Abstract
Decagonal quasicrystals have been investigated by convergent-beam electron diffraction (CBED) and transmission electron microscopy. Figure 1 shows possible pentagonal and decagonal point groups. The CBED method has revealed that the decagonal quasicrystals found so far belong to the space groups of noncentrosymmetric P10m2 and centrosymmetric P1O5mmc.Dark-field microscopy has revealed the existence of inversion domains with an antiphase shift of c/2 at the domain boundaries in the alloys with space group m2.2 High-resolution electron microscopy (HREM) has revealed the existence of specific pentagonal atom clusters in the Al-Ni-Fe, Al-Cu-Co and Al-Ni-Co alloys. Figure 2 shows a HREM image of Al70Ni15Fe15 belonging to space group m2. The atom clusters of an about 2nm diameter are clearly seen as indicated by a black circle. The clusters are polar or noncentrosymmetric due to the dark pentagon at their centers. All the clusters in domain A have one sense of polarity and those in domain B the other sense. It should be noted that the HREM image of the cluster columns was found to be pentagonal at an accelerating voltage of 200kV but nearly decagonal at higher than 300kV.
Published Version
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