Decagonal quasicrystals of melt-quenched AlCuCo and AlCo alloys have been investigated by convergent-beam electron diffraction (CBED) and transmission electron microscopy. The CBED method has revealed that the space group of the alloys is non-centrosymmetric P10m2. Al73Co27 is the first non-centrosymmetric binary quasicrystal. Dark-field microscopy has revealed the existence of inversion domains with an antiphase shift of c/2 at the domain boundaries. These results are analogous to those for the melt-quenched alloy Al70Ni15Fe15 (Saito et al., Jpn. J. Appl. Phys., 31 (1992) L109). However, structural phase transformation as a function of the transition metal composition, as observed for the AlNiFe system, does not occur in the present system, and two types of pentagonal atom cluster images are observed in high-resolution electron microscope images, but only one type in the AlNiFe alloys.
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