Two-dimensional quasicrystal with fivefold rotational symmetry and superlattice

A two-dimensional quasicrystal with a fivefold rotational axis has been found in an Al-Co-Ni-Tb alloy by means of transmission electron microscopy. An electron diffraction pattern taken with the incident electron beam parallel to the unique periodic axis shows two-dimensional quasiperiodicity. Moreover, fivefold orientation symmetry clearly appears in the pattern, when a thick sample is used in the experiment. In addition, the S1 type of superlattice has been observed in the quasiperiodic reciprocal plane. The period of the fivefold axis is suggested to be about 0.4 nm for the basic structure and 0.8 nm if chemical order in the structure is taken into account. The lattice image taken with the incident beam along the fivefold axis is related to a fivefold aperiodic tiling under certain assumptions.