COMPARISON OF HREM IMAGES AND CONTRAST SIMULATIONS FOR DODECAGONAL NI-CR QUASI-CRYSTALS

Since high-resolution electron micrographs of dodecagonal Ni-Cr quasicrystals are similar in contrast to those of several closely related periodic phases, it has been argued that dodecagonal quasicrystals can be described as a decoration of a dodecagonal quasiperiodic tiling with the same structural units as occur in these periodic phases. In order to corroborate this hypothesis, electron microscopic contrast simulations using such model structures are presented. The models considered are based on different quasiperiodic, twelvefold-symmetric tilings as well as on several periodic tilings leading to the structures of the periodic phases mentioned above. The simulated contrast is in excellent agreement with the experimental electron microscopic images, both for the periodic and for the quasiperiodic structures. It essentially reflects the structure of the underlying tiling, also for the quasiperiodic structures. It is therefore concluded that the usual interpretation of high-resolution structure images is valid not only for periodic but also for the nonperiodic dodecagonal structures, and that therefore the decoration scheme used correctly describes the atomic structure of the dodecagonal quasicrystal.