has been found in the Al-Co-Ni system in a sample quenched from 1160 degree s C at the composition Al71.5Co25.5Ni3. Its electron diffraction patterns a re characterized by a fivefold rotation axis as a unique symmetry element a s well as superstructure reflections similar to those of a related decagona l phase. The presence of this symmetry in electron diffraction patterns ori ginates from strong dynamical scattering effects of electrons in relatively thick regions of the sample, in combination with the unique parallel arran gement of fivefold symmetric atom clusters. Differently, the superstructure reflections are caused by the long-range order of this new state which can be described by a random variant of a Penrose rhomb tiling.